Multiple modalities in insect warning displays have additive effects against wild avian predators

Allocation to different components of defence has been suggested as an explanation for the existence of multiple aposematic morphs in a single population. We tested whether there are trade-offs between warning colouration and chemical defence or whether these have an additive effect when combined, using blue tits (Cyanistes caeruleus) as predators and the polymorphic wood tiger moth (Arctia plantaginis) as prey. We used artificial edible models (with and without the moths' defensive fluids) with paper wings whose colour and pattern properties matched those of real moths. When the models were presented sans defensive fluids or when the fluids were presented without colour cues, we detected no differences in initial avoidance between the two morphs. However, when the colour and chemical cues were combined, differences emerged. White wings elicited higher latency to approach regardless of the defensive fluids applied on them. After approach, however, the defensive fluids of both morphs presented on moth models elicited higher latency to attack than a water control, hinting at a repellent odour. Fluids of white moths rendered lower amounts of prey eaten regardless of wing colour, while yellow moths' fluids provoked the highest occurrence of beak wiping behaviour. Our findings highlight the importance of accounting for interactive effects between different signal modalities, as these can create patterns not detectable when examined in isolation. Understanding these interactions is vital to determine how different components of multimodal warning displays provide protection at different stages of a predation event and, potentially, how multiple morphs can co-occur in a population.Significance statementThere are many things that can stop a predator attacking a prey such as looking scary or smelling bad, but if a predator does take a bite, tasting bad can make the difference between life and death for the prey. When combined with bright conspicuous colours, both repellent odours and deterrent tastes (i.e. chemical defences) can help predators learn to avoid unprofitable prey. However, it is unclear whether it is really the sum of these visual and chemical signals that most effectively deters predators or whether one is more important than the other. Examining the effects of warning colour and chemical defence in white and yellow wood tiger moths on wild-caught birds, we show that neither aspect of the moths' defence in isolation is as effective for predator deterrence as the sum of both.