Estimation of Population Density of the Russian Wheat Aphid (Homoptera: Aphididae) from the Proportion of Grain Tillers with Different Tally Thresholds of Aphids
Sampling summer and fall populations of the Russian wheat aphid, Diuraphis Noxia (Mordvilko), on winter wheat, spring wheat, and barley in the northwestern United States (Idaho, Montana, Oregon, and Washington) from 1988 to 1991 resulted in 120 estimates of mean aphid density (m aphids per tiller) and the proportion of tillers with ≤T aphids (PT), where T is defined as the tally threshold. T was set to 0, 5, 10, 15, 20, 25, 30, and 40 for the summer populations and 0, 1... 5,8, and 10 for the fall populations. For each T, an empirically determined functional relationship between m and PT was developed using parameters estimated from the linear regression of In(m) on In[-In(PT)]. Variability associated with the m and PT relationship was found to differ greatly among the T values concerned, generally decreasing with increasing T, for the summer populations. Small T values, particularly empty tillers (T = 0), may lead to spurious estimates of m from PT. The tally threshold of 20 or 25 aphids per tiller is suggested for use in monitoring summer populations of D. noxia to reduce the variability and achieve an acceptable error level. The variability was low for fall populations on winter wheat despite a variable range associated with the values of T, but was minimized when T = 2, 3, or 4. Using these T values, half-widths of the 95% CIs are usually m] of ≈0.25). These T values are also recommended for use in estimating the aphid populations infesting spring grains during the late spring or early summer.