Abstract
Insects live in a dangerous world and may fall prey to a wide variety of predators, encompassing multiple taxa. As a result, selection may favour defences that are effective against multiple predator types, or target-specific defences that can reduce predation risk from particular groups of predators. Given the variation in sensory systems and hunting tactics, in particular between vertebrate and invertebrate predators, it is not always clear whether defences, such as chemical defences, that are effective against one group will be so against another. Despite this, the majority of research to date has focused on the role of a single predator species when considering the evolution of defended prey. Here we test the effectiveness of the chemical defences of the wood tiger moth, a species previously shown to have defensive chemicals targeted towards ants, against a common invertebrate predator: spiders. We presented both live moths and artificial prey containing their defensive fluids to female Trichonephila senegalensis and recorded their reactions. We found that neither of the moth’s two defensive fluids were able to repel the spiders, and confirmed that methoxypyrazines, a major component of the defences of both the wood tiger moth and many insect species, are ineffective against web-building spiders. Our results highlight the variability between predator taxa in their susceptibility to chemical defences, which can in part explain the vast variation in these chemicals seen in insects, and the existence of multiple defences in a single species.
Highlights
Almost all organisms are at risk of predation during some stage of their life
A total of T. senegalensis were tested with the pure pyrazines across trials, 15 with concentrations of 1 μl/ ml, 15 with 5 μl/ml and 15 with 10 μl/ml
The concentration of pyrazine in the capsules did not affect the amount of weight lost (W = 0.47, p = 0.4911), but it did significantly affect time spent feeding (W = 8.74, p = 0.0031) as higher concentrations were fed on by the spiders for longer (Fig. 5)
Summary
Almost all organisms are at risk of predation during some stage of their life. While there has been a trend in the past decades to use a greater variety of predators to test the effectiveness of chemical defences, ants and birds still predominate in such studies. Both groups are likely to be important predators in many environments, the huge variability in both sensory systems, and susceptibility to chemical defences, shown by predatory species means that over-reliance on a few taxa may produce misleading results (Zvereva and Kozlov 2016). Individual prey species are likely to fall prey to multiple predator species, it is important to test a variety of ecologically relevant predators to fully understand the strength of a species’ chemical defences (Endler and Mappes 2004)
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