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23 March 2023

Prostephanus truncatus (larger grain borer)

Datasheet Types: Pest, Invasive species

Abstract

This datasheet on Prostephanus truncatus covers Identity, Overview, Distribution, Dispersal, Hosts/Species Affected, Diagnosis, Biology & Ecology, Natural Enemies, Impacts, Prevention/Control, Further Information.

Identity

Preferred Scientific Name
Prostephanus truncatus (Horn)
Preferred Common Name
larger grain borer
Other Scientific Names
Dinoderus truncatus Horn
Stephanopachys truncatus Back and Cotton (1938)
International Common Names
English
grain borer, larger
greater grain borer
scania beetle
Spanish
barrebador de los granos
barrenador del grano mayor
barrenador grande de los graneros
Local Common Names
Germany
Bohrer, grosser Korn-
Tanzania
dumuzi
scania

Pictures

Lateral view of adult Prostephanus truncatus.
Adult
Prostephanus truncatus (larger grain borer); Adult lateral view, note 1mm scale. June 2004.
©Pest and Diseases Image Library/via Bugwood.org - CC BY-NC US 3.0
Lateral view of adult Prostephanus truncatus.
Adult
Prostephanus truncatus (larger grain borer); Adult beetle, lateral view.
©Georg Goergen/IITA Insect Museum, Cotonou, Benin
Typical cylindrical bostrichid shape of Prostephanus truncatus, declivity flattened and steep with many small tubercles over its surface.
Adult
Prostephanus truncatus (larger grain borer); Typical cylindrical bostrichid shape, declivity flattened and steep, with many small tubercles over its surface; body 3-4.5 mm long.
©NRI/MAFF

Summary of Invasiveness

Prostephanus truncatus is a facultative wood-boring beetle, native to the Neotropics, that spread to Africa beginning in the 1980s. Since then, it has become one of the most destructive insect pests of post-harvest maize and cassava due to its secondary adaptation to stored, starchy agricultural crops.
Prostephanus truncatus spreads rapidly in trade moving in infested consignments of maize and dried cassava. Trade flows have a profound effect on its speed of movement. On a local scale, the beetle will disperse by flying from environments where population density is high or food exhausted to seek new hosts, usually in subsistence farmers' granaries. It locates stored product hosts, such as maize and dried cassava either by chance or because they are already infested and harbour beetles releasing aggregation pheromone. This type of host selection can lead to very dense populations of the pest in a few farm stores surrounded by other stores showing no signs of infestation. Effective control of P. truncatus is difficult; it has shown resistance to many chemical insecticides and non-chemical controls and implementing the phytosanitary measures needed to limit spread can be challenging. In addition, the insect is particularly damaging in smallholder farm environments where resources and access to effective management may be limited.

Taxonomic Tree

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Notes on Taxonomy and Nomenclature

Prostephanus truncatus is a member of the Bostrichidae (Coleoptera), or horned powder-post beetles, which is a group of primarily wood-boring insects. P. truncatus was first described as Dinoderus truncatus in 1878 by Horn from beetles (noted as 'mutilated specimens' by Fisher (1950)) found introduced in California (Chittenden, 1911). It has also been referred to as Stephanopachys truncatus by Back and Cotton (1938). The genus Prostephanus was established by Lesne (1898) to accommodate this and three other species; P. truncatus is the only one of these species known to be associated with stored products (Hodges, 1986).
The most commonly used English name for P. truncatus is the larger grain borer. Some countries favour using ‘greater grain borer’ to convey semantic distinction from the closely related lesser grain borer (Rhyzopertha dominica) (Hodges, 1986).

Description

Prostephanus truncatus is holometabolous. Adult P. truncatus can be identified using the keys of Fisher (1950)Kingsolver (1971) and Haines (1991). Additionally, Golob and Hodges (1982) provide a key for distinguishing P. truncatus adults from other bostrichids found in stored grains. A key to both larvae and adults is given in Gorham (1991). The taxonomy, systematics and identification of P. truncatus have been reviewed by Farrell and Haines (2002).
Eggs
Eggs of P. truncatus are broadly ellipsoid, glossy, and darken from white to yellow with age (Kučerová and Stejskal, 2008). Eggs average ~670 µm long and 400 µm wide (Kučerová and Stejskal, 2008). Comparisons between P. truncatus and Rhyzopertha dominica eggs are provided in Kučerová and Stejskal (2008).
Larvae and pupae
The scarabaeiform larvae are white, fleshy and sparsely covered with hairs. They are parallel-sided, i.e. do not taper. The legs are short and the head capsule is small relative to the size of the body. Spilman (1984) includes additional description, including traits for distinguishing P. truncatus from other larvae infesting stored foods. Pupae of P. truncatus are initially white, but darken with age (Spilman, 1984). Pupae may be sexed based on the size and shape of the genital papillae (Hodges, 1986).
Adults
Adult beetles are reddish brown to dark brown, small (~4 mm in length), and have a cylindrical shape typical of bostrichids (Suma and Russo, 2005). The declivity (i.e. the apical portion of the elytra that slopes downward) is flattened and steep and has many small tubercles over its surface. The limits of the declivity, apically and laterally, are marked by a carina (i.e. a ridge of the cuticle). The antennae are 10-segmented and have a loose three-segmented club, which is densely covered with mostly short hairs (Fisher, 1950). The body is 3-4.5 mm long and 1-1.5 mm wide (Fisher, 1950). Adult females may often be distinguished from males based on greater relative height of tubercles on the clypeus (Shires and McCarthy, 1976). The type specimen (Dinoderus truncatus Horn) is located in the Horn Collection in the Academy of Natural Sciences of Philadelphia (Fisher, 1950).

Distribution

Prostephanus truncatus is considered Neotropical in origin (Athanassiou et al., 2017). Some authors note it as native only to Mexico and Central America (e.g. Borgemeister et al., 1998a; Arthur et al., 2019; Quellhorst et al., 2021), while others also include the southern United States and parts of South America (Hodges, 1986; Leos-Martinez et al., 1995). In the early 20th century, P. truncatus was reported as occasionally infesting maize in the southern United States but not as a species widely distributed (Fischer, 1950). Chittenden (1911) states that while P. truncatus has been found infesting stored maize in the United States, it “has never found permanent lodgement in the United States” and notes imports from Mexico and regions from South America as sources of introduction. Quellhorst et al. (2021) lists the United States as a country with established populations.
Prostephanus truncatus was documented in Tanzania in 1981 and has since been found throughout ~20 countries in eastern, western and southern Africa (Muatinte et al., 2014; Quellhorst et al., 2021).
Several countries have recorded interceptions of P. truncatus in trade, including Canada (Manitoba), the United States (e.g. Arizona, California, Virginia), Germany, France, Italy (Suma and Russo, 2005), Israel and Iraq (USDA-APHIS, 1982; Hodges, 1986; IIE, 1995; Suma and Russo, 2005).
A putative record of P. truncatus in Thailand is considered a misidentification (R. Hodges, Natural Resources Institute, UK, personal communication, 2006).
A record of P. truncatus in Peru (Hodges, 1986) published in previous versions of the Compendium is invalid as it is based on personal correspondence cited in Wright (1984). SENASA has developed a specific surveillance programme for grain-stored pests in Peru. P. truncatus has not been detected in any surveys conducted by SENASA since the surveillance programme began in 1999 (SENASA, 2021).
Quellhorst et al. (2021) provide an updated summary of the global distribution of P. truncatus, including countries with established populations and those with recorded interceptions.

