Abstract

Studies on the biology of Sibine megasomoides Walker (Lep. Limacodidae) associated to oil palm were carried out in Costa Rica. The life cycle of the insect was completed in 90 days, and the adults lived for 3 days. The eggs hatched after 7 days, the larval stage lasted 48 days (9 stages), and the pupal stage was completed in 32 days. All larval stages are important defoliators of the oil palm. The first 3 larval stages rasp the lower leaf epidermis; the fourth stage perforates the leaf lamina, and older stages consume all leaf tissues but the central vein. The last 2 stages eat 85% of the tissue necessary to complete the life cycle. An outbreak of the pest in approximately 185 ha was studied and a population of 12 large larvae per leaf was used as a critical level. Control was achieved by aerial spraying with Bacillus thuringiensis, and the pyretroid deltamethrin. A rather low population of natural enemies was associated with the outbreak; however, an increase in the level of parasitism and depredation was related to an efford to plant and protect several plants which were known to harbor and feed many parasitoid wasps. Some plants such as Amaranthus spinosus L. and Chamaesyce gossipyfolia Mills were particularly attractive for Cassinaria sp.; Cassia reticulata L. and Triunffeta semitriloba L. attracted Cotessia sp.. Other plants such as Baltimora recta L., Cassia stenocarpoides Standley, and Crotalaria sp. also attracted a varied entomofauna (particularly Chalcididae and Braconidae). Several of the insects visiting these plants were found as parasitoids on S. megasomoides.

Introduction
The cultivation of large areas with oil palm has created favorable conditions for the proliferation of some species of insects previously associated to wild palms (Genty 1988; Hoong and Hog 1992). Various lepidopteran in the Limacodidae family frecuently reach pest levels: Euprosterna eleasa Dyar, Natada spp and Sibine spp. (Genty et al. 1978). In Costa Rica, Sibine megasomoides is found associated with wild species of the Heliconiaceae and Musaseae families (Harrison 1963) and with commercial crops such as bananas (Lara 1970). Until 1988, the population of this insect was low in oil palm. However, in 1989 a population increase was detected in the Central Pacific area of the country. A high level of defoliation, together with the lack of knowledge about the biology of the insect motivated this study.

Materials and Methods

Larvae of S. megasomoides at the 3 first stages of development were collected in a 10-year old oil palm plantation situated in Parrita, Costa Rica in Februrary 1990. Samples of 5-10 larvae were placed in 250 ml glass flasks with perforated lids. During the study, the temperature and relative humidity were 28°C (27-29) and 70% respectively. The daily period of artificial light was 12 hours. The larvae were measured with a millimetric gauge (Vernier) and weighed on an analytical balance after each moult. To estimate the capacity of defoliation, 162 larvae were chosen, 18 at each stage of development. They were individually placed in glass flasks. The larvae were fed each day with two oil palm leaf sections measuring approximately 2.5x5.0 cm. Their consumption was estimated by measuring the remaining tissue with a piece of millimetric paper. Population of larvae was estimated by counting them on leaf 17 in two palms per hectare. Bacillus thuringiensis Berliner var. kurstaki (Dipel 2L: 0.5-0.8 l/ha) and Deltametrina (Decis 2.5 EC:0.025-0.45 l/ha) were used to reduce the population of larvae. The insecticides were also used in a mix with the lower dose of Decis. Berliner var. kurstaki (Dipel 2L: 0.5-0.8 l/ha) and Deltametrina (Decis 2.5 EC:0.025-0.45 l/ha) were used to reduce the population of larvae. The insecticides were also used in a mix with the lower dose of Decis.

Many open spaces in the plantation were treated with graminicides and planted with some plants which had been observed to attrack several species of parasitoid wasps, some of which were associated to the pest.

Results and Discussion
Life cycle. The life cycle of the insect lasted an average of 90.4 days in the laboratory conditions. The eggs hatched after 7 days; the larval stage took 48.3 days, with 9 stages of development. The pupal took 32.1 days and the adult lived for 3 days (Table 1).

The eggs are smooth, transparent and finely reticulated. The female lays the eggs on the leaves of the palms and of several epiphytic plants that grow on the palm stem. The eggs are deposited in groups of 7-15 and are covered with a yellow mucilage.

