Miticide Efficacy Against Twospotted Spider Mite in Staked Tomato, 2019

The objective of this study was to compare the efficacy of various acaricides for control of twospotted spider mite (TSSM) and compatibility with the predatory mite Phytoseilus persimilis. The trial was conducted from 11 Jul through 5 Sep 2019 at the North Carolina State University Mountain Horticultural Crops Research Station located in Mills River, NC. Tomato ‘Plum Regal’ transplants were set into black plastic mulched beds on 11 June, staked and strung as needed, and sprayed with a standard fungicide program. The experiment consisted of 8 rows, 100 ft long, with 10 ft between rows. Plots consisted of a 20-ft-long section of tomato plants, spaced 1.5 ft apart within rows, replicated four times, and arranged in an RCB design. Treatments on the same row were separated by 2–3 untreated tomato plants. On 11 Jul, plots were artificially infested with TSSM by placing TSSM infested leaves in each plot at the rate of one leaflet on every other plant. P. persimilis were never detected in the trial. The evaluated miticides are listed in Table 1. Each treatment was sprayed with a CO₂-powered backpack sprayer delivering 62 gallons per acre through four hollow cone nozzles per row at 40 psi—each treatment was sprayed with a two-nozzle wand on each side of the row. All treatments were applied on 16 August, with the exception of the Grandevo treatment that received an additional application on 8 August.

Counts of TSSM were recorded weekly from 11 Jul through 5 Sep. Mites were sampled by visually inspecting 10 terminal leaflets (from the first or second most recently fully expanded leaf) per plot and counting the total number of adult/motile mites. Season total cumulative mite days (CMD) were calculated by multiplying the average density on successive sample dates by the sampling interval (days) and adding all mite days over all sample dates. Number of mites and season CMD were log₁₀(x+1)-transformed and analyzed with ANOVA and means separations were analyzed using LSD (P = 0.05).

At the time of application of miticides on 16 Aug, mite populations averaged 3.0 ± 0.5 (SEM) mites per leaflet across all plots. TSSM populations were relatively slow to develop, reaching an average density of 0.7 ± 0.1 mites per leaflet at the time of the first application of Grandevo WDG on 9 Aug (Table 1). Within 5 days of the 16 Aug application, the most effective control was achieved with Agri-Mek, Acramite, and Nealta; neither Spear-T nor any of the Grandevo formulations were significantly reduced below the untreated check. Acramite continued to suppress mite populations below the check for 13 days (29 Aug), and Agri-Mek for 10 days (26 Aug). By 12 Sep all treatments exceeded 10 mites/leaflet and did not differ significantly from the check. While seasonal CMD did not significantly differ among treatments, values were lowest in Acramite, Agri-Mek, and Nealta.¹

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