Bagworm (Lepidoptera: Psychidae) infestation in the centennial of the Malaysian oil palm industry – a review of causes and control

In the sixties, the cause of outbreaks for leaf-eating caterpillars, particularly bagworms (Metisa plana and Pteroma pendula) and nettle caterpillars was shown to be applications of synthetic organochlorine insecticides. These disrupted the prevailing strong natural balance between the pests and their insect natural enemies, parasitoids and predators. These disruptive insecticides have long lasting contact residues and are lethal to the free flying natural enemies. Halting application of these pesticides caused many outbreaks to terminate quickly. Selective pesticides which were then brought in, killed the pests and relatively spared the enemies (integrated control). Such pesticides, in the form of lead arsenate (stomach acting), and trichlorfon (fast fading residues) were used in large scale application to suppress continuing infestation. Trunk injection (TI) which was later developed uses a suitable systemic organophosphate insecticide which is injected into palm trunks and kills only insects that eat the leaves. From the mid-sixties, outbreak became less common, but started to increase again in frequency and severity in the nineties. Formulations from Bacillus thuringiensis (Bt), and synthetic pyrethroids, such as cypermethrin and deltamethrin, were added to the range for regular use. Clean weeding could possibly reduce the survival of adult parasitoids by removing their shelter and floral food source. In parallel, oil palms in Sumatra had severe outbreaks of leaf-eating caterpillars in the 1950s and 1960s, associated with DDT dusting. When DDT was replaced by trichlorfon, parasitoid numbers increased and pest infestations became insignificant by the late sixties. In the nineties, fogging of synthetic pyrethroids caused new pest outbreaks, which more recently saw reduction in pest problems when control measures were restricted to selective applications. Despite awareness of the risks, ‘minor build up’ or ‘infestation in the locality’ may encourage ‘precautionary’ treatment. Disruptive application may be against other pests, and often outbreak can be traced back to this. A good example is Oryctes where synthetic pyrethroids are often applied in full cover sprays, and leaf-eater infestation commonly follows. In fact, there are very effective procedures to avoid significant Oryctes damage in young palm plantings without risky applications. There is an indication that some sectors are moving towards a cycle of outbreak, chemical knockdown, and re-outbreak. An investigation is suggested to put a significant area of palms onto a programme of entirely selective measures. Application would be based on action thresholds, but of equal importance is ‘no application if they are not reached’. The ultimate aim is effective and sustainable regulation of the population size of bagworms and other pests.