Plant-mediated interactions among gall forming insects

Abstract

Russian knapweed (Rhaponticum (=Acroptilon) repens (L.) Hidalgo) is one of the most troubling exotic weeds throughout the western United States invading many public and private lands. A classical biological control agents, the gall midge (Jaapiella ivannikovi Fedotova), is a parasite on Russian knapweed, forming galls on the plant, inside which J. ivannikovi broods feed and develop. This system provides an opportunity to consider plant-mediated interactions between midge individuals and considered the merits of integrating other weed management techniques (grazing, mowing, and chemical control before insect release) with biological control. To accomplish this, I conducted releases of gall midge agents at replicated sites throughout Colorado, field cage experiments with simulated grazing, and greenhouse studies. We found that J. ivannikovi initially established across Colorado, however, failed to maintain populations in subsequent years. Interestingly, grazing increases J. ivannikovi establishment. Of further note, J. ivannikovi broods compete with one another even when feeding on different parts of the plant. The results of these experiments indicate that integrating management techniques can increase biocontrol agent establishment. I also question how effective this midge is at reducing Russian knapweed flowering and vegetative growth in the field. This research resulted in recommendations regarding the most judicious use of J. ivannikovi biocontrol agents in terms of where and when agents are most effective for management, and practitioners at the Colorado Department of Agriculture Palisade Insectary have adjusted their strategies accordingly.