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 noxia (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 spring. In 1990-1991, aphids of one colony were released at three different infestation levels on each of four release dates during the growing season. Populations of all aphid species and number of mummified aphids were monitored in 1990-1991. In 1988-1989, the aphid colonies did not differ in population numbers or in their effect on yield, but fall infestations resulted in higher Russian wheat aphid numbers and greater yield loss than spring infestations. In 1990-1991, an early fall release was the only release that developed high numbers and caused yield loss. Infestations resulting from releases made in early and late winter 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 1990-1991 than in 1988-1989, but peaks in the populations during both years occurred in mid- to late April. Densities of greenbug, Schizaphis graminum (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 plants are discussed as possible factors that limit Russian wheat aphid infestations in the Texas Rolling Plains.