The occurrence and potential relevance of post-release, nontarget attack by Mogulones cruciger, a biocontrol agent for Cynoglossum officinale in Canada

Reports on nontarget attack by introduced biological control agents have caused debate over the safety of biological control of weeds. One outcome of this dialogue is the importance of monitoring for nontarget attack and its effects as part of post-release assessments. This is particularly vital in the case of the root-mining weevil Mogulones cruciger, which was approved and released in Canada, but not in the United States, to control Cynoglossum officinale. Mogulones cruciger was first released in British Columbia in 1997, following recommendations of the American Technical Advisory Group and the Canadian Biological Control Review Committee. During the same year, the US Fish and Wildlife Service raised concerns about potential nontarget effects by this insect to Boraginaceae species on the endangered species list. To assess the occurrence of nontarget attack, and its potential for nontarget effects, we identified and monitored confamilial species co-occurring with C. officinale at six M. cruciger release sites in Alberta and British Columbia over a two year period. All four co-occurring species were attacked by the weevil to varying degrees, although attack was inconsistent between years and sites. Nontarget species were attacked to a lesser degree than C. officinale, but differences were not consistent for species, sites, or years. There was a positive relationship between the probability of nontarget attack and C. officinale attack rate by M. cruciger. Our data suggest that the immigration of M. cruciger into the US may expose certain Boraginaceae to nontarget attack, but the transitory nature of that attack and consequently the risk to native species is unknown.