Callosobruchus maculatus (cowpea weevil)

There are a number of species of Callosobruchus that may be found attacking pulses, of which the most common and well known is C. maculatus. Adults of most species known from stored pulses may be identified using the bruchid key in Haines (1991).

Southgate et al. (1957) listed many synonyms of C. maculatus; the only synonym that has been used commonly in the general literature, however, is Bruchus quadrimaculatus Fabricius. Some records of C. maculatus from South Africa may represent misidentifications of C. rhodesianus.

Systematic studies of species of Callosobruchus, Acanthoscelides and Bruchus from Central Anatolia in Turkey were reported by Mergen and Cagatay (1996).

Eggs

The eggs are cemented to the surface of pulses and are smooth, domed structures with oval, flat bases.

Larva and Pupa

The larvae and pupae are normally only found in cells bored within the seeds of pulses. For descriptions and a key including C. maculatus larvae, see Prevett (1971) and Vats (1974).

Adult

C. maculatus adults are 2.0-3.5 mm long. The antennae of both sexes are slightly serrate (for details of antennal and sensilla structure see Mbata et al. (1997)). Females often have strong markings on the elytra consisting of two large lateral dark patches mid-way along the elytra and smaller patches at the anterior and posterior ends, leaving a paler brown cross-shaped area covering the rest. The males are much less distinctly marked. In common with other species of Callosobruchus, C. maculatus has a pair of distinct ridges (inner and outer) on the ventral side of each hind femur, and each ridge bears a tooth near the apical end. The inner tooth is triangular, and equal to (or slightly longer than) the outer tooth. A unique chordotonal structure in the fore coxae of adult C. maculatus and C. subinnotatus was described by Ramaswamy and Monroe (1997). The location and ultrastructure of sex pheromone glands in female C. maculatus is described by Pierre et al. (1996).

Several workers have described an active- or flight-form of adult C. maculatus which is apparently more active and is more strongly marked, with a white pygidium (Utida, 1953). The function of this form, which appears in populations as a result of genetic and environmental factors, is not understood.

The two most widespread species of bruchid beetle are C. maculatus and C. chinensis, which are distributed throughout the tropics and sub-tropics. C. maculatus originated in Africa where it remains dominant.

C. maculatus is a major pest of cowpeas, green gram and lentils. For a complete list of host plants, see Udayagiri and Wadhi (1989), and Desroches et al. (1997). Host plants vary considerably in their suitability for larval development (Wijeratne, 1998). Alpha-amylase inhibitors prevent development of C. maculatus on a number of legumes (Blanco-Labra et al., 1996; Reis et al., 1997; Ishimoto et al., 1999; Janarthanan et al., 1999) including Phaseolus vulgaris, but not the development of the bruchids Acanthoscelides obtectus and Zabrotes subfasciatus (Ishimoto and Chrispeels, 1996).

There have been many studies of host preference in C. maculatus and its ability to adapt to using hosts less suitable for larval development, for example Huignard et al. (1996); Taheri (1996); Sulehrie et al. (1998). Inheritance of aspects of host plant choice were observed by Messina and Slade (1997).

In the early stages of attack the only symptoms are the presence of eggs cemented to the surface of the pulses. As development occurs entirely within the seed, the immature stages are not normally seen. The adults emerge through windows in the grain, leaving round holes that are the main evidence of damage.

The adult beetles, which do not feed on stored produce, are very short-lived, usually no more than 12 days under optimum conditions. During this time the females lay many eggs (C. maculatus up to 115, C. chinensis up to 70), although oviposition may be reduced in the presence of previously infested seeds (Parr et al., 1998a,b). Some adult females may have the ability to distinguish their own oviposition markers and appear to ignore the oviposition markers deposited by other females (Wijeratne and Smith, 1998). The optimum temperature for oviposition is high in C. maculatus, about 30-35°C and low in C. chinensis, 23°C. As the eggs are laid, they are firmly glued to the surface of the host seed, smooth-seeded varieties being more suitable for oviposition than rough-seeded varieties (Parr et al., 1996).

