Interannual landscape changes influence plant–herbivore–parasitoid interactions

The effects of interannual changes in landscape composition on herbivory and biological control of pollen beetles on oilseed rape were experimentally tested. Parasitism by specialized parasitoids decreased following rape crop expansion, and increased following rape crop reduction, indicating interannual dilution and crowding effects. In contrast, herbivory by rape pollen beetles did not respond to these landscape changes due to crop rotation, supporting the concept that specialist natural enemies are more affected by changing environments than their host or prey. When expansion of rape crop area between years exceeded ∼5% of the landscape, parasitism rates dropped below a threshold value of about ∼35%, below which classical biological control has rarely been reported.