The larvae measured approximately 1.25 mm at the moment of hatching, and reached 24 mm at full development. The head is tucked into the first thoraci segment. The back presents 3 pairs of protusions (scoli) with urticant hairs. Each segment has one pair of red-colored fleshy lateral projections covered with fine urticant hairs.

During the first 3 stages larvae are yellow, and then turn to green, with the extremes of the body of a brown color. On the dorsal part, the larvae have a oval mark, with a white border which resembles a saddle. The abdomen is pale yellow during the 5 first stages and later turns bright pink.

The pupal cover measures 9-10x12-15 mm, is leathery, oval shaped and clear brown in color, and is wrapped in a diffuse mass of urticant threads.

Adults of both sexes are dark brown in color, their forewings are subtriangular, and darker than their back wings. The forewings have 3 gold marks. The wings form a roof above the abdomen during rest and the back wings lay covered over the front wings. The male has a wingspan of 28-43 mm, and its antenas are fasciculated. The resting wings do not completely cover the abdomen. The female has a wingspan of 40-52 mm, its antenas are filiform and the wings cover the abdomen.

Behavior
Females started laying eggs the day after mating and kept doing it for 2-3 days. During the first day a female laid up to 20 egg masses (7-15 eggs/group), but this rate declined during the following days. In other related species such as S. apicales Dyar, the females laid 26 masses of 1 to 14 eggs, with a total of approximately 324 eggs (Harrison 1963). In S. fusca Stoll the female laid 49 masses with an average of 34 eggs per mass (Genty 1972).

The newly emerged larvae ate the remainds of the afterbirth. After the first molt they fed from the lower epidermis of the leaf, and the third stage perforated the leaf laminae. From the fifth to the ninth stages, the larvae ate along the border of the leaflets and finally consumed the whole leaf but the central vein. Before each molt, the larvae stopped eating and remained immobile. On completion of the molting, it consumed some of the remainds of the teguments.

Between the fourth and ninth stages, the larvae grew rapidly and in 28 days, their length and weight was multiplied by a factor of four and 16 respectively (Table 2). The eight and ninght stages consumed 85% of the tissue necessary to complete the life cycle.

To evaluate the activity of the larvae, pieces of cloth were placed under and around the attacked palms to collect the excrements every two hours. Maximun feeding activity was detected between 14:30 and 16:30 hr., with a lower peak between 20:30 and 22:30hr. (V. Valverde, personal communication).

The larvae were gregarious, but became solitary from the seventh stage onwards. Before pupation they remained immobile for some 24 hours and stopped feeding. Latter on, they secreted a series of urticant silk threads which presumbible protect them from predators while they built the coriaceous covering.

The adults are moths with no funtional oral pieces. They were very active at night, and were attracted by artificial lights. Flight was very vigorous but not very coordinated. After completion the life cycle they fell to the ground and died after a few convulsions.

Damage caused and control methods
In February 1990, an area of near 75 ha of a 10-year old planting was affected. Later on the attack spreaded to about 185 ha. At the focus of the attack (ca. 20 ha) defoliation occurred in approximately two thirds of the palm crown (medium and lower strata). The average population on leaf 17 was 36.8 larvae, of which 78.4% belonged to the 3 first stages of development.

Several aerial spraying with Dipel 8L brought the insect under control. The economical losses caused by S. megasomoides were not determined. However, they were probably important. Wood (1982) considered the attack by a defoliator could cause losses up to 50 % in yield, and the recuperation of the palms could take up to two years. Hoog and Hoh (1992) estimated that 60% defoliation by the Limacodidae, Setora nitens, caused a loss of 27 t/ha of fresh fruit in 30 months following an attack.

In the laboratory, the economical threshold of damage was arbitrarily fixed in 10 larvae of the forth stage or larger per leaf, which could cause a 20% permisible defoliation (Table 2).

Natural enemies
At the start of the population increase in 1989, the larvae of S. megasomoides were primarely prayed upon by the Pentatomidae Mormidea ypsilon F., and to a lesser extend by Podisus sp and Alcaeorrhynchus grandis F. These predators were also observed attacking Opsiphanes sp and Stenoma cecropia Meyrick. During an outbreak of S. cecropia, the majority of the population of M. ypsilon were in the ninph stages, and between two and six ninphs per each adult of S. cecropia were commonly observed (Mexzón y Chinchilla 1991). During the outbreak of S. megasomoides early in 1990, these Pentatomidae were not found at the focus of the attack, but were observed praying on Aumomeris sp and S. fusca in a plantation closeby.