C. maculatus is easily raised in laboratories and has been used as a model organism in a number of ecological studies. Its development is strongly influenced by temperature and humidity (Xu, 1999), host substrate and population source (strain or biotype; Messina and Slade, 1999). A number of aspects of the behaviour of C. maculatus have been studied in some detail including: male mating behaviour (Giga and Canhao, 1997; Savalli and Fox, 1999; Wilson et al., 1999), investigating the effects of male size, multiple matings and interspecific interference on female fecundity; the decision making process undertaken by ovipositing females (Parr et al., 1996; 1998a; Horng et al., 1999); and the chemical ecology of host selection (Parr et al., 1998b).

The eggs are domed structures with oval, flat bases. When newly laid they are small, translucent grey and inconspicuous. Eggs hatch within 5-6 days of oviposition (Howe and Currie, 1964). Upon hatching, the larva bites through the base of the egg, through the testa of the seed and into the cotyledons. Detritus produced during this period is packed into the empty egg as the insect hatches, turning the egg white and making it clearly visible to the naked eye. When multiple conspecific eggs are laid on a single seed, larval competition may be evident (Horng, 1997).

The developing larva feeds entirely within a single seed, excavating a chamber as it grows. The optimum development conditions for C. maculatus and C. chinensis are around 32°C and 90% RH; the minimum development period for C. maculatus is about 21 days, and 22-23 days for C. chinensis. At 25°C and 70% RH the total development period of C. maculatus breeding on seeds of V. unguiculata is about 36 days, pupation taking place within the seed 26 days after oviposition (Howe and Currie, 1964). Relatively little is known about C. phaseoli although the optimum temperature for development is within the range 30-32.5°C (Howe and Currie, 1964).

Infestation commonly begins in the field, where eggs are laid on maturing pods (Prevett, 1961; Fatunla and Badaru, 1983; Messina, 1984; Fitzner et al., 1985). As the pods dry, the pest's ability to infest them decreases. Thus dry seeds stored in their pods are quite resistant to attack, whereas the threshed seeds are susceptible to attack throughout storage.

The parasitic wasps Dinarmus spp., Eupelmus spp., Uscana spp., Lariophagus distinguendus, Heterospilus prosopidis and Anisopteromalus calandrae have been associated with a number of Callosobruchus species. It is probable that several other wasps known to parasitize a wide range of Coleoptera, such as Theocolax elegans, will also use Callosobruchus species as hosts.

See Biological Control for further details.

Callosobruchus spp. are important pests of pulses. Infestation may start in the pods before harvest and carry over into storage where substantial losses may occur. In Nigeria, it has been estimated that 3% of the annual production in 1961/62 was lost due to attack by C. maculatus.

Levels of infestation in storage are strongly influenced by the type of storage structure employed and the variety of seed (Ojimelukwe et al., 1999). Storage structures that maintain high levels of moisture in seeds are more prone to high levels of infestation (Singh, 1999). Temperature of storage also influences levels of infestations in local stores (Singh, 1997).

The values of dried pulses are strongly influenced by levels of bruchid infestation in local markets, particularly in Sub-Saharan Africa.

No particular detection or inspection methods for Callosobruchus spp. have been developed.

The potential exists for the development of population monitoring by use of sex pheromones. The female-produced sex pheromone has been isolated and identified (Phillips et al., 1996); and the behavioural and electroantennogram (EAG) response to pheremonal components by males was recorded by Shu et al. (1996).

Chemical Control

Callosobruchus spp. may be controlled by fumigation treatment with phosphine, although legislation in many regions now frequently prohibits or restricts the use of these products. Controlled atmospheres of carbon dioxide can provide complete control of C. maculatus and C. subinnotatus (Mbata et al., 1996), and sealed or hermetic storage affords some protection against C. maculatus and C. chinensis (Seck et al., 1996; Singh and Yadav, 1996).

Approved grain insecticides, especially organophosphates, will protect against this pest (Lienard and Mignon, 1997; Rani, 1997; Ofuya and Lagunju, 1998).

