The evolution and ecological drivers of variation in chemical defences in the Wood Tiger Moth (Arctia plantaginis)

Aposematic warning signals and repellent chemical compounds are costly defences aimed at deterring predators’ attacks. However, despite the selective pressure from predators, the strength of chemical defences exhibits substantial and unexpected variation within and across species. This thesis aims to better understand the evolutionary drivers of this variation in chemical defence. The wood tiger moth, Arctia plantaginis, is a chemically defended species with conspicuous hindwing colouration that differs both locally and geographically. A major component of the moth’s chemical defences is produced de novo and secreted in response to attacks by avian predators. These secretions contain two methoxypyrazines: SBMP (2-sec-butyl-3-methoxypyrazine) and IBMP (2-iso-butyl-3-methoxypyrazine). In this thesis, I measured the variation in methoxypyrazine production across different wood tiger moth populations and tested how this variation influences predators’ behaviour. Furthermore, I asked whether early life resources, such as proteins, play a key role in the production of this moth’s chemical defences and warning signal. Thus, using diet manipulations, I investigated how dietary resources are distributed between growth, chemical defence, and colour pigmentations in male and female wood-tiger moths, and whether trade-offs between those traits occur. I found that the chemical defence of wild moths partly reflects local predation pressure, and both genetic and environmental components influence the strength of chemical defence. Male and female moths reared on a high-resource diet had more deterrent defensive fluids than individuals raised on low-resource or food-deprived treatments and, while the pigment components of the warning signals were only marginally influenced by food availability, there was a positive correlation between the strength of the visual component and the chemical toxins (suggesting so-called “signal honesty”). In conclusion, the resources available in early life have an important role in the efficacy of chemical defences, but warning signals are more genetically robust under variable environmental conditions.