The main parasitoids found were Cassinaria sp, Cotesia sp and some species of Tachinidae. Genty et al. (1978) mentioned similar parasitoids on S. fusca and other Limacodidae. Casinaria sp attacked larvae at the fifth and sixth stages. The parasite seemed to have a synchronized endocrine system with its host because when the caterpillar pupated, the parasite larvae, which had already completed its development, broke the host's tegument and formed its pupa externally. Such synchronization has also been observed in various Braconidae and Ichneumonidae (Beckage 1985).

The female of Cotesia sp lays its eggs on larvae of the seventh or eighth stages. Genty (1984) fond this insect completing its life cycle on S. fusca in 10-14 days: 100-250 adult wasps emerged from each larva.

The larva of the parasitoid emerged from the host caterpillar through the tegument and weaved a white silk case. After seven days, the wasps emerged in a synchronized form in around five minutes (Mexzón and Chinchilla 1991).

Other causes of mortality
During one generation, near 80% of pupae collected from the stems of the palms were found to be infected by a fungus similar to Paecilomyces sp. A large quantity of larvae also showed symptoms of a viral infection which was identified as a cytoplasmic polyhedrosis. The symptoms were loss of color, mobility and appetite, flacidity and secretions from mouth and anus.

Fig. 1 Larva of Sibine megasomoides being prayed upon by a pentatomidae bug

In Tropical America, Densonucleosis viruses have been identified in S. fusca (Genty et al.. 1978), S. pallescens Stoll (Luchini et al.. 1984), and in Sibine sp (Mexzón y Chinchilla 1991). Nuclear polyhedrosis have been found on Euprosterna eleasa Dyar and Granulosis on Mesocia pusilla Dyar (Mariau and Desmier de Chenon 1990). These viruses are specific, which make them very valuable in an IPM strategy because they can be used as bioinsecticides without causing harm to other species (Desmier de Chenon et al. 1987).

Sibine megasomoides as a pest

It is probable that disturbances in the environment which affected the population of natural enemies, caused the S. megasomoides outbreak. One possible cause of this disturbance could have been the intensive use of herbicides and insecticides in rice and melon fields nearby. The abundance of flowers and fruits which were left over from the melon harvest could have attracted a sizeable part of the entomofauna.

Fig. 2 Sibine megasomoides attacked by the parasitic wasp ( Cotesia sp)

In adult oil palm plantations, auxiliary fauna could be scarse because of the lack of vegetation which offers food sources and favorable habitats. The adult form of many parasitoids feed from the glandular and estrafloral secretions or from the pollen of many plants (Altieri 1987, Delvare and Genty 1992), were the females may obtain essential aminoacids to guarantee the viability of the eggs (Syme 1975). A correlation has been observed between the elimination of the vegetable covering (abuse of herbicides or increasing shade as the crop matures) and the increase in the prevalence of differente defoliators (Syed and Shah 1976; Hoong and Hoh 1992).

In the area of the outbreak in Quepos, herbicides were applied in the open spaces penetrated by sun light. A. meliferous vegetation was established in these sites, which was abundantly composed of Borreria sp, Hamelia patens L.,Scleria melaleuca Schlecht and Cham. and an unidentified Verbenaceae. S. melaleuca, in particular, concentrated an abundant fauna of Braconidae and Chalcididae.

Other plants such as Amaranthus spinosus L. and Chamaesyce gossypifolia Mills, were attractive to Casinaria sp. Species such as Cassia stenocarpoides (Standley) Briton, Baltimora recta L. and Crotalaria sp also atracted many beneficial insects. Similarly, two shrubs: Cassia reticulata L. and Triunfetta semitriloba L. were attractive to Braconidae (Mexzón and Chinchilla 1991). Although it is not possible to establish a cause and effect relationship, the presence of the plants referred to were associated with the reduction in the population of S. megasomoides, even in areas where no insecticides had been applied.

