ECONOMIC INJURY LEVEL FOR THE RUSSIAN WHEAT APHID (HOMOPTERA, APHIDIDAE) ON DRYLAND WINTER-WHEAT

This research was designed to determine the economic injury level for Russian wheat aphid, Diuraphis noxia (Mordvilko), on dryland winter wheat, Triticum aestivum L. Two sample units were evaluated: whole plants and individual tillers. Russian wheat aphid densities were low in the fall but increased rapidly from late February to May. The percentage of the plants infested or damaged by Russian wheat aphid feeding exceeded 90% by March 1987. The same proportion of infested or damaged tillers was not recorded in 1988 until late April. There were less-than-or-equal-to 80% and 60% yield reductions from Russian wheat aphid feeding in 1987 and 1988, respectively. Although 27% of the plants were damaged in the fall, there were too few aphids (less-than-or-equal-to 2 aphids per plant, 32 aphid-days, by December) to cause a yield loss. Regression analysis was used to compare several independent variables (aphid densities, aphid-days, percentage infested, percentage damaged, and percentage infested + damaged) with yield, number of seeds per spike, tiller length, number of tillers, and plant dry weight for plant and tiller samples. Yield was the only dependent variable that correlated with the independent variables. The best sampling unit was tillers, based on a high R2, low root mean square error, and ease of sampling. There were 0.46 and 0.48% yield losses for each 1% increase in damaged and infested tillers, respectively.