Natural Enemies of Harmful Arthropods in Oil Palm (Elaeis guineensis Jacq.) in Tropical America

In this review reference is made to 135 species of natural enemies of 27 pests associated to oil palm in Tropical America. Enphasis is placed on species found in Central America. Out of the 135 organisms mentioned, 98 are parasitoids, 15 predators and 22 are entomopathogens. Very little is known about the biology and behavior of the majority of these organisms, which limits their use in an Integrated Pest Management Program. More than 80 species of arthropods have been described as potential pests to the oil palm. Most of these do not cause any significant damage in most plantations, since they are regulated by their natural enemies. Nevertheless, it is thought that a better knowledge of the biology, behavior and interrelations between pests, their bioregulators and environment is very important to help to maintain a balanced ecosystem. The vegetation associated to the main crop is one of the most important elements of the environment; many plants are visited by parasitoids and predators in search for food and shelter. The presence of a cover vegetation may also help some diseases caused by fungi and bacteria to develop on a insect population.

The oil palm plantations of Central America have been visited by many entomologists in the past, who have documented some of the pests present. However, information about their natural enemies is sparse and only commonly found in publications with a very limited distribution. In South America, most of the information comes from Colombia, and to a lesser extent from Ecuador and Brazil (Genty et al. 1978; Reyes and Cruz 1986). Several investigators from Chiquita Brands Intnl. carried out studies on pests and their natural enemies in oil palm plantations in Costa Rica and Honduras (Stephens 1962, 1984; Evers 1976, 1979, 1982; Richardson 1979; Chinchilla 1992). More recently, MexzÂn and Chinchilla (1992) made an inventory of harmful arthropods and their natural enemies in three countries (Costa Rica, Honduras, and Panama), where 11 species of predatory organisms, 23 parasitoids, and at least 6 entomopathogens were found.

Regarding the beneficial vegetation, McKenzie (1977) in Sumatra, and Genty (1988) in Colombia, observed that the natural enemies of insect pests lived and fed on plants with nectaries in the Euphorbiaceae, Malvaceae, Solanaceae and other plant families. Mexzón and Chinchilla (1992) mentioned 18 species of common plants that were visited by parasitoids in oil palm plantations, and subsequently Mexzón (1992) added 19 more species. Delvare and Genty (1992) documented 187 species of parasitoids associated with 12 species of weeds in Colombia and Ecuador.

This literature review compiles information about the natural enemies of harmful arthropods in oil palm plantations in Tropical America.

Most harmful insects associated to the oil palm in Tropical America are lepidopterous larvae, in the families Attacidae (Automeris spp.), Brassolidae (Brassolis sophorae, Opsiphanes cassina), Limacodidae (Euprosterna eleasa, Euclea spp., Natada sp., Sibine spp.), Oecophoridae (Peleopoda arcanella), Psychidae (Oiketicus kirbyi) and Stenomidae (Stenoma cecropia). Damage is also occasionally caused by some Chrysomelidae (Delocrania cossiphoydes, Hispoleptis subfaciata, Spathiella sp. and Calyptocephala maginipennis) (Genty et al. 1987; Reyes and Cruz 1986; Genty 1989; Mexzón and Chinchilla 1992). Cutting ants (Formicidae) and the coleopterans Rhynchophous palmarum and Strategus aloeus are also very important.

In Colombia, E. eleasa Dyar, O. kirbyi Guilding and S. cecropia Meyrick have been some of the most important pests (Genty 1978; Genty et al. 1978; Reyes and Cruz 1986). Serious damage has been caused in Central America by O. cassina, S. cecropia, O. kirbyi and S. megasomoides (Ever 1976, 1979; Richardson 1979; Chinchilla 1992). O. kirbyi was an important pest in banana plantations during the '60s (Lara 1970; Stephens 1984).

At least 41 insect pests are associated with oil palm in Central America (Mexzón and Chinchilla 1992). Most of these are of secondary importance, since their populations are regulated by their natural enemies.

The best-known families are Chalcididae, Elasmidae, Eulophidae, Eurytomidae y Pteromalidae (Table 1 and Table 2).

Chalcididae. The genera Brachymeria and Conura are common in the oil palm environment, where they parasitize several lepidopterous larvae.   . The genera Brachymeria and Conura are common in the oil palm environment, where they parasitize several lepidopterous larvae.  

