Pre-release impact assessment of two stem-boring weevils proposed as biological control agents for Alliaria petiolata

Experimental studies can be useful tools to test plant responses to herbivory and to quantify the impact of potential biological control agents prior to their release. We evaluated the per-capita effect of Ceutorhynchus alliariae and C. roberti, two stem-boring weevils currently being investigated as potential biological control agents for garlic mustard, Alliaria petiolata, in North America. Weevils were released at three different densities in individual and mixed-species treatments onto potted plants of A. petiolata. Damage by C. roberti alone and by both weevils combined caused an increase in the numbers of inflorescences produced per plant. Although plants could compensate for low levels of damage, moderate to high levels of damage by both C. alliariae and C. roberti, individually and in combination, caused a decrease in plant height and a reduction in seed output per plant. The damage inflicted by both weevil species is similar so the overall impact of both species combined can be predicted by summing the impact of each species alone. Provided they are sufficiently host specific, both weevils could be released as biocontrol agents. Because reduced seed production is necessary to suppress A. petiolata populations, both species have the potential to contribute to control of A. petiolata in North America.