Aloe barbadensis rinds employ physical and chemical defense mechanisms against insect herbivores with varying success

Abstract

Plants deter herbivory through a suite of physical and chemical defenses. While most studies have focused on understanding one or the other, efficient plant defense against herbivory is often cumulative. To examine this, we used Aloe barbadensis (Asphodelaceae) as a model, since in natural settings, the species is found to be well-defended against insect herbivores. To start with, we hypothesized that the epicuticular wax potentially plays an important role in herbivore defense as the major physical defense. To test our hypothesis, we conducted bioactivity-guided extraction and polarity-based fractionation of A. barbadensis rinds. We then mixed them into an artificial diet, to determine the effect of these compounds on the growth and development of two species of caterpillars, tobacco hornworm (Manduca sexta; Lepidoptera: Sphingidae) and fall armyworm (Spodoptera frugiperda; Lepidoptera: Noctuidae). Thereafter, we identified and quantified different volatile compounds emitted by the epicuticular waxes to analyze if any herbivore deterrent compounds are emitted and might be responsible for A. barbadensis repelling insect herbivores. Our results show that the surface waxes and volatiles prevented the feeding of the larvae, and the extracts from the rinds differentially affected their growth and development. Our results do not definitively explain the entire defense phenotype; however, they do help to shed light on the combination of physical and chemical mechanisms enacted by the species.