In the genus Conura, adult wasps are normally yellow with black bands on the thorax, abdomen and legs; however, some are black with red spots. Size of these insects is very variable, ranging from very small (2 mm), as in some of the hyper-parasitoids of Cotesia, to rather large  (15mm), such as those that parasitize Automeris spp. The genus Conura also attacks other Lepidoptera, such as members of the families Brassolidae, Limacodidae, Oecophoridae, Psychidae, Stenomidae, Tinaeidae etc. It has also been reported to attack some Chrysomelidae, such as Demotispa pallida Baly and H. subfaciata (Table 2).

C. macullata parasitizes O. cassina; groups of 12-16 adults emerge through circular holes in the pupal case of the host. In the Baru region, in the North of Panama, one pupa was found to host 36 individuals of this parasitoid. Conura miniata is a parasitoid of some Limacodidae, such as E. eleasa, and a hyper-parasitoid of Cassinaria sp. (Ichneumonidae), which is a very important parasitoid of many defoliators (Genty 1989; Mexzón and Chinchilla 1992). In Costa Rica at least two species of Conura have been found associated with O. kirbyi. The level of parasitism has reached up to 17%. parasitizes O. cassina; groups of 12-16 adults emerge through circular holes in the pupal case of the host. In the Baru region, in the North of Panama, one pupa was found to host 36 individuals of this parasitoid. Conura miniata is a parasitoid of some Limacodidae, such as E. eleasa, and a hyper-parasitoid of Cassinaria sp. (Ichneumonidae), which is a very important parasitoid of many defoliators (Genty 1989; Mexzón and Chinchilla 1992). In Costa Rica at least two species of Conura have been found associated with O. kirbyi. The level of parasitism has reached up to 17%.

Adults of the Brachymeria genus are normally black, with green bands on the tibia and metafemur. The host range of Brachymeria is probably similar to that of Conura. The number of species associated with the oil palm environment is probably very large, but only a few species have been described. In Napo, Ecuador for example, B. annulata parasitizes Saliana severus Mabille (Hesperiidae) (Genty 1989). Adults of this parasitoid are common visitors of many flowering plants. At least two species of Brachymeria hyperparasitize the genus Cassinaria (Genty 1989; Mexzón and Chinchilla 1992).

Other chalcids. Other common members of this group found in oil palm plantations are in the families Elasmidae (Elasmus spp), Eulophidae (Elachertus, Euplectromorpha, Kaleva, Nesolynx), Eurytomidae (Eurytoma) and Pteromalidae (Halticopteroide) (Table 1).The species of Eulophidae are the most widely-represented, and are parasitoids of nymphs of H. subfaciata and of various Lepidoptera larvae. . Other common members of this group found in oil palm plantations are in the families Elasmidae (Elasmus spp), Eulophidae (Elachertus, Euplectromorpha, Kaleva, Nesolynx), Eurytomidae (Eurytoma) and Pteromalidae (Halticopteroide) (Table 1).The species of Eulophidae are the most widely-represented, and are parasitoids of nymphs of H. subfaciata and of various Lepidoptera larvae.

In Costa Rica, Trichospilus diatrae (Eulophinae) was found to be a parasite of pupas of S. cecropia and Peleopoda sp. with a parasitism level of 40% and 80%, respectively. More than 240 wasps emerged from the pupa of S. cecropia, and between 40 and 60 from Peleopoda sp. (Mexzón and Chinchilla 1992). In O. cassina, a metal-blue colored species of Horismelus (Eulophidae) attacked larvae which then lost their mobility and ceased feeding. Subsequently, the larval tegument was broken, leaving a packet of tubular white pupae stuck to the foliage, thereby conserving the shape of the host. This particular parasite could have been taken for Cotesia in the past.

The Braconidae are a very diverse group of wasps which are Lepidoptera parasites. The most important genera are Cotesia, Fornicia, Digonogastra, Rhysipolis and Rogas (Genty et al. 1978; Reyes and Cruz 1986; Villanueva and Avila 1987; Genty 1989).

The Cotesia wasps are common endo-parasites of Limacodidae larvae (Sibine spp., Euclea diversa, E. eleasa), Attacidae (Dirphia gregatus Creamer) and O. kirbyi (Table 3). At the end of their development, the parasited larvae emerge through the host's tegument , and, in the case of the Limacodidae, spin a cylindrical white pupa on the moribund caterpillar; or inside the bag in the case of Psychidae (Desmier de Chenon 1989). In Sibine fusca, the young wasp becomes a parasite of the larvae at the eighth or tenth stage of their development. On breaking out, 100-250 individuals have been observed on the host larvae. The larval cycle lasts 10-12 days, and the parasitism level is commonly between 30-35% (Genty 1984).

