Evaluation of Seed Treatment and Soil-Applied Insecticides Against Larval Western and Northern Corn Rootworm, 2020

The effectiveness of various seed treatments and soil-applied insecticides (granular and liquid formulations) at reducing root injury from WCR and NCR larval feeding under continuous corn production was assessed at the Eastern Nebraska Research and Extension Center (UNL-ENREC) near Ithaca, Nebraska. Local Seed Co. hybrid ‘LC0488VT2P’ was utilized in this trial and did not express rootworm-active Bacillus thuringiensis (Bt) traits. Individual plots were 4 rows × 30 ft in length with 30-inch row spacing, and planted on 27 Apr 2020 at a seeding rate ca. 32,000 seeds/acre. Eight treatments were compared to an untreated check in a randomized complete block design with four replications per treatment. Insecticide compounds, formulations, and application rates are available in Table 1. Granular insecticides were applied into the open seed furrow at plating using a SmartBox system. Liquid insecticides were applied in water with a finished volume of 5 gpa using a TeeJet® flow regulator (orifice plate, CP4916-28) with a compressed air system pressurized at 20 psi. The WCR was the predominant corn rootworm species at the trial site (>95% WCR). Initial WCR egg hatch was detected on 8 Jun and initial WCR emergence was observed on 7 Jul. Initial plant stands were recorded on 19 May (V1-V2 stages) and on 1 Jun (V3-V4 stage) by counting the total number of plants within 17.5 row-ft in each plot. Final plant stands and plant lodging were recorded prior to harvest on 23 Sep by counting the total number of plants and the number of plants leaning at ≥ 45° angle from vertical in the center 28 ft of the two middle rows per plot, respectively. Larval root injury was assessed on 23 Jul by excavating five plants from the middle two rows of each plot and rating them using the 0–3 node injury scale (NIS; 0 = no feeding, 1 = one node of roots pruned to within 1.5 inches of the stalk, 2 = two nodes of roots pruned to within 1.5 inches of the stalk, and 3 = 3 or more nodes of roots pruned to within 1.5 inches of the stalk). The center 28 ft from the middle two rows of each plot was mechanically harvested on 7 Oct with a Kincaid 2-row 8-XP combine. Data were analyzed by PROC GLIMMIX in SAS (version 9.4) with data fit to a negative binomial (stand count), beta (NIS, proportion lodged plants), or normal (yield) distribution. Yield per plot was converted to 15.5% moisture prior to analysis. The LSMEANS option was used to determine differences among treatments using Tukey’s adjustment for multiple comparisons (P ≤ 0.05).

Plant populations were similar among treatments throughout the duration of the trial. Plant stand means were not significantly different among treatments on 19 May, 1 Jun, or 23 Sep 2019 (Table 1). Relatively high larval corn rootworm pressure (untreated NIS mean = 2.01) resulted in significant differences in root injury, lodging, and yield between the untreated check and one or more insecticidal treatments. Force 2.1CS and Aztec 4.67G provided the greatest root protection. Lodging recorded from Capture LFR 1.5SC, Force 2.1CS, and Aztec 4.67G treatments was minimal and was significantly lower than lodging in other treatments. In general, the level of lodging increased with increased root injury. However, a similar relationship between root injury and yield was not apparent. Yield was similar among many insecticide treatments with Capture LFR 1.5SC, Force 2.1CS, and two Ampex 1.73SC treatments significantly greater than the untreated check.¹

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