Coconut leaf bioactivity toward generalist maize insect pests

Abstract

Tropical plants are often more resistant to insects than temperate plants due to evolution of robust defenses to cope with a more constant insect threat. Coconut [Cocos nucifera L. (Arecaceae)] has very few chewing-type leaf feeding insect pests and was tested for feeding suitability against two generalist leaf feeding caterpillar species, corn earworm, Helicoverpa zea (Boddie), and fall armyworm, Spodoptera frugiperda (J.E. Smith) (both Lepidoptera: Noctuidae). Feeding on leaf tissues from the most recently expanded leaves of a coconut variety caused significant mortality and reduced growth rates (as indicated by survivor weights) of S. frugiperda and H. zea compared to when they fed on leaves from a typical host, maize [Zea mays L. (Poaceae)], or the standard artificial diet. Proteins or other polymers did not appear to be responsible for the bioactivity noted against the caterpillars. Components responsible for activity were acetone extractable and separable by thin layer chromatography. Extracts from multiple areas of the thin layer chromatography (TLC) plates caused significant reductions in growth rates of S. frugiperda. The most bioactive TLC-separated component, identified as pheophytin a, caused oxidative browning of test diets, suggesting that cytotoxicity of reactive oxygen species is a likely mode of action against H. zea and S. frugiperda.