Evaluation of selected acaricides against twospotted spider mite (Acari: Tetranychidae) on greenhouse cotton using multispectral data.

Twospotted spider mite (TSSM), Tetranychus urticae Koch, is an early season pest of cotton in the mid-southern USA and causes reduction in yield, fiber quality and impaired seed germination. Objectives of this study were to investigate the efficacy of abamectin and spiromesifen with two divergent LC50 values against TSSM in a computer-operated spray table which simulated aerial application parameters. Combined with a pressure of 276 kPa and a speed of 8 km/h, a 650033 nozzle delivered a spray rate of 18.7 L/ha. The active ingredient rates were 1/8, 1/4, 1/2 and the lowest label recommended rates for early season cotton. The intent was to study efficacy relative to deposition characteristics at active ingredient rates equal to and lower than those recommended by the label. Spectral reflectance values from a multispectral optical sensor were used to calculate the Normalized Difference Vegetation Index which numerically described the surface reflectance characteristics of cotton canopies concomitant to damage caused by T. urticae in the greenhouse. Water sensitive paper samplers described spray droplet spectra parameters (Dv0.1, Dv0.5 and Dv0.9, µm) and percent spray coverage. The volume median diameter (Dv0.5, µm) for abamectin and spiromesifen were respectively, 218 and 258 at one-half rate of the lowest label rate. These spray droplets were well above the driftable portions of the spray volume (<141 µm) for both abamectin and spiromesifen. Efficacy evaluations indicated that spiromesifen was more effective than abamectin in controlling T. urticae on early season cotton at one-half rate of the lowest label rate. Results reported herein demonstrate that the multispectral optical sensor in lieu of manually counting T. urticae appears to be a promising tool for efficacy evaluations against acaricides for early season plants grown in greenhouses.