When grain pulses are stored at farm level the admixture of vegetable oil or essential oils including citrus peel oil can give protection (Don-Pedro, 1989; Don-Pedro, 1996; Rajapakse et al., 1997; Rajapakse and Rathnasekera, 1998), as can the admixture of dessicant dusts or ash (Apuuli et al., 1996; Mohamed, 1996; Sharshir, 1998; Appel, 1999), and certain aromatic leaves, fruits or plant extracts (Mohamed, 1997), many of which are traditionally used by subsistence farmers (Belmain et al., 1999), thereby reducing the need for, and risks associated with, the use of insecticides (Shaaya et al., 1997). The mode of action of these biologicals may be insecticidal or anti-ovipositional (Adedire and Lajide, 1999).

The list of plants and plant extracts shown to have insecticidal or anti-ovipositional effect against C. maculatus and other bruchid pests is very long (Hixing et al., 1998a, b), including for example: Aframomum melegueta (Adedire and Lajide, 1999); Aglaia elliptica (Prijono et al., 1997); Allium spp. (Adedire and Lajide, 1999); Annona spp. (Prijono et al., 1997; Aku, 1998); Bandeiraea simplicifolia (Zhu et al., 1996; Zhu-Salzman, 1998); Boscia senegalensis (Seck et al., 1996); Cassia spp. (Babu et al., 1999; Raja et al., 2000); Cinnamomum camphora (Dharmasena et al., 1998); Coleus aromaticus (Babu et al., 1999); Cymbopogon citratus (Dharmasena et al., 1998); Dennettia tripetala (Okonkwo and Okoye, 1996); Capsicum spp. (El-Lakwah, 1996; Gakuru and Foua, 1996); Citrus spp. (Onu and Sulyman, 1997; Dharmasena et al., 1998); Clerodendrum inerme (El-Lakwah et al., 1996); Cyperus rotundus (Adedire and Lajide, 1999); Duranta erecta (El-Lakwah et al., 1996); Dysoxylum cauliflorum (Prijono et al., 1997); Erythrophleum guineense (Adedire and Lajide, 1999); Eucalyptus spp. (El-Lakwah, 1996; Gakuru and Foua, 1996); Hyptis suaveolens (Adedire and Lajide, 1999); Ipomoea mauritiana (Dharmasena et al., 1998); Lantana camara (Dharmasena et al., 1998); Lippia multiflora (Koumaglo et al., 1996); Monodora myristica (Okonkwo and Okoye, 1996); Morinda tinctoria (Babu et al., 1999); Myroxlon balsamum (Dharmasena et al., 1998); Ocimum basilicum (Gakuru and Foua, 1996); Piper spp. (Lognay et al., 1996; Gakuru and Foua, 1996; Okonkwo and Okoye, 1996; Rajapakse, 1996); Pothomorphe umbellata (Adedire and Lajide, 1999); Stelechocarpus cauliflorus (Prijono et al., 1997); Syzygium aromaticum (Adedire and Lajide, 1999); Tamarindus indicus (Dharmasena et al., 1998); Tetrapleura tetraptera (Gakuru and Foua, 1996); Vitex spp. (Dharmasena et al., 1998; Raja et al., 2000); Xylopia aethiopica (Okonkwo and Okoye, 1996; Ojimelukwe and Okoronkwo, 1999); Zanthoxylum zanthoxyloides (Ogunwolu et al., 1998).

The most well known of anti-bruchid phytochemicals is azadirachtin which has been used alone, in 'botanical insecticides' or as a component of leaves or extracts (liquid or powder) of the neem tree, Azadirachta indica, particularly in the Indian subcontinent (Ogunwolu and Odunlami, 1996; Javaid and Mpotokwane, 1997; Rajapakse, 1998; Banjo and Mabogunje, 1999; El-Lakwah and El-Kashlan, 1999; El-Lakwah et al., 1999; Lale and Abdulrahman, 1999). Azadirachtin is both an anti-ovipositant and insecticide (larvicide and adulticide). Neem oil and other extracts or neem derivatives may be applied directly to seeds, where volatiles also have a fumigant effect (Reddy and Singh, 1998).

The effects of some phytochemical admixtures on bruchid parasitoid activity have been investigated (Raja et al., 2000).

Cultural Control and Sanitary Methods

Intercropping maize with cowpeas, and not harvesting crops late significantly reduced infestation by C. maculatus, C. rhodesianus, C. chinensis and Acanthoscelides obtectus in Kenya (Olubayo and Port, 1997). Patnaik et al. (1986) also reported that evels of infestation in the field are influenced by the sowing date of the seed.