Distribution Map

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Distribution Table

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History of Introduction and Spread

Establishment of P. truncatus outside of the Americas had not been recorded until outbreaks were documented on stored maize and cassava in the Tabora region of Tanzania in 1981 (Golob and Hodges, 1982). The arrival of P. truncatus to Tanzania occurred sometime earlier, though the exact date is unknown; maize imports from the Americas are the presumed pathway of spread (Golob and Hodges, 1982; Cross, 1985). After 1981, P. truncatus spread within Tanzania and further throughout eastern and southern Africa, including into Kenya (1983), Burundi (1984), Malawi (1992), Rwanda (1993), Namibia (1998) and South Africa (1999). P. truncatus was confirmed as generally widespread in Zimbabwe by 2008, likely arriving through borders with Zambia and Mozambique (Nyagwaya et al., 2010), which had their first reports in 1993 and 2007, respectively (Muatinte and Cugala, 2015; Quellhorst et al., 2021). In 1984, the first reported outbreak occurred in West Africa in Togo from which it spread into neighbouring Benin (1986), Ghana (1987), Burkina Faso (1991), Nigeria (1992) and Niger (1994) (Hodges, 1986; 1994). It was reported from Senegal in 2007, likely spread from Guinea Conakry (Gueye et al., 2008).
Much of the movement throughout Africa has been attributed to the transport and trade of infested maize grain and cassava, though natural dispersal by adult beetles probably aids in local spread within and across borders (Hodges, 1994; Nyagwaya et al., 2010; Muatinte et al., 2014). In addition, the transport of infested firewood may also significantly contribute to the spread of P. truncatus; the insect could survive and reproduce on at least three tree species sold as firewood in Mozambique (Muatinte et al., 2014).
See Hodges (1994), Muatinte et al. (2014) and Quellhorst et al. (2021) for additional details on the timing and extent of introductions through time. Quellhorst et al. (2021) and Nansen et al. (2001) summarize country-level interceptions as well.

Risk of Introduction

Prostephanus truncatus remains a quarantine threat to tropical and sub-tropical maize and cassava-growing regions. It is one of the few stored product pests that have achieved major status as a quarantine pest (Heather and Hallman, 2008). Phytosanitary measures recommended for P. truncatus in international trade have been reviewed by Tyler and Hodges (2002), with the authors noting that effective implementation of such measures has often been unsuccessful. For example, despite phytosanitary measures to reduce the risk of spread between countries, the entry of P. truncatus into Zimbabwe probably came via seed maize and food-relief maize grain distributed to farmers (Nyagwaya et al., 2010). Tshikhudo et al. (2021) found P. truncatus to be one of four quarantine pests intercepted between 2011 and 2019 from illegal and/or non-compliant plant commodities imported into South Africa.
Arthur et al. (2019) modelled the potential climatic suitability of global areas for P. truncatus in natural habitats (i.e. those outside of buffered storage structures) based on temperature and moisture. Their results highlighted tropical and sub-tropical areas as highly suitable, suggesting the potential for spread into new countries, such as tropical Asia, and further spread into the United States and South America if suitable hosts were present on the landscape.
The apparent rapid adaptation to cassava as an alternative agricultural host is considered an important factor contributing to the spread of P. truncatus in Africa, and in addition to maize production, is likely to contribute to risk elsewhere (Arthur et al., 2019).

Means of Movement and Dispersal

Natural Dispersal

Adult P. truncatus can fly and are the dispersal stage. Mated female beetles have been suggested to be the primary dispersers, and young beetles tend to have higher flight activity compared to older beetles (Scholz et al., 1997; Nansen et al., 2001). Both male and female beetles will disperse towards the male-produced aggregation pheromone, though multiple studies have found a female-bias (e.g. ~58-67%) in pheromone trap catch (Hodges, 1986; Borgemeister et al., 1998b). Assessment of the dispersal potential of adults is mixed. Studies have recorded beetles to fly in a directed manner for 50-100 m over 24 h and 250-340 m over 72 h towards a pheromone source (Muatinte et al., 2014). A tethered flight mill study considered P. truncatus to be a strong flier after observing beetles sustaining flight for 25 km over 45 h; however, evidence of such distances or flight durations occurring in natural environments is lacking (Muatinte et al., 2014). In a wind tunnel experiment, where P. truncatus was concluded to not be a strong flier, adults initiated flight most frequently under still air or low wind (e.g. no more than 20 cm/s) (Fadamiro, 1996).
Quellhorst et al. (2021) describe P. truncatus as a landscape-level pest, in that it exhibits frequent dispersal among forested and agricultural landscapes. A field study in Benin found evidence of dispersal between woody and agricultural hosts by comparing the lignin and starchy contents in the guts of captured P. truncatus beetles caught in different habitats (Borgemeister et al., 1998b).
In addition to wind, multiple abiotic factors have been evaluated for their effect on the natural dispersal of P. truncatus. In Benin, Nansen et al. (2001) found that flight activity of adults could be most explained by daylength, minimum temperature, minimum relative humidity and the interactions thereof. Flight was positively correlated with daylength when temperatures and relative humidity were low, and when minimum relative humidity was >75%. Minimum temperature was negatively correlated with flight. Precipitation was found to have little impact on P. truncatus activity (Nansen et al., 2001). In Mozambique, abundance and flight activity (as measured by pheromone-baited traps) of P. truncatus was low when temperature and rainfall were low (Muatinte and Berg, 2021). Flight activity was also greater near human settlements (i.e. areas with cultivated grain), compared to forested environments, during the rainy season.
High beetle population densities have been shown to initiate flight in adults (Quellhorst et al., 2021), which has been hypothesized to occur due to a decline in resource quality (Fadamiro et al., 1998). Though maize or maize volatiles have not been shown to elicit a flight response from P. truncatus(Fadamiro et al., 1998), volatiles from woody hosts and cues from other wood-boring insects are attractive to P. truncatus(Borgemeister et al., 1998a). However, beetle flight activity was greater near human settlements (i.e. areas with cultivated grain) compared to forested environments during the rainy season in Mozambique (Muatinte and Berg, 2021), though this could reflect a difference in local abundance rather than host attractiveness.
A field study in Honduras showed flight activity of P. truncatus following a daily bimodal pattern with a major peak at 06.00-08.00 h and a minor peak at 18.00-20.00 h (Novillo Rameix, 1991). A similar pattern was observed by Tigar et al. (1993) in Central Mexico and Birkinshaw et al. (2004) in Ghana, but in both cases, the major peak was associated with dusk.

