Size dependent predation risk in cryptic and conspicuous insects

Abstract

It is not clear which selective pressures balance the strong fecundity advantage associated with large female body size in insects. A positively sizedependent mortality risk could provide a solution. In aviary experiments with artificial larvae, we studied if larger larvae of folivorous insects are more readily found (= detectability) and/or attacked (= acceptability) by birds. As size and colouration are likely to interact in determining birds' responses, both cryptic and conspicuous prey items were used. As detectability is likely to be context-dependent, both simple (smooth) and complex (plants) backgrounds were used in respective experiments. In the conspicuous larvae, acceptability correlated negatively with prey size. However, their detectability was context dependent, being positively correlated with size on the simple background, whereas no significant effect was found on the complex background. Surprisingly, cryptic larvae showed no correlation between detectability and size, and there was only a weak tendency for birds to attack large larvae more readily. On the basis of a quantitative model, we conclude that the effect of positively size dependent bird predation, as a single factor, is not likely to counterbalance the fecundity advantage in cryptic species, and may thus not be crucial in determining the optimum for body sizes in these insects. In conspicuous species, there is a potential for different outcomes, because detectability and acceptability affect survival in different directions. The net outcome is, therefore, likely to be highly context-dependent. Furthermore, our results provide an explanation for the recently reported absence of systematic body-size differences between cryptic and conspicuous Lepidopteran larvae: although conspicuous larvae benefit from increasing their warning signal when growing larger, they also suffer a much sharper rise in detectability.

Size-dependent colouration in larvae of Orgyia antiqua (Lepidoptera: Lymantriidae): A trade-off between warning effect and detectability?

“…We find it likely that this strategy is also used by the medium sized Orgyia larvae. As an aside, it is interesting to note the weak overall effect of size on the time to be found, which confirms the results of Mänd et al (2007) suggesting a surprisingly minor overall effect of body size on the detectability of insects by visually searching predators.…”

“…In turn, conspicuous colouration also increases prey detectability by any visually searching predator. Quite obviously, detectability is higher if the body is larger, and, somewhat less obviously, there is some evidence that aposematic prey has a steeper size dependent detectability function, as compared to cryptic objects (Mänd et al, 2007;this study). This is biologically relevant because warning colouration rarely offers complete protection to the aposematic prey (Stimson & Berman, 1990;Geffeney et al, 2002;this study), and high detectability may thus be highly detrimental (Exnerová et al, 2003(Exnerová et al, , 2006Endler & Mappes, 2004).…”

Florivory as an Opportunity Benefit of Aposematism

“…The particular form of A does not affect predictions, provided that A decreases with x at a decelerating rate, as expected from observations on lepidopteran larvae (Mänd et al 2007). If the prey is cryptic, then w p 0, so the predator always attacks (Mänd et al 2007). If a herbivore is attacked, we assume that half of the current toxin load y t 1 1 is lost in defense, on the basis of observations for Pieris brassicae (Higginson et al 2011;Daly et al 2012) and our intuition that herbivores conserve some toxin to defend against future attacks.…”

“…The particular form of A does not affect predictions, provided that A decreases with x at a decelerating rate, as expected from observations on lepidopteran larvae (Mänd et al 2007). If the prey is cryptic, then w p 0, so the predator always attacks (Mänd et al 2007).…”

“…Aposematism is often not tenable for small animals because their warning signal is weak and individually they expose predators to small, ineffective toxin doses (Forsman and Merilaita 1999;Mänd et al 2007;Remmel and Tammaru 2011). If the animal is cryptic when small, thenfrom the arguments above-it should feed on interior leaves during initial instars (Smallegange et al 2007;Agerbirk et al 2010;Bandeili and Müller 2010).…”