Evaluation of Soil-Applied Insecticides for Control of the Wireworm Complex in Sweet Potato, 2019

The goal of this study was to evaluate soil-applied insecticides against common soil-borne insects that damage sweet potato. The specific objective was to document damage differences among insecticides applied at different times (preplant incorporated [PPI], soil barrier at lay-by, or their combination). We evaluated the probability of wireworm root damage and damage severity among insecticide treatments. This experiment was performed at two locations. The first trial was conducted at the North Carolina Department of Agriculture and Consumer Services Cunningham Research Farm near Kinston, NC (35.271778 latitude, −77.647505 longitude). The field site at Kinston was composed of Kalmia loamy sand (ca. 55%) and Portsmouth loam (ca. 45%). It was planted with corn in 2018. The second trial was conducted at the North Carolina Department of Agriculture and Consumer Services Horticultural Research Farm near Clinton, NC (35.02346 latitude, −78.276386 longitude). The field site at Clinton was composed of Orangeburg loamy sand (ca. 90%) and Norfolk loamy sand (ca. 10%). It was planted with soybean in 2018. On 11 and 21 Jun in Kinston and Clinton, respectively, 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, with 42-inch row spacing. Experimental blocks were separated by a 5-ft section of bare ground. Insecticides were applied as either PPI, as a post directed lay-by soil barrier treatment (PD) prior to vine running, or foliar application (F). The experimental design included insecticide treatments and an untreated check arranged in an RCB design, each replicated five times. On 10 and 19 Jun in Kinston and Clinton, respectively, PPI soil insecticides were applied directly to pre-formed hills using a CO₂-pressurized sprayer at a spray volume of 10.1 gpa at 30 psi through two flat-fan nozzles (XR8002VS TeeJet) positioned to broadcast insecticide across the center and sides of two hills (Tables 1 and 2 for Kinston and Clinton, respectively). Insecticides were incorporated after application using a hilling implement that included a sweep shank tillage tool to mix treated soil prior to final pre-plant hill shaping. Post directed lay-by treatments were applied and incorporated with a rolling cultivator on 2 Jul at both locations when the crop began to vine (Tables 1 and 2 for Kinston and Clinton, respectively). Foliar treatments were applied using a CO₂-pressurized sprayer that delivered a spray volume of 15 gpa at 30 psi directed through four flat-fan nozzles (XR8002VS TeeJet) positioned to broadcast insecticide across 6 ft of sweet potato canopy (Tables 1 and 2 for Kinston and Clinton, respectively). Foliar applications (Movento) were applied on 28 Jul and 11 Aug at both locations. Plants were mechanically dug with a two-row harvester on 25 and 27 Sep in Kinson and Clinton, respectively. Roots were hand harvested from ten linear feet of a single row. Roots were graded using a single lane Exiter Accuvision Vision Sorter located at the Horticultural Crops Research Station near Clinton, NC (https://www.exeterengineering.com). The sorter individually weighed and sized roots. Preliminary analysis showed that length, diameter, and weight measurements were all highly correlated. For this report, we analyzed total root weight from each plot. After sorting, a random subset of ~25 US-1 roots were sampled and visually evaluated for soil-borne insect damage (WDS: Wireworm spp./Diabrotica/Systena flea beetle). The number of damage sites per root were analyzed to determine 1) the probability of root damage (i.e., WDS holes ≥1) 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). Individual sweet potatoes were modeled as a binary outcome (damaged or undamaged) of treatment. In a separate analysis, the number of WDS sites present on each damaged root were then log-transformed and analyzed for treatment differences. Only damaged sweet potatoes (WDS holes ≥1) were included in the analysis. Experimental block was included as a random effect in both analyses. Means separations were conducted post-hoc using Tukey’s HSD.

Significant treatment effects were seen at Kinston (F_8,1112 = 4.11, P < 0.001) on the probability of damaged roots (Table 1). A significant treatment effect was also observed at the Clinton study location (F_8,1094 = 17.74, P < 0.001) (Table 2). Overall, WDS pressure was higher in Clinton than Kinston. Performance of insecticide combinations also differed between sites. In Kinston, Belay (PPI) + Capture LFR (PD) provided the greatest reduction in WDS damage probability, with Capture LFR (PPI) + Movento × 2 applications (F) providing the least reduction. Belay (PPI) + Capture LFR (PD) was also the only treatment to show a significantly lower probability of WDS damage relative to the untreated check (Table 1). In Clinton, Capture LFR (PPI) + Movento × 2 applications (F) provided the greatest reduction in WDS damage probability, with Belay (PPI) providing the least reduction. In Clinton, all treatments except Belay (PPI) showed a significantly lower probability of WDS damage relative to the untreated check (Table 2). There was no significant effect of treatment at Kinston (F_8,286 = 1.43, P = 0.1847) or Clinton (F_8,481 = 1.76, P = 0.0826) on the mean number of WDS holes (Tables 1 and 2, respectively). The results suggest that these treatments largely work by reducing the probability of WDS damage occurring, not by reducing the amount of feeding on damaged roots.¹

Footnotes