In Sibine sp., the larvae of Cotesia emerge through the tegument in approximately 12 days, and form their pupae in about 40 min. The pupal stage lasts 7 days, and the adults emerge in a synchronized way over approximately 5 min. The host larvae survives until the parasitoids emerge as adults (Mexzón and Chinchilla 1992). These species probably synchronize their development with that of their hosts by producing hormones or somehow manipulating the host's endochrine system (Beckage 1985).  

Members of Cotesia may also be parasites of Elasmus sp.and Conura sp, which could be negative to their role as regulators of the Lepidoptera population (Genty 1984). However, it is not always evident that the presence of a hyper-parasite considerably affects the balance (level of parasitism) between the parasitoid and its host. The presence of a hyper-parasite could be beneficial for the parasitoid, by maintaining the population of the host above a level which is critical for the survival of the parasitoid (Wahid and Kamaruddin 1993).

Digonogastra sp. (Iphiaulax) is a wasp of about 5 mm in length; it has a black thorax and a yellow abdomen; the wings are smoky black, with large cell-venation. These wasps emerge (1-6 individuals) from larvae of O. kirbyi in bags measuring between 10-20 mm. During an outbreak of O. kirbyi in Coto (Costa Rica), eight different parasitoids were found on this insect; Digonogastra being the most abundant (55%). In 380 bags gathered in an area of young palms, the combined parasitism was 90%. The adults of Digonogastra were found around Scleria melaleuca (Cyperaceae), Amaranthus spinosus (Amaranthaceae) and Flemingia macrophylla (Leguminosae). In nature plantations, parasitism did not even reach 10%, possibly due to the scarceness of melliferous flora. sp. (Iphiaulax) is a wasp of about 5 mm in length; it has a black thorax and a yellow abdomen; the wings are smoky black, with large cell-venation. These wasps emerge (1-6 individuals) from larvae of O. kirbyi in bags measuring between 10-20 mm. During an outbreak of O. kirbyi in Coto (Costa Rica), eight different parasitoids were found on this insect; Digonogastra being the most abundant (55%). In 380 bags gathered in an area of young palms, the combined parasitism was 90%. The adults of Digonogastra were found around Scleria melaleuca (Cyperaceae), Amaranthus spinosus (Amaranthaceae) and Flemingia macrophylla (Leguminosae). In nature plantations, parasitism did not even reach 10%, possibly due to the scarceness of melliferous flora.

The genus Fornicia seems to specifically attack the Limacodidae family. The adults are black and have three fused abdominal segments. The larvae that are attacked maintain their coloration and do not become mummified. Only one single endo-parasite larva emerges from the host's ventral portion and spins its pupa (Desmier de Chenon 1989). In Colombia, Fornicia pos. clathrata has been found as a parasite of up to 60% of the larvae of Euprosterna elaeasa (Reyes and Cruz 1986) and Natada spp. (Genty 1989).

The females of Rhysipolis sp. attack larvae of the fifth or eighth stages, and leave their eggs above the pleural zone of the host's thorax. The young larvae of the parasite feed externally (3-8 larvae per caterpillar), and after developing construct a series of 6 mm-cylindrical pupae. During an outbreak in Colombia, the parasitism reached only 18% (Genty 1978).

One species of wasp of the sub-family Microgastrinae was found on larvae of E. eleasa and Sibine spp. in Costa Rica. This insect is similar to Cotesia sp. in color, size, and pupal shape. Several species of Braconidae have been found in Central America as parasites of Limacodidae and Stenomidae larvae, but little is known about their regulatory activity on important pests.

The most common family found in oil palm is Ichneumonidae, which  parasites various species of Limacodidae, a few Ocophoridae, and Psychidae. The best-known genera are Barycerus, Cassinaria, Filistina and Theronia. The level of parasitism in normally low, with the exception of Cassinaria, which is common on Limacodidae larvae.