Accidental Introduction

Prostephanus truncatus is spread over long distances through the import and export of infested grain. Spread from infested grain also contributes to local dispersal. Spread from Mexico and Central America into Africa is suspected to be due from shipments of infested maize, and the movement of maize grain and dry cassava chips probably contributes significantly to the spread of P. truncatus within Africa (Muatinte et al., 2014).
Muatinte et al. (2014) summarize the spread of P. truncatus within Africa, noting it took approximately 19 years to spread and establish in nine West African countries, 16 years in five Eastern and Central African countries, and 15 years in six Southern African countries.
These rates of spread could be underestimating the potential speed of movement due to potential delays in reporting and formal documentation of the insect’s arrival to a region (Cross, 1985; Muatinte et al., 2014). In addition, the relative contributions of natural versus human-mediated dispersal to these spread rates is unknown, but movement in trade and other human transport (e.g. infested firewood (Muatinte and Berg, 2019)) is considered significant.

Pathway Causes

Pathway causeNotesLong distanceLocalReferences
Crop production (pathway cause) YesYes 

Pathway Vectors

Pathway vectorNotesLong distanceLocalReferences
Land vehicles (pathway vector)Carried by national and international grain tradeYes  
Plants or parts of plants (pathway vector) Yes  

Plant Trade

Plant parts liable to carry the pest in trade/transportPest stagesBorne internallyBorne externallyVisibility of pest or symptoms
Bulbs/Tubers/Corms/Rhizomes    
Roots
adults; eggs; larvae; pupae
Yes Pest or symptoms usually visible to the naked eye
True seeds (inc. grain)
adults; eggs; larvae; pupae
Yes Pest or symptoms usually visible to the naked eye
Wood
adults; eggs; larvae; pupae
Yes Pest or symptoms usually visible to the naked eye
Plant parts not known to carry the pest in trade/transport
Bark
Flowers/Inflorescences/Cones/Calyx
Fruits (inc. pods)
Growing medium accompanying plants
Leaves
Seedlings/Micropropagated plants
Stems (above ground)/Shoots/Trunks/Branches

Hosts/Species Affected

Natural hosts

As summarized by Nansen et al. (2004), P. truncatus is a wood-boring insect that has become secondarily adapted to stored, starchy agricultural crops. Extensive populations of P. truncatus occur in the natural environment, and the insect has been recorded from a number of tree species in Mexico and Central America (Rees et al., 1990Ramirez-Martinez et al., 1994) and Africa (Nang'ayo et al., 1993; 2002; Borgemeister et al., 1998a, b; Hill et al., 2002; Nansen et al., 2002; 2004), confirming the role of woody species as important hosts in both its native and adventive regions.
In Mexico, P. truncatus has been found infesting Spondias purpurea and Bursera fagaroides (Ramírez-Martínez et al., 1994). Since the late 1980s, numerous studies have investigated the suitability of additional woody species as hosts for P. truncatus, though listings can be inconsistent. Quellhorst et al. (2021) provide a comprehensive review of natural host plants documented since 2019 from field or laboratory conditions.
Current evidence suggests that P. truncatus only targets woody hosts that are stressed or damaged (reviewed in Quellhorst et al., 2021). Branches of woody hosts in the family Anacardiaceae girdled by other wood-boring insects (e.g. cerambycid beetles) are the only non-agricultural host on which P. truncatus has been observed in the field (Nansen et al., 2004). Laboratory studies have observed reproduction and attack on other host material such as live branches, roots and seeds, and on tree species within multiple additional families, but the relevance of these for in-field populations dynamics is unclear (Nang’ayo et al., 1993; 2002; Nansen et al., 2004; Muatinte and Berg, 2019).
Dry stalks of the grasses Hyparrhenia hirta and Acroceras macrum were found to be highly suitable hosts for P. truncatus development in a laboratory setting (Muatinte and Berg, 2019).
Prostephanus truncatus has been associated with teak (Tectona grandis) forests in Africa (Nansen et al., 2002) and laboratory studies have suggested some reproductive suitability (Nansen et al., 2004). Similarly, branches of Lannea nigritana have also been found with P. truncatus in forests of southern Benin (Borgemeister et al., 1998a) and reproduction of P. truncatus on L. nigritana branches has been demonstrated in the laboratory (Nansen et al., 2004).
Prostephanus truncatus adults have been noted to bore into myriad materials, such as leather, wooden materials/objects and plastics, among others (Hodges, 1986; Hill et al., 2002; Nansen et al., 2004; Nyagwaya et al., 2010). The role of wood structures and other materials as sources of refugia for P. truncatus around grain stores or within spread pathways is unknown, but suspected to contribute. Nang’ayo et al. (2002) note that the ability of woody substrates to simply sustain survival of adult P. truncatus, regardless of reproductive and developmental suitability, can be an important factor in the persistence of P. truncatus populations from year to year, and therefore, leading to potential unintentional spread across landscapes.
Moisture content is considered a vital factor contributing to survivorship of P. truncatus on its various hosts (Meikle et al., 1998). Moisture content and age of wood affected breeding success of P. truncatus on woody hosts in a laboratory setting, with moisture of at least 10% and young, soft sapwood being preferred (Nang’ayo et al., 1993). However, Nansen et al. (2004) found that moisture content alone was not indicative of host suitability (as measured by reproductive rate) for six non-agricultural host species. Nang’ayo et al. (2002) also found no clear correlation between the breeding success of P. truncatus and relative humidity or moisture content across 27 species of woody plants, and suggested that multiple factors, such as secondary plant compounds, are likely to be responsible for suitable breeding sites.

Agricultural hosts

Quellhorst et al. (2021) provide a comprehensive review of the known agricultural host plants documented since 2019 from field or laboratory conditions.
Maize (Zea mays) is currently the primary grain commodity on which P. truncatus can develop (Athanassiou et al., 2017); P. truncatus is reported as a serious pest of maize on the cob in the field (with moisture content as high as 40-50%), in cribs/on platforms during drying, and in stores (Hodges, 1986; Quellhorst et al., 2021). Prior to its spread into Africa, P. truncatus had already been known as a pest of field and stored maize in Mexico and parts of South America (Ramírez-Martinez et al., 1994). The variety and form of maize can have differing suitability for P. truncatus. Beetles infesting maize on the cob and maize flour have significantly higher population growth rates than whole, loose grains, though the development time on flour was longer (Athanassiou et al., 2017).
In addition, P. truncatus is an important pest of dried cassava (Manihot esculenta), including fermented and unfermented dried roots and bagged chips (reviewed in Quellhorst et al., 2021).
Laboratory studies evaluating various additional commodities (e.g. rice (Oryza sativa), wheat (Triticum aestivum), sorghum (Sorghum bicolor), cowpea (Vigna unguiculata), haricot beans (Phaseolus vulgaris), coffee beans, etc.) as potential hosts have shown variable to no suitability, though results have been inconsistent (Shires, 1977; Hodges, 1986; Athanassiou et al., 2017; Quellhorst et al., 2021). Importantly, the potential to incur damage from adult P. truncatus does not necessarily align with developmental suitability and the potential for population growth, and some non-maize commodities allowed for low levels of survival without growth. There is general consensus that P. truncatus cannot develop in small grains, such as wheat, barley and rice, but Athanassiou et al. (2017) note the potential role of ‘vehicle’ commodities in allowing for a small amount of pest persistence to occur long enough to facilitate transport to areas with more suitable hosts.
Though some attraction may occur over short distances, field studies in both Mexico and Togo suggest that there is no long-range attraction of adult P. truncatus to maize grain or cobs, or dried cassava. Laboratory tests indicate that upwind flight is mediated by a male-released aggregation pheromone and not by host volatiles (Fadamiro et al., 1998) and field studies provide evidence that host selection, in the case of maize and cassava, occurs by chance (Birkinshaw et al., 2002). Additional details of host selection can be found in Hodges (1994)Scholz et al. (1997)Hodges et al. (1998) and Borgemeister et al. (1998b).

