Yasuhiro Sato, Koichi Ito and Hiroshi Kudoh
A predator’s choice of food can play a role in maintaining the balance between different forms of a prey species. We investigated this by observing the interactions of the brassica leaf beetle (Phaedon brassicae) and a common wild perennial herb (Arabidopsis halleri) in Japan.
The herb A. halleri has two common forms or “morphs”—one with hairs on the leaf and stem and the other without. The presence of hairs is thought to be a protective mechanism preventing leaf damage by insect herbivores. Earlier studies have shown that leaf beetles prefer to lay eggs on hairless plants as their young, or larvae, grow better when they feed on hairless leaves. We sought to understand whether this demonstrated preference for hairless plants, or “optimal diet choice,” of the leaf beetle affects how the two different forms of A. halleri are represented in natural populations of this plant.
In yearly surveys conducted during mid-May from 2013-2016 using 297 monitoring plots set in 80 random plant patches, we assessed the numbers of hairy and hairless plants and the extent to which each leaf type was damaged. The leaf beetles preferred to feed off hairless plants; however, when hairy plants were abundant, the beetles consumed more hairy leaves but fed off hairless leaves to the same extent as when no hairy leaves were available. Both types of leaves could thus exist simultaneously in the population even when one type is more vulnerable to the beetles than the other.
We also used data from the first three years of the surveys to construct a mathematical model and found that this model was better able to predict the relative proportions of hairy- and hairless-leaved plants if the beetles’ diet preferences were factored into the model prediction.
There have been many laboratory studies demonstrating the importance of natural enemies in maintaining multiple forms of prey species. These new results, however, offer data-based evidence that, in natural populations, the existence of various defensive forms of prey species may be more affected than we previously thought by the behaviour of their natural enemies.
Image provided by authors.