Introduction

The bagworm, Oiketicus kirbyi is a polyphagous insect feeding on several crops: Musaceae (Musa spp.), cacao (Theobroma cacao L.), oil palm ( Elaeis guineensis Jacquin), peach palm ( Bactris gasipaes Kunth), coconut ( Cocos nucifera L.), citrus ( Citrus spp.), teek ( Tectona grandis L.), eucaliptus ( Eucalyptus spp.), Eryobothria japónica , Terminalia catappa L) and many more.

The insect became a serious problem in some commercial banana plantations on Costa Rica's Atlantic coast during the period 1962-1964. The start of the problem was associated with the irrational use of broad-spectrum, long-residuality insecticides such as Dieldrin, originally used to control an aphid (Lara 1970).

O. kirbyi was present in oil palm in Central America but was of no concern (Chinchilla 1989), until an outbreak was observed in an oil palm plantation in Puerto Armuelles, Panama, in 1990. The outbreak appeared to originate in a nearby plantain plantation. Early the following year, an outbreak occurred in another nearby oil palm plantation, this time located in Costa Rica. Initially, the insect was confined to two harvesting lots, but the area affected rapidly increased to comprise several hundred hectares. Several outbreaks fallowed during which control was attempted through the use of Bacillus thuringiensis (generally Dipel: 0.8 to 1.5 l/ha). From 1996 onwards, the population of the pest declined, apparently controlled by its numerous natural enemies. In September of 1998, a new outbreak occurred in a small area (less than 100 ha) but did not spread. From that moment on, the population of this pest has been very low.

O. kirbyi has also caused problems in oil palm in other countries. In Colombia, the pest was observed in El Cesar during 1973 and 1985, when it caused three defoliations in Palmeras de la Costa S.A (up to 353 larvae /leaf) (Villanueva and Avila 1987). It also caused damage in the Cauca Valley, in about 150 ha of plantains in 1975-1976 (García 1987).

This review summarizes part of the knowledge obtained on O. kirbyi originating in tropical America, particularly that learned in Costa Rica during outbreaks in the early nineties.

The Psychidae family

The larvae of the Psychidae family are hypognathous, measure between 8 and 50 mm, and build protective bags made from silk and fragments of plant tissue. They are cylindrical, with well-developed thoracic legs and four pairs of pseudo-legs in the abdomen and a pair at the anal extreme.

Pupation occurs within the bag; the final larval stage attaches the bag to the stratum and pupates up side down. In some more evolved genera, females do not abandon the bag, except after laying the eggs, when they drop to die on the ground. Depending on species, 200- 13 000 eggs remain within the bag. The larval stage is fairly long, but adult life is ephemeral. Sexual dimorphism is marked: adult females are neotenic with larval appearance and remain inside the bag. Males are free living moths (Stehr 1987).

There are about 600 species in the family; 500 are Old World species and 26 are found in Canada and the US. The genus Oiketicus is represented by three species in tropical America. O. kirbyi is found in lowlands from Brazil to Mexico and in the Caribbean islands.

Morphology of Oiketicus kirbyi

Adults: females are neotenic and retain the larval shape. The head is small, with no antennae and the buccal parts are atrophied. They do not abandon the protective bag and fertilization by the male occurs within the bag. The large size of the females before mating is due to a body full of ovules. After laying the eggs the body volume is reduced to about a half; the female then drops to the ground to die.

The male is a short-wing brown moth (42 mm in size). The thorax is strong, the abdomen thin and extensible and has bipectinate antennae. The buccal apparatus is also atrophied. Longevity is about four days for females and three for males.

When mating, the male tears the lower end of the female's bag and penetrates the genital opening after extending its abdomen up to 70 mm into the bag.

Eggs: these are cylindrical with rounded edges (0.34 x 0.53 mm). Initially, they are cream-colored, then orange and finally dark. They are lay within the pupal case. Incubation takes about 43 ± 1.4 days (27- 47). Viability is very high, and a female may lay between 3500 and 6000 eggs.

