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 noxia (Mordvilko) (Homoptera: Aphididae) and later treated with spores of Brauveria bassiana (Balsamo) Vuillemin or Paecilomyces fumosoroseus (Wize) Brown 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 1998. In 1995, Russian wheat aphid densities rose to an average of over 120 Russian wheat aphids per tiller in control plots compared to an average of less than 30 Russian wheat aphids per tiller in all fungus-treated plots 33 days after the first treatment. Smaller, significant reductions were observed within 14 days of treatment with B. bassiana in 1996 and 1997. Low Russian 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 tiller infestation were observed in large-plot experiments two weeks after application of a high rate of B. bassiana in spring (1997) and winter wheat (1998). Fungus-treated Russian wheat aphids suffered 32-83% mycosis when field-treated tillers were incubated in the laboratory. Persistence of fungal inocula of either species, measured by estimating CFUs obtained from washed leaf segments, decreased by 10- to 1000-fold within 3 to 5 days.