Raspberry inflicts associational susceptibility to meadowsweet in a complex food web

Abstract

Meadowsweet [Filipendula ulmaria (L.) Maxim. (Rosaceae)] has previously been reported to enjoy associational resistance in mixed populations with purple loosestrife [Lythrum salicariaL. (Lythraceae)] due to a shared hymenopteran parasitoid that provides top-down control of herbivory. Here, we report that meadowsweet suffers associational susceptibility in mixed populations with raspberry [Rubus idaeusL. (Rosaceae)] due to a shared herbivore, the raspberry flea beetle [Batophila rubi (Paykull) (Coleoptera: Chrysomelidae)]. Close to raspberry (less than 0.5 m), herbivory on meadowsweet often reached 20-25%, whereas further away (1.5-2 m) herbivory was almost always 0%. We especially highlight the different scales at which the opposing associational effects occur. The shared parasitoid forages throughout entire populations, making the level of population the focal scale of the associational resistance. The shared herbivore, however, spills over from raspberry to neighboring meadowsweets only, making the distance to individual raspberry plants the focal scale for associational susceptibility for meadowsweet. We conclude that different co-occurring plant species can mediate opposing associational effects on a focal host plant by altering the abundance and composition of herbivores and parasitoids, respectively, resulting in multiple selection layers to the geographic mosaic of herbivory.