Sociality in caterpillars: Investigations into the mechanisms associated with grouping behaviour, from vibroacoustics to sociogenomics

Abstract

Social grouping is widespread among larval insects, particularly in a number of phytophagous larval Lepidoptera (caterpillars). Although the benefits of social grouping are widely recognized, the proximate mechanisms mediating grouping behaviour, such as group formation and maintenance, are poorly understood. My Ph.D. thesis takes a pioneering approach to understanding these mechanisms, specifically, by studying the roles of vibroacoustics and sociogenomics, using the masked birch caterpillar, Drepana arcuata (Lepidoptera: Drepanoidea), as a model. There are two main objectives of my thesis- (i) to test the hypothesis that caterpillars employ plant-borne vibratory signals to recruit conspecifics to social groups; and (ii) to test the hypothesis that differential gene expression is associated with developmental transitions from social to solitary behavioural states. For the first objective, I documented morphological and behavioural changes in the larvae, showing that there are five larval instars, and developmental changes in social and signalling behaviour. Specifically, early instars (I, II) live in small social groups, and late instars (IV, V) live solitarily, with third instars (III) being transitional. Instars I-III generate four signal types (AS, BS, MS, MD), instars IV, V generate three signals (AS, MS, MD). I then used an experimental approach to test if early instars employ vibrations during social recruitment, and found that vibratory signals are used to advertise feeding and silk shelters, leading to recruitment, with higher Signalling rates resulting in faster joining times by conspecifics. For the second objective, comparative transcriptomic analysis indicates that there are 3300 transcripts differentially expressed between early (social) and late (solitary) instars, and these include transcripts potentially coding for candidate 'social' genes. One of these genes- an octopamine receptor gene- was further functionally tested using RNAi, and preliminary results suggest that its reduced expression is associated with hastened social to solitary transition. As this research contributes the first genomic data on an entire lepidopteran superfamily (Drepanoidea), I also assembled a draft genome of D. arcuata. The research is the first to test hypotheses on the roles of vibrational signalling and genomics in the social behaviour of larval insects, many of which are of great economic and ecological importance.