Host Plants and Other Plants Affected

HostFamilyHost statusReferences
Acacia mellifera (blackthorn)Fabaceae 
Acacia polyacantha (white thorn)Fabaceae 
Acacia xanthophloeaFabaceae 
Muatinte and Berg (2019), Quellhorst et al. (2021)
Acroceras macrum  
Muatinte and Berg (2019), Quellhorst et al. (2021)
Agave sisalana (sisal hemp)Agavaceae 
Detmers (1990), Quellhorst et al. (2021)
Androstachys johnsoniiEuphorbiaceae 
Muatinte and Berg (2019), Quellhorst et al. (2021)
Arachis hypogaea (groundnut)FabaceaeUnknown 
Brachystegia boehmiiFabaceae 
Muatinte and Berg (2019), Quellhorst et al. (2021)
Brachystegia spiciformis (bean-pod tree)Fabaceae 
Muatinte and Berg (2019), Quellhorst et al. (2021)
Brachystegia utilisFabaceae 
Muatinte and Berg (2019), Quellhorst et al. (2021)
Bursera fagaroidesBurseraceaeWild host
Ramirez-Martinez et al. (1994), Quellhorst et al. (2021)
Calliandra houstoniana var. calothyrsus (calliandra)Fabaceae 
Cassia afrofistulaFabaceae 
Nang'ayo et al. (1993), Muatinte and Berg (2019), Quellhorst et al. (2021)
Colophospermum mopane (mopane)Fabaceae 
Muatinte and Berg (2019), Quellhorst et al. (2021)
Commiphora africana (corkwood)Burseraceae 
Commiphora baluensis  
Commiphora campestris  
Commiphora mildbraedii subsp. mildbraedii  
Commiphora rostrataBurseraceae 
Delonix elata (creamy peacock flower)Fabaceae 
Dioscorea (yam)DioscoreaceaeUnknown 
Eulalia villosa  
Muatinte and Berg (2019), Quellhorst et al. (2021)
Hordeum vulgare (barley)Poaceae 
Athanassiou et al. (2017), Quellhorst et al. (2021)
Hyparrhenia hirta (coolatai grass)Poaceae 
Muatinte and Berg (2019), Quellhorst et al. (2021)
Julbernardia globifloraFabaceae 
Muatinte and Berg (2019), Quellhorst et al. (2021)
Lannea  
Lannea nigritanaAnacardiaceae 
Borgemeister et al. (1998a), Nansen et al. (2004), Quellhorst et al. (2021)
Leucaena diversifoliaFabaceae 
Leucaena leucocephala (leucaena)Fabaceae 
Leucaena shannoniiFabaceae 
Manihot esculenta (cassava)EuphorbiaceaeMain
Pachira quinata (red ceiba)Bombacaceae 
Detmers (1990), Quellhorst et al. (2021)
Pennisetum glaucum (pearl millet)Poaceae 
Muatinte and Berg (2019), Quellhorst et al. (2021)
Phaseolus vulgaris (common bean)FabaceaeUnknown 
Philenoptera violacea  
Muatinte and Berg (2019), Quellhorst et al. (2021)
Pinus (pines)Pinaceae 
Detmers (1990), Quellhorst et al. (2021)
Prosopis chilensis (mesquite)Fabaceae 
Prunus spinosa (blackthorn)Rosaceae 
Detmers (1990), Quellhorst et al. (2021)
Senna siamea (yellow cassia)Fabaceae 
Sorghum bicolor (sorghum)PoaceaeUnknown 
Spondias purpurea (red mombin)AnacardiaceaeWild host
Ramirez-Martinez et al. (1994), Quellhorst et al. (2021)
Strychnos spinosaLoganiaceae 
Muatinte and Berg (2019), Quellhorst et al. (2021)
Tectona grandis (teak)Lamiaceae 
Nansen et al. (2004), Quellhorst et al. (2021)
Triticale  
Athanassiou et al. (2017), Quellhorst et al. (2021)
Zea mays (maize)PoaceaeMain
Golob and Hanks (1990), Athanassiou et al. (2017), Muatinte and Berg (2019), Quellhorst et al. (2021)
Ziziphus mauritiana (jujube)Rhamnaceae 
Muatinte and Berg (2019), Quellhorst et al. (2021)

Growth Stages

Post-harvest

Symptoms

Adult P. truncatus tunnel through stored maize grain or other starchy products, such as dried cassava chips, creating large quantities of dust. They also create large holes that skeletonize the grain (Quellhorst et al., 2021). Larvae and pupae may be found in the tunnels made by the adults.

List of Symptoms/Signs

Symptom or signLife stagesSign or diagnosisDisease stage
Plants / Plants/Stems/internal feeding
Arthropods|Adults
Arthropods|Larvae
  
Plants/Seeds/internal feeding
Arthropods|Adults
Arthropods|Larvae
  

Similarities to Other Species/Conditions

Prostephanus truncatus is very similar in appearance to Rhyzopertha dominica (the lesser grain borer) and Dinoderus spp. with which it can co-infest stored grain. The most distinguishing features of P. truncatus adults are the presence of a basal row of teeth on the pronotum and the apex of the steeply inclined elytra having a strong carina (Suma and Russo, 2005). Additionally, P. truncatus is comparatively larger and has a smooth, polished surface (Back and Cotton, 1938).
See also Fisher (1950) for morphological differences among Prostephanus species.

Habitat List

CategorySub categoryHabitatPresenceStatus
Terrestrial Urban / peri-urban areas Harmful (pest or invasive)
TerrestrialTerrestrial – ManagedCultivated / agricultural land Harmful (pest or invasive)
TerrestrialTerrestrial ‑ Natural / Semi-naturalNatural forests Natural

Biology and Ecology

For further detailed information on biology and ecology, consult reviews by Hodges (1986; 1994), Markham et al. (1991)Nansen and Meikle (2002), Hill et al. (2002), Muatinte et al. (2014) and Quellhorst et al. (2021).

