Rhynchophorus palmarum (South American palm weevil)

The taxonomic position of genus Rhynchophorus and genus Dynamis has been revised by Wattanapongsiri (1966). The author classifies Rhynchophorus palmarum as Coleoptera, tribe Rhynchophorini, subfamily Rhynchophorinae. He reports 10 species in the genus Rhynchophorus, which is widely distributed in the tropics and subtropics of the Old and New World.

EggsEggs are located individually 1-2 mm inside soft plant tissue, near the apical area of the palm. Eggs are protected by a brown waxy secretion. The eggs are 2.5 x 1 mm in size, white and with rounded extremes. Old eggs often show undulatory movements of the emerging larvae, which show their darker cephalic coloration through the chorion of the egg.LarvaeThe larvae have no legs and are initially 3-4 mm long. They possess sclerotized mouth parts with strong mandibles. Larvae are cannibalistic. Their body is slightly curved ventrally and may reach 5-6 cm in length. Their colour is cream white. Prepupae become darker and before pupating they migrate to the periphery of their gallery in the trunk, floral rachis or leave stem.PupaePupae are exarate and light brown. The abdomen continuously makes undulatory movements when perturbed. Pupae inhabit a cylindrical-ovoid closed cocoon 7-9 cm long and 3-4 cm in diameter, built with vegetative fibres, organised in a spiral configuration.The eggs, larvae and pupae are described by Wattanapongsiri (1966).AdultsAdult R. palmarum have a black, hard cuticle and possess the characteristic elytra of Coleoptera, protecting the abdomen when closed. They measure 4-5 cm in length and are approximately 1.4 cm wide, weighing 1.6-2 g. The head is small and round with a characteristic long, ventrally curved rostrum. Adults show sexual dimorphism; males have a conspicuous batch of hairs on the antero-central dorsal region of the rostrum.

As reported by Wattanapongsiri (1966), the genus Rhynchophorus has an extensive worldwide distribution, but is concentrated in the tropics. R. palmarum is exclusively a New World species where its range limit to the north is the south-east of California and Texas in the USA and to Argentina, Paraguay, Uruguay and Bolivia in the south. Countries reporting the largest damage to crops in palm plantations include Central America (Costa Rica), Colombia, Venezuela and Brazil. It is common in virgin forests and in agroecosystems exploiting oil palms. The altitudinal range is from sea level up to 1200 m (Jaffé and Sánchez, 1990).Previous editions of the Compendium (1997, 1998) included records for Afghanistan, Japan and Vietnam, which were based on erroneous data and have now been removed.

R. palmarum has probably reached the limits of its natural distribution in the American continent, and seems unlikely to be able to displace closely related sister species in other parts of the world.

R. palmarum has been reported on 35 plant species from 12 different families, but is found predominantly on palms (Esser and Meredith, 1987; Griffith, 1987; Wattanapongsiri, 1966; Jaffé and Sánchez, 1990; Sánchez and Cerda, 1993). The list of hosts may give the impression that R. palmarum attacks a great number of plant species. However, the insect has only been reported as a pest in palms and on sugarcane (Arango and Rizo, 1977; Restrepo et al., 1982). When reported on other plants, R. palmarum was feeding on ripe fruits, but was not causing economic damage.

Identification of attacked plants by visual symptoms alone may lead to wrong identification. The external symptoms on infested palms are a progressive yellowing of the foliar area, destruction of the emerging leaf and necrosis in the flowers. Leaves start to dry in ascendant order in the crown; the apical leaf bends and eventually drops. Internally, the galleries and damage to leaf-stems produced by the larvae are easily detected in heavily infested plants. Pupae and old larvae are frequently found when inspecting the crown of infested plants. Affected plant tissue turns foul, producing strong characteristic odours. If the nematode Rhadinaphelenchus cocophilus is present, a transversel cut of the trunk will reveal the characteristic red-ring which consists of an brownish-red area, 3-6 cm wide and 3-4 cm from the periphery.

A diagnostic protocol for R. palmarum is given in OEPP/EPPO (2007).

The larvae of R. palmarum feed exclusively on live vegetative tissue. The females lay their eggs inside the plant tissue by making a hole in the plant with the rostrum, normally when the surface of the plant tissue presents some damage, near or on the internodal area of the palm trunk next to the crown. Studies on the biology of this species are reported in Wilson (1963), Nadarajan (1988), Sánchez et al. (1993) and Hagley (1965).Hagley (1965) reported that under laboratory conditions (70-91°F and 62-92% relative humidity), a female may lay an average of 245±155 eggs during a period of 30.7±14.3 days. The incubation period is 3.2±0.93 days and the larvae have between six and ten instars over a period of 52.0±10.0 days. The prepupal stage lasts 4-17 days, during which the larvae make a cocoon using vegetative fibres. The pupal metamorphosis period lasts for 8-23 days and the adults remain in the cocoon for 7.8±3.4 days before emerging. Adult males may live for 44.7±17.2 days and females for 40.7±15.5 days. Hagley (1965) reported that a single female may lay up to 718 eggs, whereas Sánchez et al. (1993) reported a maximal oviposition of 697 eggs.Nadarajan (1988) and Sánchez et al. (1993) studied the biology of the insect using alternative rearing methods with artificial diets. The last work describes the behaviour of the insect including courtship, mating and oviposition in the laboratory. They indicated that the females deposit their eggs into holes in the plant made by the rostrum. Eggs are then oviposited individually in randomly distributed holes. The egg rests in a vertical position in the hole which is sealed by the female with a brown waxy secretion.The adults are active during the day showing a bimodal daily activity cycle. Hagley (1965) reported major activity peaks between 7 and 11 am, and 5 and 7 pm. Sánchez and Jaffé (1993) observed flight activity in the field, confirming the binomial nature of the activity cycle, in which adults fly only with sunlight, but avoiding the hottest hours at noon and the early afternoon. Field observations showed that adults may fly at velocities of 6.01 metres per second (Hagley, 1965). When using attractive odours a distinct chemotropic and anemotropic behaviour is evident (Sánchez and Jaffé, 1993).Studies on the population dynamics of this species in Central America are reported by Chinchilla (1988), showing that the maximum adult population occurs during the dry season. Similar results were obtained by Schuiling and Van Dinther (1981) in Brazil.

