Efficacy of fungi for control of Russian wheat aphid (Homoptera : Aphididae) in irrigated wheat

Field plots of irrigated spring and winter wheat, Triticum aestivum L., in southeastern Idaho, were infested with the Russian wheat aphid, Diuraphis n oxia (Mordvilko) (Homoptera: Aphididae) and later treated with spores of Br auveria bassiana (Balsamo) Vuillemin or Paecilomyces fumosoroseus (Wize) Br own and Smith at rates of 2.5 or 5 x 10(13) spores/ha. Small plots (1.4 to 6.6 m(2)) were treated with a backpack sprayer in 1995-1998 and large plots (470 m(2)) were treated using an overhead irrigation system in 1997 and 19 98. In 1995, Russian wheat aphid densities rose to an average of over 120 R ussian wheat aphids per tiller in control plots compared to an average of l ess than 30 Russian wheat aphids per tiller in all fungus-treated plots 33 days after the first treatment. Smaller, significant reductions were observ ed within 14 days of treatment with B. bassiana in 1996 and 1997. Low Russi an wheat aphid infestation rates, averaging less than 1 Russian wheat aphid per tiller, made significant differences impossible to detect. Significant reductions in both Russian wheat aphid densities and the percentage of til ler infestation were observed in large-plot experiments two weeks after app lication of a high rate of B. bassiana in spring (1997) and winter wheat (1 998). Fungus-treated Russian wheat aphids suffered 32-83% mycosis when fiel d-treated tillers were incubated in the laboratory. Persistence of fungal i nocula of either species, measured by estimating CFUs obtained from washed leaf segments, decreased by 10- to 1000-fold within 3 to 5 days.