Some of these plant species are annual which die after fruiting. Other species are only attractive by their flowers. However, perennial species, which are attractive because of their extrafloral glands offer a more stable environment to substain a more or less permanent auxiliary fauna.

References

ALTIERI, M.A. 1987. Significado de las interacciones entre malezas e insectos en los agroecosistemas tradicionales de los trópicos. Manejo Integrado de Plagas (Costa Rica). 2:1-15.

DELVARE, G.; P. GENTY 1992. Interés de las plantas atractivas para la fauna auxiliar de las plantaciones de palma en América tropical. Oléagineux 47 (10):551-558.

DESMIER DE CHENON, R.; D. MARIAU; P. MONSARRAT; G. FEDIERE; A.SIPAYUNG 1987. Research into entomopatogenic agents of viral origin in leaf-eating Lepidoptera of the oil palm and coconut. In Proceedings of the 1987 Intl. Oil Palm/Palm Oil Confer. PORIM, Malaysia. p.471-479.

GENTY, P. 1972. Morfología y biología de Sibine fusca Stoll, lepidóptero defoliador de la palma de aceite en Colombia. Oléagineux 27 (2):65-71.

GENTY, P. 1984. Estudios entomológicos con relación a la palma aceitera en América tropical. Palmas (Bogotá) 5(1):22-31.

GENTY, P. 1988. Manejo y control de plagas de la palma africana. In Problemas Fitopatológicos de la Palma Africana. B. Ramakrishna, ed. VI Seminario. IICA-ID-Prociandino, Quito, Ecuador. 190 p.

GENTY,P; R. DESMIER DE CHENON; J.P. MORIN 1978. Las plagas de la palma aceitera en América Latina. Oléagineux (número especial) 33(7):324-594.

HARRISON, J.O. 1963. On the biology of three anana pests in Costa Rica. Ann. Entomol. Soc. Am. 56:87-94.

HOONG, H.W.; K.Y. HOH CRISTOPHER 1992. Major pests of oil palm in Sabah. The Planter (Malaysia), 68(793):193-210.

LARA, F. 1970. Problemas y procedimientos bananeros en la Zona Atlántica de Costa Rica. Trejos Hnos. San José, Costa Rica. 278p.

LUCHINI, F.; J.P. MORIN; R.L. ROCHA DE SOUZA; E.J. DE DIMA; J.C. DE SILVA 1984. Inimigos naturais de Sibine spp., Sibine nesea e Euprosterna elaeasa (Lep., Limacodidae) constatados en plantacoes de dende, Elaeis guineensis, nos estados do Pará, do Amazonas e da Bahía. EMBRAPA, Centro Nal. de Pesquisa de Serengueira e dende. 22: 1-4

MARIAU, D.; R.DESMIER DE CHENON 1990. Importance of the role of entomopathogenic viruses in oil palm leaf-eating Lepidoptera species. Prospects for developing biological control methods. Oléagineux 45(11):487-491.

MEXZON, R.G. 1992. Insectos visitantes de malezas: manejo y conservación de la vegetación para incrementar los enemigos naturales de plagas de la palma aceitera. 1er Congr. Centroamericano de Entomología y Combate Natural de Plagas, Costa Rica. 14p.

MEXZON, R.; CHINCHILLA,C. 1991. Entomofauna perjudicial, enemigos naturales y malezas útiles en palma aceitera en América Central. Manejo Integrado de Plagas(Costa Rica). 20/21:1-7.

SYED, R.A.; J. SHAH 1976. Some important aspects of insect pest management in oil palm estates in Sabah, Malaysia.In Intl. Developments in Oil Palm. D.A. Earp & W. Newall eds. The Incorporated Soc. of Planters, Malaysia. p. 577-590.

SYME, P.D. 1975. The effects of flowers on the longevity and y fecundity of two native parasites of the european pine shoot moth in Ontario. Environ. Entomol. 4:337-340.

VALVERDE, V.H. 1990. Evaluación de la actividad diaria de larvas de Sibine megasomoides en palma aceitera. Palma Tica. Palm Research Program, Quepos. Comunicación personal.

WOOD,J. 1982. The present status of pests on oil palm estates in South Asia. In The oil palm in agriculture in the eighties. Pushparajah, E. and C.P. Sooh, eds. The Incorporated Soc. of Planters, Malaysia. p. 499-518.