Larva: newborn larvae are yellowish but turn gray with black spots as they grow; females are darker than males. Mandibles are strong. The thorax has three strong pairs of legs and the abdomen (with 8 segment) bears four pairs of pseudo-legs. The anal segment is dark, chitinous and has a pair of pseudo-legs.

After hatching, newborn larvae abandon the maternal bag, and produce a silken thread to be wind dispersed. Upon reaching an appropriate substratum, they start building their protective bags, which are made larger as they grow in size from about 1.5 mm to near 40 mm in the male and 55 mm in females. Males' bags are light brown or gray (40- 65 mm), and females' bags are darker and measure 58- 85 mm (Campos et al. 1987).

Larval cycle last between 145 and 185 days: mean of 140 days for males and 151 for females. Males and females have 8 and 9 larval stages respectively. Stephens (1962) mentions from 15 to 20 larval stages in males and 12-15 in females.

Pupa: the female pupa has both extremes rounded, and has a segmented appearance and has no external signals of legs, antennae or other structures. The posterior end of the male pupa is pointed and curved toward the ventral part. The plaques corresponding to external organs in adults are clearly marked. Females are darker than males. The pupal stage lasts for about 38.2 ± 2.0 días. Dimensions for males are 6.7 ± 3.4 x 27.7 ± 1.4 mm and 9.3 ± 0.8 x 35.7 ± 2.1 mm for females (diameter x length) (Campos et al. 1987).

The life cycle of this insect has been studied by several authors (Stephens 1962; Campos et al. 1987; García 1987), but results vary widely. The reasons for these discrepancies are varied, and may be found in the different substrata used to feed the insects, climatic conditions and, more than anything, in the difficulty involved in studying an insect that remains hidden within a bag. Data on life cycle are shown in Table 1 .

Insect behavior

Upon "emerging" from pupa, females impregnate the scales or hairs at the extreme of the abdomen with a blend of sex pheromones. At least five compounds have been identified (chiral esters), and 1-methyl-butyl decanoate is the most active and produced in largest quantities (Rhainds et al. 1994).

Proportionately, more females pupate on the youngest leaves. Females pupating on the extremes of youngest leaves of the oil palms (leaves with a more erect position) have a greater success in being fertilized by males than those that choose to pupate the leaves of the middle canopy (leaves with a more horizontal habit). Nevertheless, males prefer to mate with larger females regardless their position on the plant (larger females not necessarily pupate on youngest leaves).

Males probably prefer large females because they produce larger quantities of pheromone and eventually more eggs. Small females may compensate for these disadvantages by pupating on youngest more erect leaves where the dispersion of the pheromone and the newborn larvae (by wind) may be more efficient. It is also known that males tend to fly over the upper strata of the canopy.

Despite these apparent advantages for small females, most of them pupate (along with large ones) on leaves of the intermediate canopy. Based on these results, it was concluded that selection of a site to pupate for females may depend on other factors besides their size. An important element to be considered is the greater exposure to predators of those females pupating on the external more exposed leaves (Rhainds et al. 1995a; 1995b).

Mating takes about 30.7 ± 4.6 minutes (23-61 min). Oviposition soon fallows and lasts 1.8 ±0.6 days (Campos et al. 1987).

Males are nocturnal and are attracted by light. Most females emerge about three weeks before most males, so initially there is a very high proportion of females to males (from 10:1 to 2:1). During this period, the probability of a female mating is limited by the low availability of males. The final result is that many females do not mate, and eventually (2-4 days) drop to the ground to die (Rhainds et al. 1995b).

Damage

Larvae may feed on many plants besides oil palm, including weeds and forest trees. Newborn larvae hang from silk threads and are readily dispersed by wind, animals or vehicles. Small larvae do not move too much, but large ones may move between leaves of the same tree or even between tress. The ability to fast in well-developed larvae and the protective bag make this pest difficult to control. Other important characteristics of the insect are its long life cycle and its high fecundity.

One single larva may consume about 304.5 cm² of leaf tissue. Table 2 gives information on damage caused by the different larval stages.