Cassinaria sp.lays an egg in the interior of its larval host, and the egg's development seems to be synchronized with that of the larva. During pupation, the larva of the parasitoid breaks the host's pupal tegument and forms its own pupa externally. The pupa is oval-shaped, and clear-brown with black bands or spots. The adult wasp has a black thorax and an orange-colored abdomen. Cassinaria sp.attacks species of Euclea, Euprosterna, Natada, Sibine and Megalopyge (Table 4). In Costa Rica and Colombia, there is a high level of hyper-parasitism by Conura biannulata Ashm, Brachymeria sp.and Neotheronia sp.(Reyes and Cruz 1986). sp.lays an egg in the interior of its larval host, and the egg's development seems to be synchronized with that of the larva. During pupation, the larva of the parasitoid breaks the host's pupal tegument and forms its own pupa externally. The pupa is oval-shaped, and clear-brown with black bands or spots. The adult wasp has a black thorax and an orange-colored abdomen. Cassinaria sp.attacks species of Euclea, Euprosterna, Natada, Sibine and Megalopyge (Table 4). In Costa Rica and Colombia, there is a high level of hyper-parasitism by Conura biannulata Ashm, Brachymeria sp.and Neotheronia sp.(Reyes and Cruz 1986).

During an outbreak of S.megasomoides in Costa Rica, 14 apparently healthy pupae of Cassinaria sp.were collected, but only one adult wasp of the species was obtained; Brachymeria sp.wasps emerged from the remaining 13 pupae. The parasitized pupae presented a hole through which the hyper-parasitoid emerged. Using the criterion of the presence of such holes, it was subsequently determined that 95% of the pupae had been affected by hyper-parasitoids.

Parasitoid flies are common in populations of Attacidae, Brassolidae and Limacodidae (Table 5). The female flies lay their white eggs on the caterpillar's tegument during the last larval stages. Upon hatching, the larvae penetrate the host's tegument and continue their development as endo-parasites. At the point of penetration there appears a necrotic spot. Normally, only one adult fly emerges, by breaking the pupa of the host, in Brassolidae and Limacodidae, but 3-4 adult flies may emerge from larger hosts such as Automeris sp.

The more widely-known species belong to the Bombylidae, Sarcophagidae and Tachinidae families (Table 5). Several tachinid flies have been found attacking E.eleasa, Sibine spp. O. cassina and Automeris sp. (Genty 1972; Mexzón and Chinchilla 1992). In young and nature plantations, Palpexorista coccyx Walker caused a parasitism on Sibine fusca which varied between 36 and 64% respectively (Genty 1972).

Many species of flies visit plants of the Euphorbiaceae family. The majority of these plants grow in open areas, and rarely inside a mature oil palm plantation.

Little is known about the parasitoids that develop in eggs, possibly because very small eggs are not always taken into account during pest sampling.

Members of the Scelionidae, Eucyrthidae and Eulophidae families parasitize the eggs of O. cassina, Automeris spp and Leptopharsa gibbicarina (Table 6). O. cassina eggs, when attacked, lose their normal coloring (reddish bands) and take on a black tone. In the case of Telenomus sp., 6-11 parasitoids emerge from each egg (Reyes and Cruz 1986). In the case of Ooencyrtus sp., 1-4 wasps emerge from each egg.

Erytmellus sp.caused a parasitism of up to 15% in eggs of L.gibbicarina (Villanueva 1985, Reyes 1988). One Trichogrammatidae caused a high parasitism in eggs of Delocrania cossyphoides (Reyes and Cruz 1986). sp.caused a parasitism of up to 15% in eggs of L.gibbicarina (Villanueva 1985, Reyes 1988). One Trichogrammatidae caused a high parasitism in eggs of Delocrania cossyphoides (Reyes and Cruz 1986).

Rhinostomas barbirostris (Culculionidae) is commonly associated to stressed-old oil palms, and probably is not a primary pest.

The higher level of parasitism commonly observed in young plantations may be due, in part, to the presence of an abundant melliferous flora. As the palms grow, their shade impedes the development of many of these plants, which provide food and shelter for numerous arthropods which regulate the population of many pests. As a result, the level of parasitism may decrease, and insect damage to the palm increases in some plantations (Root 1973; Mexzón and Chinchilla 1992, Delvare and Genty 1992).

Predators consist of a very heterogeneous group comprising mites, spiders, frogs, reptiles, birds etc. Some of these species may abound during population explosions of arthropod pests.

Spiders

Sibine sp. pupae infected by a fungus.

Insects

Bugs of the Pentatomidae family are probably the most common predatory insects within an oil palm plantation. Alcaeorrhynchus grandis Dallas, Mormidia ypsilon Fab., Podisus sp., and Proxys pos. punctulatus have been observed preying on larvae of O.cassina, Sibine spp., Stenoma cecropia, and Talima pos. straminea, on the North Atlantic coast of Honduras and the Pacific coast of Costa Rica, in Panama, and in Colombia (Genty et al. 1978; Reyes and Cruz 1986; MexzÂn and Chinchilla 1992) (Table 7).

