Twospotted Spider Mite Control in Soybeans, 1991

The twospotted spider mite is the principal mite species infesting soybean fields in IL. During the summer of 1991 we established a trial within a farmer's soybean field near Tolono to evaluate the efficacy of 2 rates of Comite 6.55EC and one rate of Cygon 4E against this mite. The effect of mite control on pod weight and lot yield was also assessed. The field was planted on 2 May. Individual plots were 4 rows × 100 ft long with 30-ft alleys between plots. Treatments were replicated 4 times and arranged in a randomized complete block design. Treatments were applied with 3 hollow cone nozzles (Conejet TSXS-8)/row from a 4-row sprayer delivering 30 gal/acre at 40 psi on 29 Jul when soybeans were at the R5 stage of growth. Ten plants were sampled in each plot to determine the mite densities at 3, 10, 16, and 23 days after miticide treatment (DAT). Three leaves were separated from each plant in the bottom, middle, and top strata. A 9.0-cm2 disk was cut from the center of the middle leaflet from each stratum. Each disk was examined with a dissecting microscope to determine the number of eggs, immatures, and adults. On 23 Aug, all pods on 10 plants (R6 growth stage) per plot were separated from the plants and weighted. On 11 Sep, plot yields were determined from 25-ft sections of each of the two middle rows in each plot, and yields were adjusted to 13% moisture. Mite counts were transformed by log (x + 0.5) before ANOVA.