Efficacy of Miticides Applied at Tassel Stage for the Control of Efficacy of Miticides Applied at Tassel Stage for the Control of Spider Mites in Corn, 2020 Spider Mites in Corn, 2020

Spider mite populations peaked during the week of August 19 and declined rapidly by August 26. Mite populations were made up entirely of two-spotted spider mites ( Tetranychus urticae ). The Portal with Exponent, Oberon, and Zeal treatments provided the best season-long control in this trial while Portal, Onager


Western Kansas Agricultural Research
Efficacy of Miticides Applied at Tassel Stage for the Control of Spider Mites in Corn, 2020 A. Zukoff Summary Spider mite populations peaked during the week of August 19 and declined rapidly by August 26. Mite populations were made up entirely of two-spotted spider mites (Tetranychus urticae). The Portal with Exponent, Oberon, and Zeal treatments provided the best season-long control in this trial while Portal, Onager, and Mitomax II provided the most control 14 to 21 days after treatment. The lowest overall reduction in predatory mites during the season, when compared to control plots, occurred in the Portal and Onager treatments.

Experimental Procedures
A center pivot irrigated field near Garden City, KS, in Finney County was planted to Mycogen MY10Z28R8 corn hybrid on May 29, 2020, at a rate of 35,000 seeds per acre. Agronomic practices for irrigation, fertilizer, and herbicide were standard for corn production in the southwest Kansas region. Average maximum air temperature between onset of tasseling and the end of the trial was 89.8°F and 6.99 inches of precipitation was recorded. The trial was arranged in a randomized complete block design with 8 treatments (includes untreated check) replicated four times. Plots were 4 rows wide (30-in. center) by 30-ft long. Replications were separated by 4 rows of buffer corn while individual plots within replications were separated by 10-ft alleyways. All treatments were applied on August 5 at 15.0 gpa with an air pressurized (30 psi) 10-footwide high-clearance spray boom. All applications were mixed with a .25% v/v rate of non-ionic surfactant (except for the Mitomax II treatment). The boom was equipped with 8 (XR TeeJet 8002VS) nozzles on 18-inch centers. At application the temperature was 78°F (64% RH) with a wind speed of 10 mph from the south.
Samples were taken 1 day before the miticide applications and at 7, 14, and 21 days thereafter. On each sample date, half the leaves were taken from each of two plants randomly selected from the middle two rows of the plot. Leaves were bagged within the plot and transferred to the laboratory and placed in 5-gallon Berlese funnels equipped with 60-watt incandescent light bulbs for 3 days. Mites exiting the leaves fell into 50-mL centrifuge tubes filled with 70% methanol. Mites were separated from the methanol using a vacuum aspirator and counted under a stereoscope. Insect data were log transformed (Log (x + 1.0)) and analyzed using PROCMIX (SAS 9.4) and then back-transformed to mites per plant for presentation. All "percent reduction" calcula-tions were made using Henderson-Tilton's formula. Grain yield data were not collected because mite infestation never reached economic levels.

Results and Discussion
In untreated plots, two-spotted spider mite populations peaked at 24.8 mites per plant on August 19 and rapidly fell to 1.8 per plant by August 26 (Table 1). On September 2, active mite colonies were no longer present and evidence of the entomopathogenic fungus Neozygites sp. was widespread throughout the plots. Natural enemy populations were present during each week of sampling before and after treatments and predatory mites (Neoseiulus sp.) were abundant (Table 2). Overall, spider mite pressure during this trial was low, but the number of mites per plant prior to treatments were not significantly different (Table 1).
Portal with Exponent, Zeal, and Oberon all provided the highest season long control with up to 98%, 100%, and 97%, respectively, and all three provided a significant reduction of spider mites 7 DAT (Tables 1 and 3). Oberon's control began to wane after two weeks, while Portal with Exponent along with Zeal held up for 3 weeks (Tables 1  and 3). Onager provided limited control 7 days after treatment (DAT) but increased significantly to 70% 14 DAT and offered no additional control 21 DAT (Table 3). Significant control with Mitomax II was not achieved until 14 DAT (Table 3). Control by Portal alone was modest 7 DAT but steadily increased to a peak of 59% 21 DAT (Table 3). Initial control by Portal with Zeal was better (51%) but declined steadily and offered no additional control 21 DAT (Table 3). Predatory mites were present in all plots both before and after treatment and their population peaked and declined in step with the spider mite population (Table 2). A significant reduction in predatory mites 7 DAT when compared to untreated plots occurred in the Portal with Exponent (60%), Portal with Zeal (77%), Zeal (74%) and Oberon (75%) treatments (Table 4). Predatory mite reductions steadily continued through 21 DAT in the Portal alone, Onager, and Mitomax II treatments (Table 4). Average overall reduction in predatory mites through the season was lowest in the Portal alone and Onager treatments (21% and 26%, respectively) while the highest overall reduction in predatory mites through the season occurred in the treatments containing Zeal, up to 68% (Table 4).

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Persons using such products assume responsibility for their use in accordance with current label directions of the manufacturer.   Highlighted cells are the mite counts from untreated plots used in calculations. DPT = days pretreatment. DAT = days after treatment. UTC = Untreated control.