Evaluation of Soil-Applied Insecticides for Control of the Wireworm Complex in Sweet Potato, 2018

The overall goal of this study was to evaluate soil-applied insecticides against common soil-borne insects that damage sweet potato. The specific objectives were to document damage differences among insecticides applied at different times (preplant incorporated, soil barrier at lay-by, or their combination). We evaluated wireworm root damage and damage severity among insecticide treatments. This experiment was performed at North Carolina Department of Agriculture and Consumer Services Cunningham Research Farm near Kinston, NC in 2018 (GPS Coordinates: 39.298308, −77.572707). To increase wireworm pressure, the experiment was planted in a wireworm nursery where corn was grown in the previous season. On 14 Jun, sweet potato slips, cv. Covington, were planted at a density of 1 slip per ft using a tractor-mounted transplanter. Plots consisted of four rows each 30-ft long and rows were spaced 42 inches apart. A 5-ft section of bare ground separated experimental blocks. Insecticides were applied either preplant incorporated (PPI) or as a lay-by soil barrier treatment [post directed (PD)] prior to vine running. The experimental design included insecticide treatments and an untreated check arranged in a randomized complete block design replicated four times.

On 11 Jun, PPI soil insecticides were applied directly to preformed hills using a CO₂-pressurized sprayer that delivered a spray volume of 10.1 gal per acre at 30 psi directed through two flat-fan nozzles (XR8002VS TeeJet) positioned to broadcast insecticide across the center and sides of two hills. Insecticides were immediately incorporated after application using a hilling implement that included a sweep shank tillage tool to mix treated soil prior to final hill shaping. PD lay-by treatments were applied and incorporated with a rolling cultivator 2 Jul when the crop began to vine. All best management practices were used for sweet potato fertility and weed management. Plots were not fumigated prior to planting; however, nematode and disease pressure was very low across the experiment.

Plants were mechanically dug with a two-row harvester on 5 Oct. All roots were hand harvested from ten linear feet (two row digger = 20 total feet of row sampled). Roots were weighed (kg) by grade: Jumbos, US-1, US-2, and culls (data not presented). After weighing, a random subset of 50 US-1 roots were separated, washed, and evaluated for soil-borne insect damage (WDS: Wireworm spp./Diabrotica/Systena Flea Beetle). The number of damage sites per root was counted and analyzed to determine 1) the probability of root damage and 2) the number of WDS feeding sites per damaged root.

Data were analyzed using logistic regression with PROC GLIMMIX in the SAS System, version 9.4 (SAS Institute, Cary, NC). Because some treatments included both a PPI and PD treatment, we compared these treatment combinations as separate main effects of treatment timing during the growing season. Individual sweet potatoes were modeled as a binary outcome (damaged or undamaged) of PPI, PD, and the PPI x PD interaction (Table 1). Experimental block was included as a random effect and means separations were conducted post hoc using Tukey’s HSD.

The percent of damaged sweet potatoes in Table 1 is based on the outcome of the effects of the PPI treatments only (F_3,18 = 14.92, P < 0.01). There was no significant difference in the percent of damaged sweet potatoes based on the PD treatment (F_1,18 = 0.27, P = 0.61) and there were no significant differences in the PPI x PD interaction (F_1,18 = 0.05, P = 0.82). This result suggests that the probability of detectable root damage in this study is associated with use of a PPI.

The average number of WDS sites present on each damaged root was then analyzed with a model containing main effects of PPI and PD, along with the PPI x PD interaction (Table 1). Experimental block was included as a random effect. Means separations were conducted post hoc using Tukey’s HSD. Only damaged sweet potatoes (WDS holes ≥ 1) were included in the analysis. There was no significant treatment effect of PPI (F_3,363 = 1.13, P = 0.34), PD (F_1,363 = 2.46, P = 0.12), or the PPI x PD interaction (F_1,363 = 0.64, P = 0.43). This result shows that the average number of feeding sites was not statistically different when a sweet potato root was damaged.

This research was supported in part by funding from industry.