No. 17 Article 4/July 15, 2005

Preliminary Results from a Spider Mite Control Experiment

The recent, long-hoped-for rains may reduce the impact that twospotted spider mites could have on soybean yields in some areas. Let's hope so. Nonetheless, we have collected some preliminary information from one research trial that may help those who missed the rain or may help growers in the future.

On June 24, Ron Estes (coordinator for the Insect Management Program in the Department of Crop Sciences) and his squad established an insecticide efficacy trial near Tolono, about 10 miles south of Champaign. The field had been planted no-till into corn stalks. Infestations of twospotted spider mites had developed in patches within the field, and the patches were closely associated with areas in which henbit had been killed by a spring application of Roundup. The spider mites apparently had established on the henbit and then moved to soybean plants when the henbit began to die.

The design of the plot was a split plot with four replications. The main plots were insecticide treatments (Dimethoate and Lorsban); the subplots were different application scenarios (no application [untreated check], early application [June 24], late application [July 7], and two applications [June 24 and July 7]). Numbers of spider mites were counted 6 and 13 DAT (days after treatment). The samples 13 DAT (July 7) were taken before the late application of either Dimethoate or Lorsban. Therefore, the plots that were to receive the late application of either product were essentially untreated checks on July 7 before the samples were taken. This same logic means that the insecticide treatments to be applied both early and late had only been applied early. So I have lumped the data to represent the effects of applications of Dimethoate and Lorsban on June 24.

Table 2 provides preliminary information from the plots, as of July 7. It is important to note that because of the spotty nature of the infested patches in the trial area, counts of mites varied considerably. Also, because we did not have the right sampling equipment before the plot was sprayed, the pretreatment counts are suspect. (They are not provided here.)

Both Dimethoate and Lorsban provided noticeable control of twospotted spider mites in the trial. Although the numbers of mites increased somewhat from 6 to 13 DAT in the plots treated with Lorsban (+8%) and Dimethoate (+35%), the percentage increases in the miticide-treated plots were quite a bit smaller than the percentage increases in the untreated plots (+156% and +89%). Although spider mites were not eliminated in the treated plots, their rate of increase was slowed significantly during the 2-week period from application to the second sampling.

The second applications for the designated plots were applied on July 7, and the next set of samples will be taken on July 14, 20 days after the first application on June 24. The rains began in Champaign County on July 12. The data from this week's sampling may tell us a little about the effects of rainfall on spider mite populations within the plot area. Also, yields will be estimated from these plots later this season.

Continue scouting for twospotted spider mites in soybeans, despite the welcome rainfall. The soybean plants should receive a noticeable boost from the rain, and the effects of spider mite feeding should be blunted. But a return to hotter, drier conditions will enable the existing spider mite population to increase in size again unless a fungal organism establishes itself in the mite population and begins an epizootic. We'll keep you informed of our findings.--Kevin Steffey

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