Darna trima (nettle caterpillar)

In oil palm the lacerations caused by the early instar D. trima larvae will soon become necrotic; portions of some of these patches may eventually drop off leaving elongated holes surrounded by brown tissue. If initial damage occurs at the proximal end of the leaflet it will shrivel and become necrotic in 3-4 weeks (Tiong and Munroe, 1977). The feeding habit of later instars results in leaflets stripped to their midribs. In severe infestations entire fronds become necrotic and many of the leaflets have only their midribs with bits of lamina attached. In cocoa, repeated attacks result in dieback of lateral branches and excessive production of lateral buds (Entwistle, 1972).

Integrated Pest Management

Pardede (1992) found that an IPM approach integrating the application of Bacillus thuringiensis, conservation of the natural enemy population (in particular Cantheconidea javana) and collection and destruction of cocoons and adults of D. trima, can rapidly bring an outbreak under control.

Cultural Control

Many cultural practices are modified towards enhancing the effectiveness of indigenous natural enemies (see Biological Control).

Biological Control

Tiong and Munroe (1977) studied the effects of spraying a virus suspension on an outbreak of D. trima in Sarawak. Dead larvae were macerated and strained to produce an inoculum containing a mixture of granulosis virus and small RNA viruses. The suspension was more effective when applied with mistblowers than with knapsack sprayers: more than 77% of the larvae died within 4 days, with almost complete mortality by the 12th day. The cost of using the virus was about 40% of that using chemical treatment; virus suspension appeared to provide a more durable control.

Another approach for utilizing viruses to control leaf-eating caterpillars infesting oil palm in North Sumatra was described by Sipayung et al. (1990). For general field treatment at least 300 g diseased caterpillars are required per hectare. During a pest outbreak a mixture of a sublethal dose of a pyrethroid insecticide (less than 10% of the normal dosage) and the virus is recommended; the pyrethroid increases the susceptibility of the caterpillars to the virus, resulting in faster infection and higher mortality.

Various workers have suggested the conservation or establishment of suitable plants to enhance the numbers and reproductive potential of parasitoids and predators. Syed and Shah (1977) believe that extensive destruction of weeds, either deliberately or inadvertently, is a major indirect cause of pest outbreaks in oil palm in Sabah. They recorded several beneficials visiting the weeds Euphorbia geniculata, E. prunifolium, Ageratum conyzoides and A. mexicanum. They argued for the maintenance of certain weeds, especially the flowering ones, to help support the natural enemy fauna. Desmier de Chenon et al. (1990) has made several recommendations in the same vein; less competitive non-crop plants, such as members of the Euphorbiaceae, should be grown in the borders of plantation blocks to provide sources of food for the parasitoids. In Sumatra larvae of the sawfly Neostromboceros luchti is an important source of food for predators like Eocanthecona furcellata and some reduviids. These sawfly larvae are frequently found feeding on the fern Diplazium asperum which is common in oil palm plantations, and the conservation of this plant should provide an alternative source of food for the predators to tide them over periods when leaf-eating caterpillars of oil palm are scarce. The tachinid Chaetexorista javana with its high fecundity, good searching ability and wide host range can be a very useful parasitoid. However, the adults like to feed on flowers in sunny situations and this feeding is necessary to enable their eggs to mature. The lack of suitable flowering plants in sunny locations should be rectified in oil palm plantations. Pardede (1992) recommends the planting of Pueraria javanica [P. phaseoloides] in young oil palm plantings. In addition to its other beneficial effects, this leguminous cover crop acts as a host for caterpillars of Lamprosema [Omoides] diemenalis, which in turn, is an alternative prey for Cantheconidea javana, an important predator of D. trima.

Although limacodids are largely polyphagous, most reported outbreaks occur on palms, in particular oil palm and to a lesser extent coconut (Wood, 1987) and sago palms (Kimura, 1979). Outbreaks of limacodids on oil palm in Malaysia were mainly reported in the mid-1960s and mid- 1970s. Wood (1976) observed that limacoids were scarce in Peninsular Malaysia, more common in Sabah and common in some estates in Sarawak. From a survey of oil palm estates in Malaysia conducted in 1990, Norman and Basri (1992) concluded that limacodids were less important in the 1980s; an average of only five outbreaks per year were recorded during the 10-year period covered by the survey.Of the various species of limacodids known to infest oil palm, D. trima is the commonest found in outbreaks (Holloway et al., 1987). Young plantings are more severely attacked, but mature palms are also defoliated. In one outbreak it was reported that as many as 2000 larvae per frond were found and the leaf area of some palms was reduced up to 60% (Young, 1971). It has been estimated that a single larva consumes 20 cm² of leaf tissue over its entire life (Kimura, 1974).Defoliation of oil palm by leaf-eating caterpillars can have a drastic effect on yield. In a trial in which palms were artificially defoliated, there was a 43% drop in yield a year after 50% defoliation of the upper part of the crown, followed by a 17% drop in the year after that (Wood et al., 1973a). The main effect was expressed 4-6 months after defoliation and complete recovery took some years (Wood, 1982). Even limited damage can lead to some crop loss and remedial action is beneficial, provided that measures taken are well-timed and selective in action (Wood, 1987). In immature palms the effect of artificial defoliation was less marked.Outbreaks of D. trima have been reported on cocoa in Java, Indonesia and West Malaysia (Entwistle, 1972).