FACTORS REDUCING THE PEST STATUS OF THE RUSSIAN WHEAT APHID (HOMOPTERA, APHIDIDAE) ON WHEAT IN THE ROLLING PLAINS OF TEXAS

Effects of field infestations of the Russian wheat aphid, Diuraphis no xia (Mordvilko), on yield of winter wheat in the Texas Rolling Plains were studied. In 1988-1989, colonies established from three different population isolates were released into small plots in the fall and spr ing. In 1990-1991, aphids of one colony were released at three differe nt infestation levels on each of four release dates during the growing season. Populations of all aphid species and number of mummified aphi ds were monitored in 1990-1991. In 1988-1989, the aphid colonies did n ot differ in population numbers or in their effect on yield, but fall infestations resulted in higher Russian wheat aphid numbers and greate r yield loss than spring infestations. In 1990-1991, an early fall rel ease was the only release that developed high numbers and caused yield loss. Infestations resulting from releases made in early and late win ter and early spring did not affect yield. Initial infestation levels of 330, 660, and 1,000 aphids per 12-m2 plot had no significant effect on Russian wheat aphid numbers during the late part of the season or on yield. Higher numbers of Russian wheat aphids developed in 1990-199 1 than in 1988-1989, but peaks in the populations during both years oc curred in mid- to late April. Densities of greenbug, Schizaphis gramin um (Rondani), and bird cherry-oat aphid, Rhopalosiphum padi (L.), and aphid mummies were positively correlated with high Russian wheat aphid densities. Unsuitable wind direction and lack of alternate host plant s are discussed as possible factors that limit Russian wheat aphid inf estations in the Texas Rolling Plains.