The importance of specialist natural enemies for Chrysomela lapponica in pioneering a new host plant

Abstract

Abstract. 1. The Salicaceae have been suggested as ancestral host plants of Chrysomela species (Coleoptera: Chrysomelidae). In Chrysomela lapponica, some populations are specialised on salicaceous plants, but others have switched to birch. This study aimed to elucidate the significance of natural enemies as possible selective forces for the host plant shift of C. lapponica from willow to birch.2. Two C. lapponica populations were studied, one specialised on willow Salix borealis in Finland, and another one specialised on birch Betula pubescens in the Czech Republic. Abundances of predators and parasitoids on birches and willows were recorded at both population sites. Furthermore, field and laboratory experiments were conducted.3. Field data do not support the hypothesis that generalist predators affected the host shift from willow to birch in C. lapponica.4. Parasitism of C. lapponica (pre)pupae by a specialised phorid fly was significantly stronger in specimens living on willow than in birch‐living ones.5. The predatory syrphid Parasyrphus nigritarsis specialised on Chrysomelinae was only detected on willows. The syrphid preferred to orient towards substrates treated with defensive larval secretion or faeces of the willow‐specialised C. lapponica specimens compared with the birch‐specialised ones.6. The data suggest that specialised parasitoids and predators might have been driving forces for C. lapponica to leave willows and to pioneer birches as sites with a lowered risk of predation and parasitism. This hypothesis is discussed with respect to results of earlier studies on the impact of bottom‐up effects by the plant.