Sampling

Since most larvae feed on the apical and sub apical portions of the young and intermediate leaves, Rhainds et al. (1996) developed a sampling procedure based on the fact that total population of larvae in the whole tree correlated with the partial population on a leaf in an intermediate position in the canopy. Sampling 160 terminal leaflets on leaf 17 (80 on each side of the rachis) represents a compromise between costs, efficiency and reliability of the sampling. A sampling done during an outbreak in Coto, Costa Rica, gave an average number of larva per leaf of 45.22 ± 4.21 larvae of the first stages. During this study, sampling one palm per hectare gave a good estimate of total population in the affected area.

Natural enemies

The different stages of O.kirbyi are attacked by numerous natural enemies ( Table 3 ). Among these are lizards, birds, and other vertebrates and invertebrates. However, other insects are probably the most important. Parasitoid wasps include Digonogastra diversus (= Iphiaulax pos. psychidosphagus ) (Braconidae), Conura brethesi , Conura oiketicusi (= Psychidosmicra sp.), Brachymeria sp. (Chalcididae), Ateleute sp. and Filistina sp (Ichneumonidae). The order in which these wasps were mentioned reflects their order of importance. Among Diptera there are some members of Sarcophagidae and Tachinidae. Diseases caused by entomopathogens occur some times and the bacteria Klebsiella oxitoca has also reduced the population in some occasions (Stephens 1962; Lara 1970; Gravena and Almeida 1982; García 1987; Villanueva and Avila 1987)

During the outbreak in Costa Rica in 1991-92, parasitism by D.diversus and Conura spp. was very high. Several hundreds of field-collected larvae were classified according size and parasitism level ( Table 4 and Table 5 ). The most abundant parasitoids were D. diversus (57.7%) and two species of Conura (possibly C.oiketicusi ) (17.15%) and C.brethesi .

Wild plants as host of natural enemies

Known parasitoids (adult wasps) of O.kirbyi feed on flowers and extra floral glands of several plant species. In young oil palm plantations plenty of solar radiation reaches the ground, which allows the development of a varied associated flora. High levels of larval parasitism (up to 95 %) are common under these conditions. As palms get older, less and less solar radiation reaches the ground and only shade-loving species survive, which do not necessarily support adult forms of most parasitoids. Parasitism is dramatically reduced under these new conditions (as low as 10%).

In young plantings growing on the southwest of Costa Rica, many plant species flower between December and April. During this time is common to observe individuals of D.diversus on Amarantus spinosus (bledo), Baltimora recta , Cassia tora , Scleria melaleuca and Vitis sycioides . The parasitoid wasps Conura spp. are common on A. spinosus , C. tora , Melanthera aspera , Solanum jamaicense and other species. Ateleute sp. and Filistina sp. share the same plant species ( Table 6 ).

Integrated pest management

The integrated management of the pest is based on sampling leaf 17 looking for larvae (healthy, parasitized, diseased…) and predators. Natural enemies are also looked for on the associated flora.

Chemical control. Aerial applications of broad-spectrum, long-residuality insecticides in both bananas and oil palm in the past disrupted natural control. The protection afforded the larva by its bag and its ability to fast make this pest difficult to control with most insecticides. Trunk injection of monocrotophos (14-18 cc/palm) has been used with success (up to 98% larval mortality in two weeks) in some occasions. However, the use of this technique is laborious, costly and not risk-free in terms of its affects on the natural enemies of the pest.

Results obtained with the use of Bacillus thuringiensis (2-3 kg/ha:Thuricide, Dipel…) have been variable (up to 70 % larval mortality), depending on several factors, including climatic conditions. Normally, effective control requires at least two applications of the product spaced 3-4 weeks. Chitin inhibitors (triflumuron: 0.45- 0.75 g a.i./ha) and the nereistoxin (Padan) have also been used with relative success.

Cultural control. Given that larvae tend to concentrate toward the tip of leaves, they can be collected manually on young palms. Cutting the tips of leaves is not a good idea since defoliation can be greater than that caused by the larvae. Manual removal of larvae can be costly and most of bags taken could be males, since many females tend to position themselves toward the tips of the youngest taller leaves where a normal person can not reach them.

