Watermelon Whitefly Insecticide Efficacy Trial, 2015

The objective of the study was to evaluate the efficacy of various insecticides, insecticide rotations, types of application (soil injections or foliar sprays) and timing of applications for insect control and for suppression of watermelon vine decline (WVD) disease fruit symptoms caused by the SPW-transmitted Squash vein yellowing virus (SVYV) on watermelon under desert growing conditions. An insecticide trial was established at the University of California Desert Research and Extension Center, Holtville, CA using a stand of watermelon variety ‘Sugar Baby’ direct seeded into single row beds, 80 inch centers, on 25 Mar 2015. Stand establishment was achieved using drip irrigation, which was used thereafter. Plots were two beds wide (13.33 ft) by 50 ft long and bordered by one untreated bed. Included were an untreated check and nine insecticidal treatments, applied as soil injection or foliar spray, in a RCB design experiment with four replicates. All at planting soil injection insecticide treatments were applied 25 Mar using an 8001.5 nozzle, delivering 5.18 gpa @ 30 psi, concentrated into a 1 inch band applied 1.5 inches below the soil surface in the seedline. Foliar sprays were applied using a Lee Spider Spray TracTractor setup to treat two rows with three TJ-60 11003VS nozzles per row delivering 30.6 gpa @ 40 psi. The application dates for each foliar insecticide are shown in Table 1 . Whitefly adults were sampled by examining the 5 ^th leaf back from a cane tip on ten random plants per plot. Whitefly eggs and nymphs were counted using a binocular microscope in the laboratory within 1-leaf disk of 2-cm ² on the abaxial leaf surface in the lower left quadrant from five melon leaves extracted from random plants per plot. Season averages were calculated for each treatment for adults, eggs and nymphs and only the averages are shown in the table. Phytotoxicity from insecticides was evaluated using a 0 to 10 scale. Harvest of mature fruit was taken from 0.002 acre or 13.1 ft of row. The first harvest was 10 June and continued through 23 June; data from the five harvest dates are pooled. The total numbers of fruit and weight of fruit in lb were recorded for each treatment replicate. Watermelon fruit were bisected and examined for rind discoloration typical of WVD (SVYV infection verified by Rt-PCR at USDA ARS, Salinas, CA and at UC Davis, Davis, CA) and numbers of fruit showing WVD symptoms were recorded for each replicate. The percentages of fruit showing WVD symptoms were calculated. Brix were measured for each fruit using a refractometer (American Optical Corp., Buffalo, NY). Data was analyzed using ANOVA. Differences among means on each sampling date were determined using Least Significant Difference Test (LSD) ( P  <   0.05).

All of the insecticide treatments had lower SPW adult seasonal averages than the untreated check ( Table 2 ). There were no differences among the treatments for SPW egg seasonal means. The only insecticide treatment that did not have a seasonal average for SWP nymphs compared to the untreated check was Venom injected at planting and not followed with additional foliar insecticide treatments. There were no differences ( P  <   0.05) among the treatments for numbers of fruit, for weight of fruit or for Brix. The incidence of WVD was low, no differences were detected among the treatments for numbers of fruit or percentages of fruit expressing WVD symptoms, and there did not appear to be any suppression of WVD by any of the insecticide treatments ( Table 3 ). There were no symptoms of phytotoxicity following any of the insecticide applications. This research was supported by industry gifts and seed donated by Golden Valley Seed, El Centro, CA.

Author notes