In 1990, in Costa Rica and Honduras, A. grandis and M. ypsilon were the most numerous predatory bugs observed during an outbreak of O. cassina and S.cecropia (Mexzón and Chinchilla 1992). Posada (1988) found that A. grandis controlled an outbreak of Euprosterna eleasa in Cesar, Colombia. This author also documented eight species of Chrysopidae preying on the lacy bug, Leptopharsa gibbicarina. These species were: Ceraeochrysa cubana, C.scapularis, C.smithi, C.claveri, Nodita sp.and Chrysoperla externa.

Mormidia sp. (Pentatomidae) attacking an adult of Stenoma cecropia

Larvae of Crysopidae, camouflaged in vegetable and insects remains, search through the foliage preying on scales and eggs of many insects. Other common predators in the oil palm environment include several species of ants and wasps of the Polistes group.

Mites

Several predatory mites are associated with Retracrus elaeidis (Phytoptidae) (Rojas et al. 1993). The most frequently-found species belong to the Phytoseiidae (Amblyseius sp.) and Cunaxidae (Cunaxoides sp.) families. Other predators identified were in the Bdellidae, Stigmaeidae and Ascidae families (Table 8).

Vertebrates

Some small frogs (Hyla spp.) and various reptile species (small lizards) feed on insects living above the foliage. Several bird species can also consume large quantities of larvae during an outbreak of defoliators such as O.cassina and S. cecropia. In Central America, the Great-Tailed Grackle (Quiascalus mexicanus), the Oropendola (Psaracolices monctezuma), and the Pia-Pia (Psylochynus norio) are common insect-predators (Evers 1982; Mexzón and Chinchilla 1992), but the first one seems to be the most efficient.

Pupae of Cotesia sp. (Braconidae) on a larva of Sibine sp. (Limacodidae)

ENTOMOPATHOGENS

Viruses

It is possible that all defoliators may be susceptible to one or more diseases caused by viruses. However, some species such as Stenoma cecropia are little affected by them (Desmier de Chenon 1990). This last author reviewed the status of the viral diseases of oil palm pests in Asia and Tropical America. Previously, Genty and Mariau (1975) reported the effectiveness of a Densonucleosis in regulating the population of Sibine fusca in Colombia. In Honduras, a viral preparation obtained from diseased larva of Sibine sp. was successfully used  to control an outbreak of this pest (R. Aragón, cited by Chinchilla 1992). In Central America, several viral diseases have been observed in O.casina and Automeris spp. In Panama and Costa Rica, a Densonucleosis and a Cytoplasmic Polyhedrosis were found in Sibine sp. and S. megasomoides respectively (Mexzón and Chinchilla 1992). In Brazil and Colombia, a Densonucleosis was reported in S. pallescens Stoll (Luchini et al. 1984), and a Nuclear Polyhedrosis in E. eleasa (Reyes and Cruz 1986).

Symptoms of a viral infection in Sibine spp. larvae include reduced activity and loss of gregarious behavior. The larvae cease feeding and secrete body fluids through the mouth and anus. The tegument becomes paler and later turns black. The internal organs disintegrate and the larvae take on a flaccid appearance (Genty 1972; Orellana 1986).

Pupa of Opsiphanes sp. parasitized with several tachinid larvae

Several authors have provided recommendations about the preparation and use of viral suspensions in the field (Table 9). The application of a viral solution can be a cheaper and more environmentally-friendly alternative than the use of commercial insecticides (Sipayung et al. 1989).

Fungi

Among the most common fungi causing epizootic diseases are Beauveria bassiana (Bals.) Vuill., B. tenella (Sacc.) Petch., Metarhyzium anisoplae (Metsch.) Sorokin, Hirsutella sp. and Paecilomyces sp., Beauveria tenella may control populations of Antaeotricha sp. and Talima stramina Schauss. Paecilomyces farinosus Holm and Grey has been found infecting larvae of Euclea diversa Druce and Natada sp. (Genty et al. 1978; Reyes and Cruz 1986) (Table 10).

MANAGEMENT OF NATURAL ENEMIES OF PESTS

The use of some natural enemies to reduce the damage caused by harmful arthropods is very promising. The following discussion details some of the strategies that could increase the populations of these beneficial organisms in oil palm plantations.

Scleria melaleuca (Cyperaceae) being visited by numerous chalcids (Conura sp.)

