Biology and host specificity of Aulacobaris fallax (Coleoptera: Curculionidae), a potential biological control agent for dyer's woad, Isatis tinctoria (Brassicaceae) in North America

Dyer's woad, Isatis tinctoria, a plant of Eurasian origin is a problematic weed in western North America against which a classical biological weed control programme was initiated in 2004. Three European insect species were selected as candidate agents to control this invasive species, including the root-mining weevil Aulacobaris fallax. To determine its suitability as an agent, the biology and host specificity of A. fallax were studied in outdoor plots and in the field between 2004 and 2006 in its native European range. Aulacobaris fallax is a univoltine species that lays its eggs from March to August into leaf stalks and roots of dyer's woad. Larvae mine and pupate in the roots and adults emerge from August to October. Up to 62% of the dyer's woad plants at the field sites investigated were attacked by this weevil. In no-choice host-specificity tests, A. fallax attacked 16 out of 39 species and varieties within the Family Brassicaceae. Twelve of these are native to North America. In subsequent multiple-choice tests, seven species, all native to North America, suffered a similar level of attack as dyer's woad, while none of the European species were attacked. Our results demonstrate the importance of including test plant species that have not co-evolved with the respective candidate agent. In sum, we conclude that the risk of non-target effects is too high for A. fallax to be considered as a biological control agent for dyer's woad in the United States.