Evaluation of foliar pesticides for the control of spider mites in field corn, 2023

Spider mites are an important pest of corn in the Midwestern United States. Two distinct species typically infest corn in Nebraska: the Banks grass mite and the two-spotted spider mite. Species identification is important because Banks grass mites tend to appear earlier in the season, feed mostly on the lower leaves of corn, and are still moderately susceptible to many commonly used miticides. In contrast, the two-spotted spider mite appears mid to late season, feeds over the entire plant, and is not consistently controlled by available pesticides. While it is not uncommon to find a mixture of the two spider mite species in field corn in west central Nebraska, for this study, the population was identified as entirely Banks grass mite.

This field trial was established to evaluate the efficacy of foliar insecticides and miticides on populations of spider mites in field corn. The trial was conducted in Perkins County (40.852119° N, −101.705801° W) near Grant, Nebraska, United States. A randomized complete block design with 7 treatments (including an untreated check) and 4 replications was used. Mite pressure was limited to the edge of the field, so plots were arranged along the edge of the field to maximize pest pressure. Each plot was 8 rows (20 ft) wide by 35 ft long. The trial was planted on 1 May 2023 at 29,500 seeds per acre, and the hybrid planted was Pioneer P1366Q. The treatments were applied on 11 Aug when corn was at reproductive stage R1-R2, between 9:00 and 11:00 am. The temperature at application was approximately 66‒71°F, with wind speed of 1‒5 mph. All treatments were applied with a backpack sprayer with TeeJet AIXR 11002 Yellow nozzles at a rate of 15 gpa and pressure of 40 psi at a walking rate of 3 mph. Assessments of spider mite populations were made 1-day pre-treatment (PRE) and 7, 15, and 21 days after treatment (DAT) by collecting 10 leaf samples just below ear height from each plot and counting the number of spider mite eggs and active stages. Data were analyzed using a generalized linear-mixed model with negative binomial distribution and means comparisons were completed by least squares means with Tukey adjustment in SAS 9.4.

There was a significant effect of treatment at the PRE sampling date (P = 0.0422); however, means comparisons showed no significant differences (Table 1). Surprisingly, there was not a significant effect of treatment at 7 DAT (P = 0.6593), with the populations failing to increase in the untreated check. Populations stayed very low in all treatments across sampling dates, other than an increase in the untreated check by 21 DAT. Statistical models did not converge for the 15 DAT and 21 DAT sampling dates; therefore, means are presented without analysis (Table 1).¹

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