Eustenopus villosus (Coleoptera : Curculionidae) feeding of herbicide-resistant yellow starthistle (Centaurea solstitialis L.)

A population of yellow starthistle (Centaurea solstitialis L.) near Dayton, Washington developed herbicide resistance in response to repeated applicat ions of picloram and other auxin-type herbicides. Laboratory and field expe riments were conducted in 1998 to determine host acceptability and suitabil ity of this herbicide-resistant yellow starthistle population to the biolog ical control weevil Eustenopus villosus (Boheman) (Coleoptera: Curculionida e). In choice and no-choice feeding and oviposition experiments using excis ed buds, the weevil did not demonstrate a consistent preference for either herbicide-resistant (R) or -susceptible (S) yellow starthistle. When caged on buds of intact plants, the E. villosus feeding rate of 97% did not diffe r between R and S types. Host plant suitability, measured as larval damage and development to adult weevils, was equivalent in R and S types, with wee vils maturing in 46% of the R and in 32% of the S capitula bearing oviposit ion sears. The number of viable achenes per capitulum was reduced by 87% du e to larval feeding, with no difference between R and S types. Observations at the field site where resistance was found revealed oviposition scars on 78% of the late-bud-stage capitula on 23 June 1998 and 73% of the flowerin g and postflowering capitula on 15 August 1998. Selection for herbicide res istance has not created host incompatibility for E. villosus nor reduced th e effectiveness of E. villosus as a biological control agent. (C) 2001 Acad emic Press.