Efficacy of in-furrow insecticides for corn rootworm larvae near Colman, South Dakota, 2024

A corn rootworm (NCR/WCR) management efficacy trial was established on 13 May 2024 at a site near Colman, South Dakota. Experimental plots were arranged in an RCBD with 4 replications. Each plot was 4 rows wide and 48 feet long with 30-inch row spacing. A non-rootworm traited seed corn, Dekalb hybrid (DKC45-35RIB VT2P), was planted using an SRES Step 4 Planter with a seeding rate of 30,800 kernels per acre. The Aztec HC 2.1G and Force 6.5G insecticides were applied at planting using the SmartBox metering system. The SmartBox insecticide application meter was adapted to fit the Noble meter mounting brackets on the planter. At planting, in-furrow liquid insecticides were applied at 5 gallons per acre using a compressed air R&D sprayers system with solid stream spray tip spraying into the open seed furrow after seed drop. Plant stand count (3 Jun) and lodging (15 Aug) data were collected from the center 2 rows of each plot. Stand was determined by counting the number of plants in 17.5 feet of row (1/1,000th of an acre), and percent lodging was calculated as the total number of plants in the rows that were at a 30-degree or greater angle then divided by total number of plants. Root rating means were derived from 6 roots dug randomly from the 2 outer rows at the VT growth stage on 22 Jul. Roots were washed then rated for severity of rootworm feeding injury using the Oleson 0–3 root injury scale. Yield assessments in bushels per acre were taken from the center 2 rows of the 4 row plots on 16 Oct. Plant stand, root rating, lodged plants, and yield were analyzed using analysis of variance means separation according to Duncan’s new multiple range test in the ARM research management software.

Corn root injury in the untreated control was significantly worse than the insecticide treatments and exceeded the 0.25 economic injury level (EIL) by 2.2-fold (Table 1). The insecticidal treatments all provided adequate root protection at this field site with 2.3- to 3.1-fold below the 0.25 EIL (Table 1). The percentage of corn plants lodged at 30% or greater was very low at this site due to the low root node injury score (0.55). This was influenced by excessive moisture during the larval hatch which contributed to decreased larvae survival. Yield in insecticide treatments range from 0.1 to 20.7 bu/A greater than the untreated plot. However, due to the variability of harvested grain in bu/A by replication, yield was not significantly greater than the untreated (Table 1).¹

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