Good store hygiene plays an important role in limiting infestation by these species. The removal of infested residues from last season's harvest is essential, as is general hygiene.

Solarization (sun drying and heating) can be used to control infestations without affecting seed germination (Mohamed and Ismail, 1996). When small lots are stored, sun-drying the beans periodically in a thin layer for periods of up to 4 hours can give substantial protection. Solar heaters or tranparent bags of seeds left in the sun can provide excellent control of infestations (Ntoukam et al., 1997; Ghaffar and Chauhan, 1999).

Irradiation

Irradiation by ionising gamma radiation has proved effective in controling C. maculatus in stores, although the practice is not widely allowed and may be costly. Hatching of eggs appears to be prevented at 10 Gy; larval development is arrested at 20 Gy; pupal development is arrested at 150 Gy. Up to 1500 Gy may be required to kill adults (Dongre et al., 1997), although sterilization of adult males and females may be achieved at doses of less than 100 Gy (Diop et al., 1997; Pajni et al., 1997a, b)

Host-Plant Resistance

Cultivars of host seeds vary considerably in their susceptability to insect attack (Ghadiri and Sohrabi, 1999; Padmavathi, 1999), and there has been much interest in screening varieties of seeds for resistance (Kalyan and Dadhich, 1999; Lambrides and Imrie, 2000), including artificially generated mutant lines (Wongpiyasatid et al., 1999) and resistant wild progenitors of commercial varieties (Dongre et al., 1996). The biochemical basis of resistance in seeds has been elucidated in some cases (Miura et al., 1996; Xavier-Filho et al., 1996). The seed chemicals responsible for resistance (mainly in cowpea varieties) include trypsin inhibitors (Shade et al., 1996), tannins (Oigiangbe and Onigbinde, 1996), chitinases and beta-1, 3-glucanases (Gomes et al., 1996), and insecticidal lectins ((Zhu et al., 1996; Omitogun et al., 1999).

Adzuki beans have been genetically modified to express alpha-amylase inhibitors (alphaAI), making them resistant to C. maculatus and C. chinensis (Ishimoto, 1996).

Biological Control

Biological control has not been widely used against Callosobruchus species, although natural populations of C. maculatus are often subject to high levels of parasitism, particularly in West Africa (Ouedraogo et al., 1996). It seems likely that studies conducted on the complex biology of plant-host-parasitoid interactions (for example, Monge and Cortesero, 1996) and the effect of climate (Ouedraogo et al., 1996) and host density (Tuda, 1996, 1998; Sanon and Ouedraogo, 1998) on parasitoid behaviour may be more likely to influence changes in storage practices to encourage parasitoids than encourage the implementation of classical biological control programmes. Nontheless, mass rearing techniques for parasitoids have been developed (Islam, 1998) and proceedures for control by innoculation of parasitoids have been suggested (Sanon et al., 1998). Much research has focussed on the larval and pupal parasitoids Dinarmus acutus (Gupta et al., 1998), Dinarmus basalis (Ohashi, 1996; Ouadraogo et al., 1996; Ahmed, 1997; Gupta et al., 1998; Islam, 1998; Sanon and Ouedraogo, 1998; Sanon et al., 1998; Schoeller, 1998; Rojas-Rousse et al., 1999), Eupelmus orientalis (Schoeller, 1998; Rojas-Rousse et al., 1999), Eupelmus vuilleti (Terrasse, 1996; Sanon and Ouedraogo, 1998; Schoeller, 1998; Rojas-Rousse et al., 1999) and Heterospilus prosopidis (Tuda, 1996, 1998; Tuda and Iwasa, 1998); and the egg parasitoids Uscana lariophaga (Van-Alebeek, 1996; Van-Alebeek et al., 1996; Van-Alebeek and Van Huis, 1997; Van-Huis et al., 1998; Zaghloul and Mourad, 1998) and Uscana mumukerjii (Pajni et al., 1996).

Entomophagus fungi (Beauveria bassiana and Metarhizium anisopliae) and bacteria (Bacillus thuringiensis) exhibit only a limited amount of efficacy against C. maculatus (Lopez-Meza and Ibarra, 1996; Vilas-Boas et al., 1996).