Risk assessment of the root-weevil Mogulones crucifer, a biocontrol agent released against Cynoglossum officinale in Canada and prohibited in the United States

Mogulones crucifer is a root-feeding weevil that was released in Canada for the biological control of the noxious rangeland weed, Cynoglossum officinale, in 1997. The weevil has since become successful in suppressing C. officinale populations in Canada. It has not, however, been permitted for release in the United States, instead being declared a plant pest by USDA APHIS in 2010 due to concerns about nontarget attack of threatened and endangered plant species in Boraginaceae. To predict the realized host range of the weevil and consequently assess the risk of nontarget attack, we are investigating the host selection behavior of M. crucifer in response to olfactory cues of C. officinale and selected native rare or T&E (federally listed threatened and endangered) plant species using laboratory host-finding bioassays. Results indicate that during host finding, M. crucifer greatly prefers volatile cues of C. officinale to all tested native confamilial nontarget plant species. In tests comparing responses to volatiles from test plant species vs. purified air, the weevils showed no attraction for any plant species tested, other than C. officinale. Volatiles of some native confamilial species were even repellent to the weevil. The findings suggest that the weevil is unlikely to locate and therefore colonize rare or endangered native Boraginaceae, and that these species are therefore not at risk of attack and injury by M. crucifer.