Effects of sowing date and insecticides on cereal aphid populations and barley yellow dwarf virus on barley in Kenya

The effects of the date of sowing and insecticide sprays on aphid populatio ns and barley yellow dwarf virus (BYDV) incidence in barley was studied in Mau Narok, Kenya. Rhopalosiphum padi (L.) and Metopolophium dirhodum (WLK.) were common aphid species, but other cereal aphids present were Rhopalosip hum maidis (Fitch), Stiobion avenae (F.), Schizaphis graminum (Rond.) and H ysteroneura setaria Thom. The incidence of BYDV was significantly decreased in plots sown with seed that had been treated with imidacloprid (NTN-33893 , Gaucho) and subsequently sprayed with foliar insecticide (Cypermethrin). Yield loss due to BYDV was also significantly different between the treatme nts and between the early-planted and the late-planted crop (P < 0.05). Gra in yield and 1 000-grain weight were not significantly different among inse cticide treatments in the early-planted crop. In the late-planted crop, the yield increase with seed treatment alone was highly significant (P < 0.001 ), with a yield increase of 36-43 %, more than that of the untreated contro l. Grain yield was significantly (P < 0.05) negatively correlated with the total number of cereal aphids, as well as with the numbers of R. padi alone .