Management and preservation of beneficial plants (auxiliary flora)

Knowledge of the auxiliary fauna and its feeding habits can permit a more rational management of the vegetative covering in commercial plantations. Diverse populations of beneficial arthropods can increase, favoring the development of certain "weeds" inside the plantation. The places where these plants are allowed to grow should be carefully chosen so they do not interfere with normal cultural practices within the plantation.

Plants with extra-floral nectaries or glands such as those found in Malvaceae, Solanaceae, Tilliaceae, Verbenaceae, and Euphorbiaceae, among others, permit the feeding and refuge of many of the main pest's natural enemies (Mckenzie 1976; Altieri 1983; Desmier de Chenon et al. 1989; Genty 1989, 1992; Mexzón and Chinchilla 1992; Mexzón 1992).

A diverse vegetation creates many microclimates, a heterogeneous chemical atmosphere, and a complex structural diversity, which can be more hostile to the development of pests within a particular crop (Tahveneinnen and Root 1972).  A complex vegetation in the plantation also helps to maintain a favorable microclimate (high humidity) for the appearance and development  of some epizootic diseases caused by fungi and bacteria (Evans 1982; Papierok et al. 1993).

Urena lobata (Malvaceae) has nectaries which attracts many parasitoids wasps

The preparation of a descriptive manual, with photographs of the main pests alongside the plants where their natural enemies take refuge, could serve to initiate the active participation of field workers in the vigilance and control of important pests.

Breeding of predators and parasitoids

In Sumatra, the bug Eocanthecona furcullata was artificially bred with frozen larvae of Setothosea asigna (Desmier de Chenon 1989). This experience gives rise to the possibility that other important predators can also be artificially reproduced. The biology of the diverse predators and parasitoids should be studied in detail in order to design artificial diets that would permit their mass reproduction. Prior to this, the diet of the host larvae would have to be investigated, as it was done with the fruit flies Anastrepha ludens (Spisakoff and Hernandez-Davila 1968), Dacus spp. and their parasitoids (Ahmad et al. 1971).  

The introduction of exotic species of predators and parasitoids should not be made without a previous study looking at the quality of the material that would be imported into the new environment. Three latent dangers are the inadvertent introduction of a hyperparasite, or of a morphologically-similar species which would be unrecognizable to the non-specialist, or lastly, of possibly inefficient variants of the parasitoid (Hoy et al. 1991).

Urena lobata (Malvaceae) has nectaries which attracts many parasitoids wasps

Use of entomopathogens

Several viruses can be used as bio-insecticides against defoliators in the Limacodidae family, Opsiphanes cassina, and other important pests. Fungal preparations of B.bassiana and M.anisopliae, among others, have been successfully tried  on various crops to control important insect pests.

Rational use of pesticides

The indiscriminate use of pesticides (particularly insecticides of wide spectrum) has probably had a very harmful effect on the environment. The control obtained has been erratic, and has given rise to secondary pests of increasing importance. The rational use of insecticides requires detailed knowledge about the behavior of the pest, its natural enemies, climatic conditions, and any other aspect which could affect the regulation of the pest population.

CONCLUSIONS

The study of the biology and behavior of native parasitoids and predators of common pests is considered an important step in the development of successful breeding methods for these organisms. Equally, more should be known about the local entomopathogenic agents and the negative effect that some commonly used agro-chemicals could be having upon them. The objective is to promote an environment in which more individuals of any particular pest could be infected by entomopathogens, or attacked by some type of predator or parasitoid.

Studies should look at the relationships between the population fluctuation of beneficial organisms and the climatic variations within the plantation. Information is also needed about the relation between the plant phenology, and the microclimate's requirements of the organisms. These are some of the factors that determine the vertical distribution of beneficial organisms in different strata of the vegetation, and the horizontal distribution within the different species of plants.

The multiple interactions which occur between the predators, parasitoids, and vegetation are complex and rather specific in some situations. For example, some plants could attract valuable parasitoids, but also their hyper-parasitoids. If such particular plants proliferate in the plantation, this could tilt the balance against the parasitoid. The bridge to be crossed to understand these relations is certainly long, and requires the close cooperation of people working in Entomology, Botany, Micro-climatology, Ecology and other disciplines.

The fact that there are more than 80 potential  pests identified in oil palm plantations, and that only a few of them have occasionally caused economic damage, should convince us that the role of the biological regulators has been and continues to be, tremendously efficient. Plantation managers at all levels are morally obligated to commit the necessary resources to support any effort, whose objective is not just the temporary control of pests, but the maintenance of an environmental balance, in which the use of synthetic chemicals is reduced to a minimum.

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