BIOLOGICAL AND MICROBIAL CONTROL Inundative Release of Aphthona spp. Flea Beetles (Coleoptera: Chrysomelidae) as a Biological “Herbicide” on Leafy Spurge in Riparian Areas

Inundative releases of beneÞcial insects are frequently used to suppress pest insects but not commonly attempted as amethod of weed biological control because of the difÞculty in obtaining the required large numbers of insects. The successful establishment of a ßea beetle complex, mixed Aphthona lacertosa (Rosenhauer) and Aphthona nigriscutus Foundras (87 and 13%, respectively), for the control of leafy spurge, Euphorbia esula L., provided an easily collectable source of these natural enemies that enabled us to attempt inundative release as a possible leafy spurge control method in a sensitive riparian ecological zone where chemical control is restricted. Our target weed populations were small isolated patches of leafy spurge along three streams in southwestern, central, and northeastern Idaho. This study assessed leafy spurge and associated vegetation responses to inundative releases of 10 and 50 beetles per spurge ßowering stem over two consecutive years. Releasing 10 beetles per ßowering stem had inconclusive effects on spurge biomass, crown, stem, and seedling density. Alternatively, releasing 50 beetles per ßowering stem resulted in a reduction of biomass, crown and stem density in the range of 60Ð80% at all three study sites, and about an 60% reduction of seedling density at one site, compared with untreated plots. In contrast to leafy spurge, associated vegetation did not conclusively respond to beetle release, indicating that it may take more than two years for desired riparian vegetation to respond to reductions in leafy spurge competition.