Weed control should be selective (or suspended) during an outbreak of this or any other defoliator: patches or bands of known beneficial plants can be left to provide refuge and food to parasitoids and predators. Nurseries of these plants can be prepared and planted in areas where they can be protected without interfering with normal agronomic practices (Table 6).

Biological control. Parasitized larvae can be collected and placed in containers that allow the adult parasitoids to leave. However, protecting and planting those plants that harbor these natural enemies of the pest is more effective.

Pheromones. The main blend of pheromones produced by the female to attract males has been identified and can be used to monitor male population or even in mating disruption. Light traps do not attract many males, and no trap with food will work since adult males do not feed.

Concluding remarks

Emergence of females of O. kirbyi prior to males, in a highly uniform crop such as the oil palm, combined with a slight overlapping between generations may lead to a reduction of the genetic pool of a particular population. Under these circumstances, the males that emerge first (and find more females available) are favored, as well those females that emerge late and will have more males available to mate with (Rhainds et al. 1995).

A combination of a reduced genetic pool and an increase in the population of natural enemies may lead to the final dismissal of this insect as a pest. During the outbreaks of the pest in Costa Rica, it was quite evident (no data is available) that the average size of bags became smaller with time, which could be an indication of inbreeding.

References

Campos Arce, J.J.; Peres, O.; Berti, E. 1987. Biologia do bicho do cesto Oiketicus kirbyi (Lands- Guilding 1827) (Lepidoptera: Psychidae) en folhas de Eucalyptus spp. Anais Esc. Super. Agric. Luiz de Queiroz 44: 341-358.

Chinchilla, C. 1989. Fauna perjudicial en palma aceitera. Palma Tica, Programa de Investigación en Palma Aceitera (Mimeo). 41 p.

Davis, D.R. 1987. The Psychidae. pp. 366-369. In. Immature insects. Stehr, F.W., editor Kendall/Hunt Publishing Co, Inc., USA

García, F. 1987. Aspectos biológicos y manejo del gusano canasta Oiketicus kirbyi . Instituto Colombiano Agropecuario. Ministerio de Agricultura. Bol. Tec.149. 23 p.

Genty, P. 1989. Manejo y control de las plagas de la palma aceitera en América tropical. Curso ASD para agrónomos y administradores de Palmas de Oriente. Colombia. 11 p. (mimeo).

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

Mexzón, R.G. 1991. Informe de actividades entomológicas en el período de Julio 23 a Agosto 20 de 1991. Palma Tica S. A. División de Coto., PIPA. 7 p. (mimeo).

Mexzón, R.G.; Chinchilla, 1996. Enemigos naturales de los artrópodos perjudiciales a la palma aceitera ( Elaeis guineensis Jacq.) en América tropical. ASD Oil Palm Papers Nº 13: 9-33.

Ponce, T.; Ines Pelaez, H.; De La Cruz, L. 1979. Estudio biológico del gusano canasta Oiketicus kirbyi Lands-Guilding (Lepidoptera: Psychidae) en plátano y reconocimiento de sus principales parasitoides. Acta Agron. 29: 41-46.

Rhainds, M.; Gries, G.; Li, J.; Gries, R.; Slessor, K.N.; Chinchilla, C.M.; Oehlschlager, A. C. 1994. Chiral esters: sex pheromone of the bagworm, Oiketicus kirbyi (Lepidoptera: Psychidae). J. Chem. Ecol. 20 (12): 3083-3096.

Rhainds, M.; Gries, G.; Castrillo, G. 1995b. Pupation site affects the mating success of small but not large female bagworms, Oiketicus kirbyi (Lepidoptera: Psychidae). OIKOS 74: 213-217.

Rhainds, M.; Gries, G.; Chinchilla, C. 1995a. Pupation site and emergence time influence the mating success of bagworm females, Oiketicus kirbyi. Entomol. Exp. Applicata 77: 183-187.

Rhainds, M.; Gries, G.; Chinchilla, C. 1996. Development of a sampling method for first instar Oiketicus kirbyi (Lepidoptera: Psychidae) in oil palm plantations. J. Econ. Entomol. 89 (2): 396- 401.