Biology, phenology, and host range of a seed-feeding weevil, Ceutorhynchus constrictus (Coleoptera: Curculionidae), a biological control agent for garlic mustard, Alliaria petiolata, in North America

Ceutorhynchus constrictus Marsham is a seed-feeding weevil and potential biocontrol agent of the invasive plant garlic mustard, Alliaria petiolata (Bieb.) Cavara & Grande (Brassicaceae), in North America. Ceutorhynchus constrictus is native to Europe, has a single generation per year and eggs are laid into developing seeds within the seedpods of bolting garlic mustard plants. Females use their prolonged rostrum to bore holes through seedpods into the developing seed, deposit a single egg and then cover the opening with secretion. Larvae feed on ripening seeds and progress through three instars before exiting seedpods in late June, to pupate and overwinter in the soil. In Europe, C. constrictus development from egg to mature larva requires 30-to-40 days. At densities of up to three mating pairs of C. constrictus per garlic mustard inflorescence, there was no evidence of intraspecific competition among adult females or larvae. Up to 79% of seeds were destroyed on individual plants. When placed in the most conservative no-choice tests, C. constrictus has a physiological host range that includes two Brassica, one Boechera and one Braya species (all Brassicaceae). However, these plant species are not within the ecological host range of C. constrictus because they are not accepted as hosts in single-choice or open-field tests. Our results, and lack of field records on non-target plants in its native range, indicate that the ecological host range of C. constrictus is restricted to garlic mustard. We conclude that C. constrictus is a highly specialized herbivore that exclusively attacks field populations of garlic mustard.