Reproductive Biology

Romano et al. (2020) used ethorobotics (i.e. the use of robotics to investigate animal behaviour) to explore the mutual pushing behaviour seen by adults during courtship and in aggressive intrasexual interactions.
Adult P. truncatus frequently initiate their attack on stored maize cobs with intact sheaths by boring into the base of the maize cob cores, although they eventually gain access to the grain via the apex of the cob by crawling between the sheathing leaves (Hodges and Meik, 1984). Beetles bore into the maize grains, generating large quantities of dust while making neat round holes that skeletonize the grains. Adult females lay eggs in chambers bored at right angles to the main tunnels (Guntrip et al., 1996). Reported oviposition rates vary, from up to 20 eggs at a time (Quellhorst et al., 2021), to a mean of 4 eggs/day (with a range of 0 to 10 eggs/day), to a mean of ~1 egg/day (Scholz et al., 1997). Female beetles can live for several months and lay a mean of 430 eggs in their lifetime (Hill et al., 2003). Once eggs are laid, the female will cover them with grain dust (Vowotor et al., 1998; Quellhorst et al., 2021). The number of eggs laid and the amount of dust are positively correlated, though the purpose of the dust (e.g., larval food, concealment, etc.) remains unconfirmed (Quellhorst et al., 2021).
Larvae hatch from the eggs after about 4 days at 32°C (70% RH) and go through three larval instars in an average of 16.1 days (Bell and Watters, 1982). On maize, first instars feed on the floury endosperm tissue, whereas second and third instars prefer the germ tissue (Vowotor et al., 1998). The last instar constructs a pupal case from frass and larval secretion and stays within the grain or the surrounding dust (Bell and Watters, 1982; Hodges, 1986). At 32°C (70% RH), the mean pupal development period is 4.7 days, with the mean total development time from egg to adult being ~25 days (Hodges, 1986). Across all life stages, the average lower developmental threshold of P. truncatus is ca. 15°C; stage-specific thresholds may be lower (reviewed in Quellhorst et al., 2021). With this threshold, P. truncatus requires 404-435 degree-days to complete development from egg to adult (Quellhorst et al., 2021). Development time can vary by host, though, with development requiring slightly longer on cassava compared to maize. Studies have been contradictory regarding whether male or female beetles emerge first (Hodges, 1986). When reared on maize, the sex ratio of emerged adults are typically equal and female beetles tend to weigh more than males when raised on either maize or cassava (Hodges, 1986).
Prostephanus truncatus development has been investigated in laboratory settings across a range of temperature and moisture conditions (Shires, 1979; 1980; Bell and Watters, 1982Hodges and Meik, 1984; Quellhorst et al., 2020). These studies suggest optimal developmental conditions occur at 32°C and 70-80% RH, which result in adult emergence within ca. 27 days when reared on maize grain (Shires, 1980). However, humidity within 50-80% RH does not greatly affect the development period or level of mortality; at 32°C, a drop in RH from 80 to 50% extended the mean development period by just 6 days and increased the mean mortality by only 13.3%. This tolerance of dry conditions was confirmed during field studies in Nicaragua and Tanzania in which maize at 10.6 and 9% moisture content, respectively, was heavily infested (reviewed in Hodges, 1986).
The success of P. truncatus as a storage pest may be partly due to its ability to develop in grain at low moisture. Many other storage pests are unable to increase in number under low moisture conditions, allowing P. truncatus a competitive advantage. For example, Sitophilus oryzae, a species occurring in the same ecological niche, needs a grain moisture content of at least 10.5% for development (Hodges, 1986).
Adult P. truncatus can survive on woody substrates otherwise unsuitable for breeding; survival for up to 3.5 months has been reported in wood (Nang’ayo et al., 2002).

Population Size and Density

Under ideal conditions, the intrinsic rate of population increase of P. truncatus has been estimated as 0.7-0.8/week when fed on maize grain or cobs (Hodges, 1986).
Studies evaluating population development in grain stores have shown that very low initial infesting population sizes (e.g., ~one beetle per 17 maize cobs) can result in high storage densities within one storage season, even without additional immigration (Scholz et al., 1997).

Associations

See Quellhorst et al. (2021) for a review of studies on the interspecific competition of P. truncatus in grain store environments. Within grain storage structures, there are many interacting insect species that may compete. The presence of another stored grain pest, Sitophilus zeamais, has been found to suppress P. truncatus in most conditions; S. zeamais seeks out grain with P. truncatus infestations and S. zeamais larvae will consume competing P. truncatus(Vowotor et al., 2005; Ngom et al., 2020; Quellhorst et al., 2020). However, both species experience direct competitive costs when in mixed colonies compared to single-species colonies (Quellhorst et al., 2020). Moreover, at warmer temperatures (e.g. 35°C) P. truncatus is capable of producing more progeny and destroying more maize than S. zeamais(Quellhorst et al., 2020). P. truncatus also shows a competitive advantage at warmer temperatures (e.g. 30°C) over another common, co-occurring insect pest of stored maize, S. oryzae(Baliota et al., 2022).
Few studies have evaluated the competitive interactions of P. truncatus with other insects outside of laboratory environments (Quellhorst et al., 2021). In a grain storage trial in Zimbabwe, the grain damage level from P. truncatus was found to be significantly correlated with the presence of the red flour beetle (Tribolium castaneum) (Nyabako et al., 2020).
In natural environments in both its native and invaded range, P. truncatus has been associated with woody plant species previously girdled by other wood-boring beetles (Cerambycidae) (Nang’ayo et al., 1993; Ramírez-Martínez et al., 1994; Nansen et al., 2004).

Environmental Requirements

Prostephanus truncatus can thrive at higher temperatures and lower moisture conditions than what is optimal for most other insects found infesting stored grains, resulting in a competitive advantage for P. truncatus (Quellhorst et al., 2021). For example, S. oryzae, a frequently co-occurring species with P. truncatus, needs a grain moisture content of at least 10.5% for development (Hodges, 1986), whereas heavy infestations of P. truncatus have been seen in grain with moisture content of 9% (Hodges, 1986). P. truncatus is highly tolerant to desiccation, reported to survive ∼0% RH for ∼20 days (Mutamiswa et al., 2021).
In laboratory studies, P. truncatus adults showed greater thermal tolerance (both heat and cold) than larvae when reared on maize or sorghum grain. For example, adults show higher basal heat tolerance (CTmax) and heat knockdown time than larvae (Machekano et al., 2020; Mutamiswa et al., 2021). Heat acclimation to 37°C results in higher CTmax of both larvae and adults by 5.6 and 2.3%, respectively. However, heat acclimation comes at the cost of water conservation and more significantly so for larvae than adults (Mutamiswa et al., 2021). Desiccation acclimation generally improves heat tolerance (i.e. CTmax and heat knockdown time), suggesting a cross-stress tolerance for P. truncatus(Mutamiswa et al., 2021).
Machenkano et al. (2020) measured various cold tolerance traits of P. truncatus, finding that mean supercooling points were -19.45°C and -16.65°C for larvae (third instars) and adults, respectively. 100% mortality of adults could be achieved by exposure to -9°C for 4 h or to -15°C for 30 min.

