Herbivory induced aggregation of tortoise beetles on morning glory plants

Abstract

Tortoise beetles (Chiridopsis spp.) are herbivores of morning glory plants (Ipomoea spp.) and they exhibit high level of host specificity. In nature, we have observed these beetles aggregating only on host plants with a founder population present on them. Through this study, we tried to understand the role of founder beetles, origin, and the composition of the aggregation signal by conducting behavioural assays, GCMS analysis, and SPME-HS analysis followed by an attempt at reverse genetics. Using Chiridopsis nigropunctata and Ipomoea elliptica as our model system, we report that signal for C. nigropunctata to aggregate originates in the plant itself and a founder population of beetles is not necessary to bring about aggregation on I. elliptica. These beetles aggregate similarly on a plant with just mechanical damage and no founder beetle, as they do on plants having herbivory and a founder beetle. It means beetles do not differentiate between these two treatments. GC-MS and SPME-HS data suggest that the said aggregation signal may be composed of plant VOCs induced in response to beetle herbivory as well as mechanical damage. α-copaene, β-copaene and δ-cadinene were the three compounds that show significant induction in concentrations 12 hours post herbivory and mechanical damage alike. Hence, they are the top candidates to be (or constitute) the said aggregation signal. We also attempt to PCR amplify and identify the genes responsible for producing these sesquiterpenes in I. elliptica and ultimately silence them using the VIGS technique and observe the effect on aggregation in silenced plants.