Comparison of Soil-Applied Insecticides for Control of Silverleaf Whitefly on Watermelon, 2022

The silverleaf whitefly, Bemisia tabaci, is a serious pest of watermelon because it transmits several viruses including Squash vein yellowing virus (SqVYV), which causes watermelon vine decline, Cucurbit leaf crumple virus (CuLCrV), and Cucurbit yellow stunting disorder virus (CYSDV) which all can seriously reduce yield. The objective of this trial was to evaluate soil-applied insecticide regiment to evaluate their effects on SWF populations. Two raised beds 32 inches wide and 400 ft long on 12 ft centers covered with a white-on-black polyethylene film mulch locate at the Southwest Florida Research and Education Center in Immokalee, FL, were planted with watermelon seedlings 16 Mar at 24-inch spacing. Seedlings were obtained from a commercial greenhouse.

Four insecticide treatments and an untreated check were distributed in an RCB design experiment with four replications among plots of 16 plants each. The plots were separated by 10 feet.

Prior to planting Verimark was applied to the transplants on 14 Mar in the trays by applying 3 ml of insecticidal solution to each cell in the tray via pipette and allowed to sit overnight. Material that ran through tray was collected and then reapplied as a foliar spray via a 500-ml Dynalon HDPE spray bottle to ensure all insecticide was applied to the plants. Soil drenches as per Table 1 were made 16 Mar by delivering a 120 ml suspension to the base of each plant using an EZ-Dose sprayer operating at 45 psi (Table 1). Verimark and venom were later injected through the drip tape (Table 1) by sectioning off each treated plot with a ball valve and pressurizing the tape using a 12-volt pump operating at 0.23 gpm to inject 2 gallons of water, followed by 2 gallons of suspension and finally 2 gallons of water to clear the lines (Table 1).

Adult Whitefly were counted weekly from 1 Apr to 6 May on the underside of the leaves by carefully inverting four fully developed leaves in a row near the edges of the raised beds on five different vines in each plot. Preimaginal stages were counted under a stereoscopic microscope on 1 Apr and 13 May by collecting five leaves from each plot and randomly removing 8, 0.5 square inch circles from each leaf and counting the immatures on a total of 4 square inches per leaf. All data were subjected to ANOVA and means separated using LSD (P = 0.05) (SAS Institute).

This trial was terminated prior to harvest because of hot and wet conditions typical of southwest Florida in late May which led to the rapid decline of the plants from gummy stem disease.

All treatment programs reduced adult whitefly populations when compared to the untreated check from 15 to 29 Apr (Table 2). Reduction of adults was more variable in the early stages of the trial with Venom and Sivanto drenches showing a reduction in numbers on 1 Apr but not on 8 Apr while the Verimark Tray application had no reduction on 1 Apr but did on 8 Apr and the Verimark drench had no reduction on either of those dates. On the last sample date of 6 May the Verimark and Sivanto drench programs that were followed by Venom significantly reduced the adult population but the Tray drench or the Venom followed by Verimark did not.

Significant differences in the amount of SWF eggs seen on the leaf surface were observed from 1 Apr to 15 and on 22 Apr but no differences were observed in the latter stages of the trial (Table 3) as the insects and plants matured. Only the Venom drench followed by the Verimark and the Sivanto drench followed by Venom had significantly lower numbers on eggs on all those sample dates. The Verimark tray application showed no reduction in the number of eggs on any of the sample dates. However, all the treatment programs except for the Verimark drench on the first sample date of 8 Apr showed a significant reduction in the total number of SWF nymphs counted (Table 4). No phytotoxicity was observed.¹

Footnotes