Notes on Natural Enemies

Historical surveys in the native range of P. truncatus have found few natural enemies associated with the beetle, and most were considered to have a cosmopolitan distribution (Quellhorst et al., 2021). However, Teretrius (formerly Teretriosoma) nigrescens, a beetle endemic to Meso-America, was found to be a common and effective predator of P. truncatus eggs and larvae (Quellhorst et al., 2021). T. nigrescens also attacks other stored product species, like the weevil Sitophilus zeamais, but prefers P. truncatus in laboratory choice tests (Holst and Meikle, 2003).
In flight, T. nigrescens is attracted to the male-released aggregation pheromone of P. truncatus. The predator is also broadly attracted to the frass of P. truncatus (Stewart-Jones et al., 2004; 2006).
Several hymenopteran parasitoids have been associated with P. truncatus in its native and invaded range, such as the wasp, Anisopteromalus calandrae (Hodges et al., 1983; Helbig, 1998; Quellhorst et al., 2021). The polyphagous hemipteran, Xylocoris flavipes, has been observed predating on all three larval instars of P. truncatus in West Africa; however, populations of X. flavipes declined as P. truncatus numbers increased. It is presumed that the conditions created by P. truncatus infestation (i.e. large quantities of dust) are unfavourable to X. flavipes and hence this predator probably does not play an important role in the control of P. truncatus in stored product environments (Helbig, 1999).
In a survey in Kenya, Odour et al. (2000) found that the entomopathogenic fungus Beauveria bassiana occurred on only 0.08 to 0.94% of the total P. truncatus collected. In a total immersion bioassay, isolates of Beauveria, Metarhizium and Paecilomyces obtained in Ethiopia where all found to be virulent against P. truncatus (Kassa et al., 2002).
Quellhorst et al. (2021) provides a list of known natural enemies of P. truncatus.

Natural enemies

Natural enemyTypeLife stagesSpecificityReferencesBiological control inBiological control on
Anisopteromalus calandraeParasite     
Beauveria bassiana (white muscardine fungus)Pathogen     
Metarhizium anisopliae (green muscardine fungus)Pathogen     
Pteromalus cerealellaeParasite     
Teretrius nigrescensPredator
Eggs/Larvae/Pupae
  Africa; TogoStored maize
Theocolax elegansParasite     
Xylocoris flavipesPredator     

Impact Summary

CategoryImpact
Animal/plant collectionsNone
Animal/plant productsNone
Biodiversity (generally)None
Crop productionNegative
Environment (generally)None
Fisheries / aquacultureNone
Forestry productionNone
Human healthNone
Livestock productionNone
Native faunaNone
Native floraNone
Rare/protected speciesNone
TourismNone
Trade/international relationsNegative
Transport/travelNone

Impact

P. truncatus is a pest of maize and dried cassava roots after harvest in sub-Saharan Africa and also from time to time in Central America. Infestations in maize may start on the mature crop in the field, i.e. when moisture content is at or below 18%. Weight losses of up to 40% have been recorded in Nicaragua from maize cobs stored on the farm for 6 months (Giles and Leon, 1975). In Tanzania, up to 34% losses have been observed after 3 months storage on the farm, with an average loss of 8.7% (Hodges et al., 1983). P. truncatus is a much more damaging pest when compared to other storage insects including Sitophilus oryzae, S. zeamais and Sitotroga cerealella, under similar conditions; maize losses due to these other species were 2-6, 3-5 and 2-5%, during a storage season in Zambia, Kenya and Malawi, respectively. Losses caused by P. truncatus in dried cassava roots can be very high; the dried roots are readily reduced to dust by boring adults and a loss of 70% has been recorded after only 4 months of farm storage (Hodges et al., 1985). A group of 25 farmers from five villages in Togo sustained average cumulative losses of 9.7% after 3 months storage, this figure rose to 19.5% after 7 months (Wright et al., 1993).Not all problems with this pest are restricted to farmers' granaries. In the early days after the arrival of P. truncatus in East Africa, countries with the pest found their maize exports banned. For example in 1987-88, it is estimated that Tanzania lost US$634,000 in export earning. This situation improved following efforts to upgrade phytosanitary procedures in the region but such procedures, involving fumigation, have their own continuing costs (Boxall, 2002).A grain injury model for P. truncatus infesting farm-stored maize in West Africa has been developed at the International Institute of Tropical Agriculture. It can be used in conjunction with predictive models of pest population dynamics to guide the development of integrated pest management strategies (Holst et al., 2000a). The models are conveniently displayed, together with information on sampling routines, on a web site ().A detailed review of the damage and loss caused by P. truncatus has been prepared by Boxall (2002). He considers loss of value, nutrition as well as impact at the national level, effect on international trade and modern methods of rapid loss assessment. In the early days after the arrival of P. truncatus in East Africa, countries with the pest found their maize exports banned. For example, in 1987-1988, it is estimated that Tanzania lost US$634,000 in export earning. This situation improved following efforts to upgrade phytosanitary procedures in the region but such procedures, involving fumigation, have their own continuing costs.

Impact: Economic

Prostephanus truncatus is primarily a pest of post-harvest maize and dried cassava roots, but damage to maize cobs or shelled grain can also occur in the field when moisture content is at or below 18% (Hill et al., 2002; Machenkano et al., 2020). Infestation by P. truncatus can reduce the storage viability of dry maize and cassava to as few as 6-8 months from a more typical 10-12 month period (Muatinte and Berg, 2021). P. truncatus feeding reduces maize germination, increases the grain’s moisture content, and facilitates contamination by microorganisms (Ngom et al., 2020). Fungal contamination has included the species Aspergillus flavus, which can produce aflatoxins (Ngom et al., 2020; Quellhorst et al., 2020).
Recorded loss estimates for P. truncatus can vary. Typically, damage caused by P. truncatus in stored dry maize or dry cassava chips is estimated as a percentage of infested kernels and/or damage or loss in mass during storage of these commodities, rather than as economic losses (Muatinte and Berg, 2021). In stored maize, mean losses are often reported between 7 and 41% of maize stored over 3 to 9 months (Boxall, 2002; Ngom et al., 2020; Quellhorst et al., 2020). For example, weight losses of up to 40% have been recorded in Nicaragua from maize cobs stored on the farm for 6 months (Giles and Leon, 1975). In Tanzania, up to 34% losses have been observed after 3 months storage on the farm, with an average loss of 8.7% (Hodges et al., 1983). However, in smallholder granaries in Mozambique, 100% loss of maize grain was attributed to P. truncatus (Muatinte et al., 2022).
Losses caused by P. truncatus in dried cassava can also be very high; mean weight loss in fermented and unfermented cassava root caused by P. truncatus ranges between 52 and 74% (Hodges et al., 1985; Quellhorst et al., 2021). In Benin, P. truncatus has been reported to infest 100% of dry cassava chips in storage, rendering them unsuitable for consumption (Muatinte and Berg, 2021). Mutamiswa et al. (2021) summarize that cumulative field and storage losses from P. truncatus can be up to ~80%. Machekano et al. (2020) summarize that field and storage damage from P. truncatus can account for 10-30% cob/grain/seed loss in about 3 months.
Compton and Sherington (1999) describe a rapid method for estimating the weight losses caused by P. truncatus to maize cobs.
Indirect economic losses can also occur from quarantine measures and other restrictions to international trade. For example, after the arrival of P. truncatus into East Africa, countries with the pest found their maize exports banned. In 1987-88, Tanzania lost US$634,000 in exports. Though the situation improved following efforts to upgrade phytosanitary procedures in the region, such procedures, like fumigation, have their own continuing costs (Boxall, 2002). Further documentation quantifying the extent of indirect cost impacts is otherwise lacking (Muatinte and Berg, 2021).
Compared to other common stored grain insects, P. truncatus is often more damaging. For example, P. truncatus and Sitophilus zeamais co-occur in grain stores in Central and South America and Africa. Mixed colonies of the beetles will have a reduced impact to grain compared to single species colonies, but especially at warmer temperatures (e.g. >25°C), P. truncatus will be capable of destroying more maize in a shorter period of time compared to S. zeamais(Quellhorst et al., 2020). P. truncatus differs from other beetle pests in maize storage in that the level of damage can vary significantly between local grain stores, which can complicate management (Birkinshaw et al., 2002).
Prostephanus truncatus can also infest and damage many other non-food products, such as stored timber, wood products and household goods of numerous materials (e.g. shoes, plastic, fabric) (Boxall, 2002; Nang’ayo et al., 2002; Muatinte et al., 2014; Muatinte and Berg, 2019).

