The price of safetySubmitted by editor on 10 October 2018.Get the paper!
The world is a dangerous place for a small insect. With so many would-be predators, many insects turn to chemical defences in order to protect themselves. These chemicals range from the toxic to the merely smelly or bad tasting. They all have one purpose however, to make a predator think twice before eating the bearer. But if chemical defences are so useful, why doesn’t everyone have them? The answer may lie in their costs.
Chemical defences are thought to be costly because acquiring them from the diet requires machinery to process them; or because the prey must produce them from scratch. Thus, it is expected that only animals that have enough resources can afford the cost of handling, or producing, defensive compounds. While there are many studies showing that diet has a big impact on chemical defence in species that sequester their defences directly from their food, less is known about how it influences species that produce their defensive chemicals themselves. We tested this with the aposematic wood tiger moth Arctia plantaginis.
Figure 1. A female wood tiger moth (Arctia plantaginis) resting on the vegetation. Photo: Bibiana Rojas
The wood tiger moth is a common species across the northern hemisphere. This day flying moth advertises its chemical defences with brightly-coloured hindwings (Fig. 1). When attacked by a predator, such as a bird, the moth produces a specialised defensive fluid from glands behind its head. This fluid is distasteful to birds, and can be enough to make them spit the moth back out. The bad taste comes from chemicals called pyrazines, which the moths manufacture themselves.
We tested how costly this fluid is to produce by manipulating the moths diet during development. Wood tiger moths do not feed as adults, so must gain all their resources as larvae. We food deprived growing larvae for one day per week, and then tested the effectiveness of their defensive fluids against wild-caught blue tits. We found that moths that had been food deprived had less effective chemical defenses than those that were allowed to eat uninterrupted (Fig. 2).
Figure 2. Blue tits took longer to eat prey soaked with the fluids of well-fed moths
Our results show that resource limitation during early life can have negative consequences for these moths, as they are less protected from predators. This finding may explain why some individuals are able to survive bird attacks while others cannot.
Authors: Emily Burdfield-Steel, Bibiana Rojas
Twitter handles: @MothPostDoc @biobiiana @JohannaMappes