Studies by Moura et al. (1993) in plantations of the oil palm Elaeis guineensis in Brazil, showed a median rate of parasitism by Paratheresia menezesi of 51.0%, and an average of 18.33% of R. palmarum pupae infested with pupae of P. menezesi.Griffith and Koshy (1990) reported entomopathogenous nematodes of the families Rhabditidae and Heterorhabditidae on adult R. palmarum. Nickle (1970) reported that the nematode Praecocilenchus rhaphidophorus is an obligate parasite of species in the genus Rhynchophorus.

Since the beginning of twentieth century R. palmarum has been reported as one of the most important pests on ornamental palms and on oil palms, mainly in commercial plantations of Cocos nucifera and Elaeis guineensis (Griffith 1968, 1970; Dean 1979; Fenwick 1967; Sánchez and Cerda 1993).

The larvae feed on the growing tissue in the crown of the palm, during which it makes a gallery, often destroying the apical growth area and causing eventual death of the palm. Economic damage depends on the palm species and on the number of larvae infesting the plant. Fenwick (1967) and Griffith (1968) reported that populations of 30 larvae are sufficient to cause the death of an adult coconut palm.

In addition to the direct damage caused by this pest, R. palmarum is an active vector of the nematode Rhadinaphelenchus cocophilus, which in turn is an obligate parasite distributed in all tissues of the plant. This nematode causes the plant illness known as red-ring disease which has reached epiphytotic levels in the past (Griffith, 1968). Coconut palms of 3-10 years of age die during the first 2 months after inoculation (Griffith, 1987). Thurston (1984) and Brathwaite and Siddiqi (1975) reported that infested plants take 23-28 days to show the symptoms of red-ring disease, and die 3-4 months after showing the first symptoms.

Esser and Meredith (1987) estimated that several millions of US dollars are lost annually due to the association of red-ring disease and R. palmarum. They estimated that 800 hectares of coconut plantations were abandoned in 1923 due to this disease, and that in Grenada 22% of the coconut palms were infested with red-ring disease. A similar situation seems to be common in other countries in America.

R. palmarum primarily attacks the apical region of palm crowns, and larvae remain inside the galleries they build. Thus, the pest is only detected when damaged plants start to die, or by using pheromone baited traps (Jaffé et al., 1993; Chinchilla and Oehlschlager 1992a, b; Sánchez and Jaffé, 1993). Similar damage by Dynamis borassi, a sympatric and morphologically similar looking weevil, may occur.

Control strategies have to take into account that R. palmarum is both a pest in its own right and a vector of the nematode Rhadinaphelenchus cocophilus. Bain and Fedon (1951) determined that contamination of healthy plants with red-ring disease occurs only if insect vectors are present. The most important vector in America is R. palmarum. At the moment, the control of red-ring disease is by control of the insect vector as no efficient control of the nematode exists. Chemical control of the insect, although often attempted, is not successful (Hagley, 1963). Cultural control consisting of the burning of affected trees reduces infestation. Chemical killing and drying of infected plants also reduces infestation (Victoria et al., 1970; Blair, 1970; Griffith, 1987), as larvae need living plant tissue in order to survive. The use of natural enemies against this pest may be possible, but has yet to be established. Moura et al. (1993) suggested that P. menezesi may be used to regulate populations of R. palmarum.

The most widely used control methods are based on the capture of adults with traps baited with rotting plant materials, such as palm tissue, pineapple and sugar cane (Griffith, 1987; Dean, 1979; Morin et al., 1986; Genty, 1988; Moura et al., 1990). Various different types of traps have been proposed in order to attract the insects and kill them in the trap with chemicals (e.g. triclorfon and pirimifos-ethyl) (Dean, 1979). Yellow traps seem to be more efficient than those of other colours (Camino, 1975).

The most modern versions of the trap use natural or synthetic aggregation pheromones to help attract the insects. Moura et al. (1989) and Rochat et al. (1991a) showed that males produce an aggregation pheromone, attracting males and females equally. Rochat et al. (1991a, b) identified the pheromone as 2(E)-6-metil-2-hepten-4-ol, calling it Rhynchophorol. It was found that male insects only release the pheromone when feeding. Jaffé et al. (1993) showed that it was the smell of the appropriate plant odours, mainly ethyl-acetate, that started the release by males of the aggregation pheromone, and that the aggregation pheromone alone only attracts insects up to a certain distance, after which plant odours are required to attract the insect into the trap. Oehlschlager et al. (1993) and Chinchilla and Oehlschlager (1992a, b, 1993) evaluated pheromone baited traps in the field. Various efficient trapping methods have been proposed (Moura et al., 1990, 1993; Chinchilla and Oehlschlager 1992a, b; Oehlschlager et al., 1992a, b; Vera and Orellana, 1988; Jaffé et al., 1993; Sánchez and Jaffé, 1993), all based on containers which attract the insect with odours produced by plant tissue (mostly sugarcane) and the aggregation pheromone. The pheromone can be obtained either by commercial synthesis or by filling the trap with males, activating them with ethyl-acetate odours to induce production of the pheromone (Sánchez and Jaffé, 1993). Captured insects can then be killed with insecticide or by other means.