Impact: Environmental

Impact on Habitats

Prostephanus truncatus has no known environmental impacts. Negative impacts to non-agricultural hosts and habitats are undocumented.

Impact on Biodiversity

Prostephanus truncatus has no known effects on biodiversity.

Impact: Biodiversity

P. truncatus has no known effects on biodiversity.

Impact: Social

Prostephanus truncatus can be particularly damaging to granaries of subsistence farmers and, in some regions (e.g. sub-Saharan Africa), the losses from P. truncatus can be significantly more than those caused by other storage pests. The depredation by P. truncatus can result in farmers having to purchase maize or having no maize available to generate income. The insect is thus a significant threat to food security in many countries.

Risk and Impact Factors

Invasiveness

Proved invasive outside its native range
Long lived

Impact outcomes

Host damage
Infrastructure damage
Negatively impacts agriculture
Negatively impacts livelihoods
Damages animal/plant products
Negatively impacts trade/international relations

Impact mechanisms

Herbivory/grazing/browsing

Likelihood of entry/control

Highly likely to be transported internationally accidentally

Detection and Inspection

Methods for detection and monitoring of P. truncatus has been reviewed in detail by Hodges (2002). Flight traps, such as funnel, delta or wing traps baited with the male-released aggregation pheromone of P. truncatus, are highly effective for monitoring this species. These traps should be placed at least 100 m from stores, which contain maize or dried cassava, or from the standing maize crop to avoid attracting the beetles to these food sources. A detailed leaflet giving recommendations on the use of pheromone traps to monitor P. truncatus has been prepared by Hodges and Pike (1995). For long-term, routine trapping programmes in both East and West Africa the Japanese beetle flight-trap baited with P. truncatus pheromone is now the method of choice while pheromone baited Delta traps, made from cardboard, are used in short-term programmes.The risk of stores becoming infested by P. truncatus is known to be related to the number of P. truncatus that are flying (Birkinshaw et al., 2002). There are big variations both within and between years in the numbers of beetles taking flight. In many locations this difference between years is noticeable by the extent to which farmer's stores become infested, i.e. there are good and bad years. A computer-based model has been developed that uses climate data to predict P. truncatus flight activity and in this way it can predict the years when P. truncatus infestation will be bad (Hodges et al., 2003).Monitoring for the presence of P. truncatus in farm maize stores themselves is difficult because the pest is not attracted to its pheromone when present on its food. A sequential sampling plan for inspecting stores in West Africa has been described by Meikle et al. (2000). Compton and Sherington (1999) have described a rapid method for estimating the weight losses caused by P. truncatus to maize cobs.

Prevention and Control

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.
Control

Chemical Control

Historically, management of P. truncatus has relied on using contact chemical insecticides to reduce populations, often applied as a dilute dust in many smallholder farm settings (Singano et al., 2020). Synthetic pyrethroids typically provide control of P. truncatus. However, these insecticides are relatively ineffective against other pests of stored products such as Sitophilus spp. and Tribolium castaneum, which often occur in the same pest complex as P. truncatus and are more susceptible to organophosphate compounds. This difference in susceptibility led to the development of various formulations that contain a mix of pyrethroids and organophosphates to control multiple species in the same application (Golob et al., 1985Golob, 2002). However, resistance to organophosphates and pyrethroids is a concern for P. truncatus, though few studies exist that investigate the nature and frequency of resistance (Golob et al., 1990; Boukouvala and Kavallieratos, 2021; Quellhorst et al., 2021).
Fumigation with gaseous insecticides, such as phosphine, has been reported to successfully manage P. truncatus, but the circumstances under which these chemicals can be used safely and effectively are increasingly limited (Nayak et al., 2020; Quellhorst et al., 2021).
Recent studies have found effective control of P. truncatus with novel insecticides, such as the anthranilic diamide, chlorantraniliprole (Boukouvala and Kavallieratos, 2021) and a formulation of an insect growth regulator with pyrethroid, deltamethrin and a synergist (Quellhorst et al., 2022). The bacterial insecticides, spinosad and spinetoram, have also shown promising efficacy against P. truncatus(Pozidi-Metaxa and Athanassiou, 2013; Athanassiou and Kavallieratos, 2014). Numerous botanical and plant derived insecticides have been used against P. truncatus, some with promising effect. However, challenges remain for the use of these products due to lack of investigation into safe, standardized and effective synthesis and application (Muatinte et al., 2014; Quellhorst et al., 2021).

Biological Control

Reviews on the control of P. truncatus by the predatory beetle, Teretrius nigrescens, have been published by Meikle et al. (2002) and Borgemeister et al. (2003). Known as an effective predator against P. truncatus in Meso-America, both the larvae and adults of T. nigrescens predate on P. truncatus eggs and larvae (Holst and Meikle, 2003). Initial releases of T. nigrescens as a classical biological control agent were in Togo in 1991 and in Kenya in 1992. In both countries T. nigrescens became well established and spread. Subsequent releases occurred in Benin, Ghana, Tanzania, Mozambique and Malawi (Schneider et al., 2004; Muatinte and Cugala, 2015). Only in Tanzania does it appear that there has been any difficulty in the predator becoming quickly and easily established. However, despite the successful introductions, suppression of P. truncatus populations is not enough to prevent regular outbreaks of P. truncatus and subsequent crop losses (Muatinte and Cugala, 2015). Ultimately, the use of T. nigrescens as a classical biological control agent is considered unsuccessful from most perspectives. However, the predator can nonetheless be effective at reducing P. truncatus densities that allow the delay and/or reduction in application of other control tactics (Holst and Meikle, 2003; Quellhorst et al., 2021).
Several hymenopteran parasitoid species have been identified parasitizing P. truncatus in both the pest’s native and invaded range (reviewed in Quellhorst et al., 2021).The cosmopolitan wasp, Anisopteromalus calandrae, has shown high efficacy against P. truncatus and other pests of stored grain and has been suggested to be used to augment the pest suppression from T. nigrescens(Bonu-Ire et al., 2015).
Entomopathogens, especially fungi, against P. truncatus and other stored product pests have shown promise (Rumbos and Athanassiou, 2017). For example, application of the fungus Beauveria bassiana in maize storage settings can reduce P. truncatus infestations. However, research evaluating other commodity settings, in-field applications and other entomopathogenic taxa (e.g. bacteria, nematodes, viruses) remains needed (Quellhorst et al., 2021).
Quellhorst et al. (2021) note that a challenge with biological control of P. truncatus is that natural enemies that may be effective in a stored-product setting may not be equally effective at attacking P. truncatus in a forested environment, and vice versa.

