Optimal foraging by herbivores maintains polymorphism in defence in a natural plant population

Many species of plants and animals exhibit polymorphism for defensive traits. Adaptive foraging by natural enemies has long been hypothesized to maintain such polymorphism, but this has not been clearly demonstrated in a natural prey or host population. The purpose of this study was to address whether the brassica leaf beetle Phaedon brassicae promotes the maintenance of defence polymorphism in the trichome‐producing (hairy) and trichomeless (glabrous) morphs of Arabidopsis halleri subsp. gemmifera . Here, we modelled foraging behaviours of herbivores and demography of the host plant. Then, we estimated the model parameters based on the likelihood of observed data from a natural A. halleri population. The patterns of leaf damage to hairy and glabrous plants were well explained when we presumed the optimal diet choice (ODC) by P. brassicae . The observed dynamics in the plant number and morph‐frequency were well supported by the model with the estimated parameter values. Our numerical analysis showed that the ODC by P. brassicae caused a negative frequency‐dependent selection on trichrome dimorphism. The coexistence of two morphs was allowed over a wide range of herbivory pressure and the cost of defence. These results indicate that the ODC by P. brassicae contributes to the coexistence of hairy and glabrous A. halleri . While species interaction and stochastic dispersal are both involved in polymorphism dynamics in the field, our findings suggest that the role of consumer behaviours in the maintenance of defence polymorphism may be more important than currently appreciated. A plain language summary is available for this article.