Technical Abstract: Inundative releases of beneficial insects are frequently used to suppress pest insects, but not commonly attempted as a method of weed biological control because of the difficulty in obtaining the required large numbers of insects. The successful establishment of a complex of Aphthona spp. flea beetles for the control of leafy spurge 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 ecological zone where chemical control is restricted. Our target weed populations were small isolated patches I of leafy spurge along three streams in southwestern, central and northeastern Idaho. Riparian areas are high-value environmentally sensitive habitats where use of many herbicides is prohibited and flea beetles are often unable to become permanently established because periodic spring flooding drowns the overwintering larvae in the soil. This study assessed the impact of inundative releases of 10 and 50 beetles per leafy spurge flowering stem of a mix of Aphthona nigriscutus, and A. lacertosa (87 percent A. lacertosa, and 13percent A. nigriscutis) over two consecutive years on leafy spurge and associated vegetation. Results indicate that releasing 10beetles per flowering stem had inconclusive, potentially small negative effects on leafy spurge biomass, crown, stem, and seedling density. Alternatively, based on point estimates, releasing 50 beetles per flowering stem resulted in an approximate 80% reduction of biomass, crown and stem density and 60%