INSECTICIDAL CONTROL OF CEREAL APHIDS AND ITS IMPACT ON THE EPIDEMIOLOGY OF THE BARLEY YELLOW DWARF LUTEOVIRUSES

Imidacloprid is a seed-applied nitroguanidine insecticide that has bot h contact and long-lasting systemic properties. Its direct effects on cereal aphid populations and indirect effects on barley yellow dwarf l uteovirus (BYDV) incidence were examined in laboratory and field studi es. Adult longevity and fecundity of three aphid species caged on vari ous aged, imidacloprid-treated oat or wheat plants was reduced, althou gh the quantitative efficacy of the compound differed among aphid spec ies. The transmission efficiency of BYDV by aphids to imidacloprid-tre ated plants was lower, relative to nontreated plants, when plants were 10 days old, but similar to nontreated when plants were 24 days old. In three years of field trials using winter wheat and two years using spring oat, aphid populations were reduced significantly in imidaclopr id-treated plots relative to nontreated plots. Although the number of alighting alate aphids did not differ significantly between imidaclopr id-treated or nontreated plots, the number of apterous aphids remained significantly lower in the treated plots for the majority of the grow ing season. BYDV epidemics did not develop in spring oat in either of the two years. BYDV epidemics did develop in the fall in the emerging winter wheat crop. Rhopalosiphum maidis was the predominant aphid migr ating into the 1991-92 crop. The incidence of the BYDV-RMV serotype wa s three times higher in nontreated plots than in imidacloprid-treated plots. In the 1992-93 and 1994-95 seasons, R. maidis and R. padi were the predominant aphids migrating into the crop, although R. padi was t he predominant colonizing aphid. The incidence of BYDV-RMV was similar in imidacloprid-treated and nontreated plots in 1992-93, but the inci dence of the BYDV-PAV serotype was significantly less in the treated p lots. In 1994-95, the incidence of both BYDV-PAV and BYDV-RMV was high er in the nontreated plots than in the imidacloprid-treated plots. Imi dacloprid offers several advantages both in terms of its long-lasting systemic activity and its mode of applicaiton. As a seed treatment, th e amount of material applied is minimized, there is little waste and t he environmental impacts are reduced. The effectiveness of the compoun d at reducing the fecundity or reproductive rate of several aphid spec ies may be advantageous in controlling secondary spread of viruses. Ho wever, the contact properties of the compound, important in repelling aphids or preventing phloem feeding, may be short-lived, thus reducing its effectiveness in preventing primary spread of the virus into the crop, especially under high aphid and inoculum pressure.