Evaluation of seed spacing, insecticide choice, and application methods on systemic first-generation Colorado potato beetle control on potato in Wisconsin, 2024

This trial was conducted at the University of Wisconsin’s Hancock Agricultural Research Station (GPS: 44.11417°N, −89.53487°W) on a Plainfield sand soil in 2024. Potato, Solanum tuberosum cv. ‘Snowden’ B-size tubers were hand planted on Apr 24 with an approximate seed spacing of either 12 inches (20 per 20-ft plot row or 14,520 per acre) or 8 inches (30 per plot row or 21,780 per acre) as specified in the treatment protocol. Rows were spaced on 3-ft centers and fertilizer granules containing no insecticide were introduced at planting per standard commercial practices. Rows were closed after in-furrow applications were completed. Plots were separated by 6 ft across rows and 8 ft along rows. Plots were arranged in an RCB design with 4 replicates. Plots were maintained according to standard commercial practices including fertilizer, fungicide, and herbicide applications.

Three experimental factors consisting of 2 levels each were crossed in a full factorial design resulting in 8 treatments. Factors included seed spacing at planting (12 or 8 in), insecticide product (Platinum 75 SG or Verimark 1.67 SC), and insecticide application method (seed treatment, ‘ST’, or in-furrow, ‘IF’). Platinum (active ingredient thiamethoxam) was applied at 2.67 oz/ac for in-furrow treatments and 0.16 oz/cwt (12-inch seed spacing) or 0.11 oz/cwt (8-inch seed spacing) for seed treatments. Verimark (active ingredient cyantraniliprole) was applied at 13.5 fl oz/ac for in-furrow treatments and 0.75 fl oz/cwt (12-inch seed spacing) or 0.5 fl oz/cwt (8-inch seed spacing) for seed treatments. There was no untreated control treatment included in this experiment. Seed treatments were applied the day prior to planting by tumbling seed in a cement mixer and introducing the application slurry. Treated seed was then transferred to 50-pound crates and allowed to dry. In-furrow treatments were applied using a CO₂-pressurized backpack sprayer operating at 30 psi and delivering approximately 8.4 gpa through 1 flat-fan nozzle (Tee Jet 8001VS) while traveling at 3 mph.

Colorado potato beetle (CPB), Leptinotarsa decemlineata, populations were assessed on 10 randomly selected plants in the center of each plot for the following life stages: adults, egg masses, small larvae (1^st to 2nd instars), and large larvae (3^rd to 4th instars). CPB counts and whole-plot defoliation estimates were performed on Jun 4, Jun 16, and Jul 1 (41, 55, and 68 d after planting). For simplicity, the total counts across these three dates are presented here. A single 20-ft-long row from each plot was harvested on Sep 11 (140 d after planting) using a mechanical potato digger to determine yield and tuber quality. Tubers were graded using an AgRay Vision X-Ray grading machine which generated electronic measurements of tuber size and weight. The second-largest value of a tuber’s dimension in the X, Y, and Z directions was considered its ‘diameter’ and compared against the following USDA classification scheme: A-size tubers are larger than or equal to 1.875 inches diameter, B-size tubers fall between 1.875 and 1.5 inches, and undersize tubers fall below 1.5 inches diameter.

Evaluation data were analyzed in R version 4.4. Insect counts were log(x + 1) transformed, and defoliation percentages were arcsine square root transformed prior to statistical analysis. Treatment main effects were determined using ANOVA. Means separation letter codes were generated using Tukey’s HSD procedure (α = 0.05, R package ‘agricolae’). Effect tests for seed spacing, insecticide product, and application method on each evaluation metric were determined using linear modeling.

Generally, differences between treatments were minor and not statistically significant, but some trends emerged indicating differential performance between seed spacing, insecticide choice, and application method. There were no statistically significant differences between treatments in total CPB adults, egg masses, or large larvae, but total small larvae and final defoliation did vary between treatments (Table 1). Total small larvae were lowest with 8-inch seeding with the Verimark seed treatment, but this was only significantly lower in pairwise comparisons relative to the 12-inch seeding with the Verimark seed treatment. Defoliation ratings were generally similar across all treatments for the first 2 evaluation dates but rose significantly by Jul 1 as systemic control began failing. However, on Jul 1, defoliation was significantly lower in plots planted with 8-inch spacing, perhaps due to the higher density of plants resulting in less defoliation per plant given the same beetle populations as the plots planted with 12-inch spacing.

Despite relatively modest differences in beetle counts, the significant differences in defoliation starting Jul 1 likely contributed to substantial differences in yield between treatments (Table 2). Total yield and the total yield of A-size tubers were significantly higher with 8-inch seed spacing, Verimark, and in-furrow application. However, individual tuber weights were significantly higher with 12-inch seed spacing relative to 8-inch seed spacing in this experiment, although this difference was more than offset by the significantly higher number of harvested tubers from plots with 8-inch seed spacing (no effect of insecticide or application method on tuber count was observed). This experiment did not include any insecticides targeting late first-generation or second-generation beetles, so perhaps if the experiment were continued through the full season, the smaller tubers from plots planted with 8-inch seed spacing would bulk up to parity with the 12-inch spacing plots.

Using linear modeling, some subtle effects of spacing, product, and application method on each evaluation metric were observed (Table 3). Adult counts were lower with 12-inch seed spacing, Platinum, and seed treatment application, relative to 8-inch spacing, Verimark, and in-furrow application. Egg mass counts, small larvae, and defoliation were lower with 8-inch seed spacing, Verimark, and in-furrow application. Finally, large larvae were lower with 12-inch seed spacing, Verimark, and in-furrow application. Most of these effects were minor and not statistically significant.

Our hypothesis when designing this experiment was that denser seeding rates, increasingly common among potato growers in Wisconsin, may result in weaker systemic control of CPB. This is due to insecticide labels specifying maximum application rates on a per-acre basis, so when planted at higher densities, there will be less active ingredient applied to the field per individual plant. However, this experiment revealed no such pattern, in fact, we observed lower defoliation pressure in the higher-density plots, perhaps because similar amounts of beetles were distributed over more plants resulting in lower per-plant defoliation pressure from beetles and larvae. A question remains as to whether there are in fact lower in-plant concentrations of these systemic insecticides when planting densities are increased, and what impacts this might have on the development of insecticide resistance down the line.¹

Footnotes