Not just the sum of its parts: Geographic variation and nonadditive effects of pyrazines in the chemical defence of an aposematic moth

Abstract

Chemical defences often vary within and between populations both in quantity and quality, which is puzzling if prey survival is dependent on the strength of the defence. We investigated the within‐ and between‐population variability in chemical defence of the wood tiger moth (Arctia plantaginis). The major components of its defences, SBMP (2‐sec‐butyl‐3‐methoxypyrazine) and IBMP (2‐isobutyl‐3‐methoxypyrazine), are volatiles that deter bird attacks. We hypothesized that (1) variation in the chemical defences of male wood tiger moths reflects the local predation pressure; (2) observed differences in quantity and quality of defence among populations have a genetic basis; and (3) increasing concentrations of SBMP and IBMP will elicit greater aversive reactions in predators, with the two pyrazines having an additive effect on predators' avoidance. We found that (1) the chemical defence of wild moths partly reflects local predator selection: high predation pressure populations (Scotland and Georgia) had stronger chemical defences, but not lower variance, than the low‐predation populations (Estonia and Finland). (2) Based on the common garden results, both genetic and environmental components seem to influence the strength of chemical defence in moth populations; and (3) IBMP alone did not provide protection against bird predators but worked against bird attacks only when combined with SBMP, and while SBMP was more effective at higher concentrations, IBMP was not. Altogether this suggests that, when it comes to pyrazine concentration, more is not always better, highlighting the importance of testing the efficacy of chemical defence and its components with relevant predators, as extrapolating from chemical data may be less than straightforward.

Diet influences resource allocation in chemical defence but not melanin synthesis in an aposematic moth

“…However, we did not find any differences between the predators’ hesitation time (i.e., latency) to “attack” the bait soaked with fluids from male moths raised on high-protein and the latency to attack baits with water (control), which suggests that the predators may not perceive a difference in the odour (volatile compounds) of both. These results agree with previous studies suggesting that the defensive fluids of wood tiger moths may contain several repulsive compounds, not only pyrazines, that influence defence efficacy in concert (see also Winters et al 2021; Ottocento et al 2022) and affect predator response in different ways (Rojas et al 2019; Winters et al 2021). Moreover, as Ottocento et al show, the relative amount of the two methoxypyrazines (SBMP, IBMP) present in the wood tiger moth defensive fluids is more relevant to predator deterrence than the total amount of pyrazines.…”

“…Unfortunately, however, it is not possible to quantify the amount of pyrazines and conduct bird assays with the defensive fluids of the same individual. Interestingly, the same study (Ottocento et al, 2022) showed that predator responses are stronger towards the chemical defences of individuals originating from populations with high predation pressure (e.g., Scotland; Rönkä et al 2020) than towards individuals from populations with lower predation pressure, even if the total amount of pyrazines in the defensive fluids does not differ among populations. Our results show that highly melanised male moths from both diet treatments have more effective chemical defences than those with less melanin, hinting at a positive correlation between costly melanin pigments and chemical defences.…”

“…The birds used for the experiment were caught at Konnevesi Research Station (Central Finland) from February to April 2018. The birds were trapped (Ham et al 2006) then housed individually in the aviary (see Ottocento et al 2022).…”

“…Birds were first familiarised with the experimental boxes and trained to eat a bait (oat flakes) (see Ottocento et al 2022). Each bird then experienced four sequential trials, at 5-minute intervals, in which they were presented with a plate containing one bait (see Fig.…”

The evolution and ecology of multiple antipredator defences

“…Testing theoretical predictions about the effects of multiple defences on predator cognition will require carefully designed experiments involving manipulation of prey defensive traits (Ottocento et al, in press; Rojas et al, 2019) as well as costs and benefits associated with predator decisions to attack or avoid (Sherratt & Holen, 2018). As cognition and hunting strategies of predators, and their sensitivity to prey defences can vary greatly, model predators should be as ecologically relevant as possible.…”