Cultural Control and Sanitary Methods

Implementing sanitary measures in grain storage facilities is critical to mitigating stored product pest infestations like P. truncatus(Morrison et al., 2019). For P. truncatus, some examples include removing infested residues or minimizing cracked kernel containment and grain dust (Athanassiou et al., 2017). Monitoring or removal of surrounding wooden structures or sources of host refugia is also advised, though may be less feasible in many small shareholder settings (Quellhorst et al., 2021). Relatively few colonizing individuals are needed to cause economically-damaging infestation levels in stored grain, so Scholz et al. (1997) note that preventing early arrival is more effective than the potential diminishing returns of inhibiting further immigration throughout the storage season.
Dry maize plant materials left in fields after harvest can sustain large population of P. truncatus, which can then continually infest nearby grain storage facilities. Removal of stalks and other plant residues from fields after harvest through slashing, ploughing, disking and burning are suggested as a cultural control against P. truncatus. However, these field sanitation methods can have adverse effects to soil moisture, soil organic content and reduce forage availability, which led many to doubt the sustainability of such cultural controls for small-scale farming systems (Muatinte and Berg, 2019).

Physical/Mechanical Control

Temperature treatments of stored grain have shown effective control of P. truncatus.
Heat treatments of moderate temperature and long duration (e.g. 45.5°C for 4 h or 47°C for 2 h) can cause complete mortality of P. truncatus adults, which appear to be the most heat tolerant life stage, while also minimizing negative effects to maize or sorghum grain germination (Machekano et al., 2020). Similarly, cold exposures of at -9°C for 4 h, -11°C for 2 h, -13°C for 1 h, or -15°C for 0.5 h resulted in 100% mortality of adults (Machekano et al., 2020).
The use of hermetically-sealed containers (e.g. bags or underground pits) for grain designed to limit oxygen can be effective against P. truncatus, especially with the development of new containment materials (McFarlane, 1988; Mutambuki et al., 2019; Singano et al., 2020; Quellhorst et al., 2021). However, the ability of P. truncatus to bore into plastics, wood, and many other materials poses a challenge with this technology and any other bagging or barrier designed to physically exclude insects from stored products (Morrison et al., 2019; Mutambuki et al., 2019). Ngom et al. (2020) also note that many subsistence farmers cannot afford or access hermetic storage products.

Host Resistance

Quellhorst et al. (2021) summarize that host plant resistance traits relevant to store product insects like P. truncatus could be anatomical, biochemical, or genetic in target. In general, there is limited work using host plant resistance to manage P. truncatus specifically (Quellhorst et al., 2021), though some success has been seen. Maize landraces from southern Mexico have been identified with high resistance to P. truncatus damage (García-Lara and Bergvinson, 2013). In Senegal, Ngom et al. (2020) identified three maize varieties with high relative resistance to P. truncatus to damage (based on the Dobie Index of Susceptibility of the grain) - DMR-ES, Early-Thaï and Tzee-Yellow - and found that high relative grain hardness, moisture and amylose content were associated with these varieties. Moderately resistant varieties also had high relative phenolic acid content.

IPM

Morrison et al. (2021) discuss recommendations for the use of semiochemicals and other behaviourally-based tactics in post-harvest integrated pest management (IPM) programmes. Morrison et al. (2019) detail the critical role that sanitation within storage facilities plays in the effectiveness of many IPM tactics used against stored product insect pests, including P. truncatus.
McFarlane (1988) reviews pest management tactics of P. truncatus in Africa, emphasizing the need to consider the regional differences in both P. truncatus impact and socio-economic factors in developing and implementing IPM. More recently, Muatinte et al. (2014) suggest one of the biggest challenges for IPM of P. truncatus in Africa is the effective engagement of farmers to implement and evaluate different methods of control, rather than the need for additional research on the integrations of tactics themselves.

Monitoring and Surveillance (incl. remote sensing)

Quellhorst et al. (2021) and Hodges (2002) provide reviews of major trapping methods used for monitoring and detection of P. truncatus.
Prostephanus truncatus populations are known to have sporadic and aggregated distributions in natural and stored products environments (Vowotor et al., 2005; Nyabako et al., 2020). This can make infestations difficult to accurately detect, especially at low densities (Scholz et al., 1997; Hodges, 2002). Further, in grain stores, the insect tends to aggregate at the base of storage containers beyond the reach of most sampling probes and adults feeding inside kernels may further avoid detection (Nyabako et al., 2020). Direct sampling of commodities or the use of probe traps to sample for P. truncatus is not advised (Quellhorst et al., 2021).
A sequential sampling plan for inspecting grain stores in West Africa is described by Meikle et al. (2000).
Flight traps, such as funnel, delta, or wing traps baited with the two-component male-released aggregation pheromone of P. truncatus, are highly effective and are commonly used for monitoring (Leos-Martinez et al., 1995; Hodges et al., 2004; Quellhorst et al., 2021). Both male and female beetles are attracted to the pheromone and just a single male emitting aggregation pheromones is sufficient to cause a high infestation in stored grain later in the season (Scholz et al., 1997). The aggregation pheromone is also attractive to the predatory beetle, Teretrius nigrescens (Hodges et al., 2004). There is some conflicting evidence on the role of host volatiles in the movement and attraction of P. truncatus, but generally, host odours are not considered to play a role in long range attraction of P. truncatus. Host odours can influence short range behaviour, but more likely result in arrestment of movement (reviewed in Quellhorst et al., 2021). Studies evaluating the optimal daily and seasonal timing for trap placement are summarized in Quellhorst et al. (2021).
The risk of grain stores becoming infested by P. truncatus is known to be related to the number of P. truncatus adults that are flying in the area and there can be large variations both within and between years in the numbers of beetles taking flight (Birkinshaw et al., 2002; Hodges et al., 2003). Hodges et al. (2003) developed a climate-based model to forecast annual dispersal activity of P. truncatus in Ghana. Nyabako et al. (2020) used a machine learning approach to forecast potential P. truncatus infestations and subsequent damage in smallholder grain stores in Zimbabwe.

Links to Websites

NameURLComment
GISD/IASPMR: Invasive Alien Species Pathway Management Resource and DAISIE European Invasive Alien Species Gatewayhttps://doi.org/10.5061/dryad.m93f6Data source for updated system data added to species habitat list.
Global register of Introduced and Invasive species (GRIIS)http://griis.org/Data source for updated system data added to species habitat list.

References

Adda, C., Borgemeister, C., Meikle, W.G., Markham, R.H., Olaleye, I., Abdou, K.S., Zakari, M.O., 1996. First record of the larger grain borer, Prostephanus truncatus (Coleoptera: Bostrichidae), in the Republic of Niger.Bulletin of Entomological Research, 86(1) 83-85.
Anon., 2004. Mozambique Food Security Update. December 2004. FEWS NET. http://www.fews.net/centers/files/Mozambique_200411en.pdf
Arthur, F.H., Morrison, W.R., Morey, A.C., 2019. Modeling the potential range expansion of larger grain borer, Prostephanus truncatus (Coleoptera: Bostrichidae).Scientific Reports, 9(1) 6862.
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