Toadflax invasion into natural areas has prompted interest in weed management via biological control. The most promising biological control agent currently available for the control of Dalmatian toadflax is Mecinus janthinus, a stem-boring weevil that has been shown to significantly reduce toadflax populations. Some land managers, however, are reluctant to release approved weed biological control agents based on concerns about possible nontarget impacts. Few postrelease examinations of biocontrol impact and host specificity have been performed, despite the call for such information. This study examined the host specificity of Mecinus janthinus, postrelease, in relation to Colorado sites to provide information to managers about its relative safety as a weed biological control agent. This study employed three components: (1) greenhouse choice and no-choice experiments; (2) no-choice caged field experiments; and (3) release-site evaluation of nontarget use of native plant species where this weevil has been released and has established. Both greenhouse and field experiments failed to demonstrate nontarget use of native plant species by M. janthinus in the region where it was studied, even in no-choice starvation tests. We found no evidence of nontarget herbivory on native plants growing at toadflax sites where M. janthinus was well established. These results support the continued use of M. janthinus as a low-risk biological control agent for the management of toadflax in the Rocky Mountain Front Range.

The effects of larval crowding on growth, development and reproductive output of the Parthenium beetle Zygogramma bicolorata Pallister were investigated. Experimental manipulations of larval densities indicated that crowding creates intraspecific competition among the larvae, which negatively affected their growth and development. Larvae at high densities grew more slowly and exhibited longer larval periods compared with those at lower densities. Best-fit curves of developmental rates against larval densities were drawn, which revealed maximum theoretical density for 95% (6 or 15 larvae/63.6 cm2) and 50% developmental rate (1.75 larvae/63.6 cm2), respectively. The reproductive potential of the beetle reared at different larval densities was also observed, but no effect of increased larval densities was observed.