Performance of Seed Treatments and In-Furrow At-Plant Insecticides for Protection Against Cry3bb1-Resistant Western Corn Rootworm, 2016 Open Access

The western corn rootworm is an important pest of corn that can compromise yield by feeding on plant roots during its larval stage. WCRW management has been complicated by the development of resistance in some regions, including Nebraska, to transgenic Bacillus thuringiensis (Bt) traits, particularly the protein Cry3Bb1, which confers cross-resistance to mCry3A. A field trial was established to evaluate the efficacy of neonicotinoid seed treatments in combination with in-furrow insecticides on a corn hybrid expressing Cry3Bb1 and Cry34/35Ab1 Bt proteins against corn rootworm in an area with a history of rootworm resistance to CryBb1. The trial was conducted in a farmer’s field in Keith County near Ogallala, NE (41.117228° N, −101.652686° W). Damage from WCRW to corn expressing Cry3Bb1 proteins was documented in the field during the previous seasons. An RCB design with four replications and 10 treatments (including an untreated check) was used. Each plot was six rows by 35 ft. The trial was planted on 6 May 2016 using a small plot research planter at 32,000 seeds/acre at an approximate depth of 1.4–1.75 inches in 30-inch rows. The hybrid planted was Munson 6642SS (Munson Hybrids, Galesburg, IL) with the SmartStax RIB Bt trait package, which expresses Cry3Bb1 and Cry34/35Ab1 along with other Bt toxins that target lepidopteran pests. All seeds, including the untreated checks (UTCs), were treated with Maxim Quattro fungicide at 0.064 mg AI/seed. The tested insecticides were applied in-furrow, with calculations based on an application volume of 5 gal/acre. The at-plant insecticide treatments were applied on 6 May 2016 at rates described in Table 1. Soil type at the experimental site was Lex loam with low water percolation capacity. The plots received irrigation, fertilization, and weed management inputs identical to the commercial field surrounding the plots following standard agronomic practices for the region. An aerial application of 2.6 fl oz/acre Mustang Maxx (zeta-cypermethrin) and 1 pt/acre dimethoate insecticide targeting western bean cutworm (Striacosta albicosta Smith) was made on 26 Jul 2016. On 25 Jul 2016, eight plants from the central two rows and interior 22 ft of each plot were randomly chosen and removed along with roots. The roots were washed and rated for damage using the Iowa State 0 to 3 Node Injury Scale, where a rating of 1 would indicate one node of root injury due to rootworm feeding. The data were analyzed using PROC MIXED with mean separation by least square means (P = 0.05) in SAS version 9.4.

Low-to-moderate WCRW damage was indicated by the UTC, where node-injury ratings ranged from 0.00 to 3.00 with a mean of 0.49. All seed treatments, with or without in-furrow insecticide treatment, provided statistically significant root protection when compared with the UTC but did not differ from each other (Table 1). These data show that when an effective Bt protein (Cry34/35Ab1) is expressed in the corn plant, seed treatments alone (Poncho 600 0.50 mg AI/seed or Lumivia + Cruiser 5FS 0.25 + 0.25 mg AI/seed) provide satisfactory root protection against Cry3Bb1-resistant WCRW. The application of in-furrow insecticides provided no additional benefit to seed treatments, and therefore, their use should be limited (when an effective Bt protein is present) in the absence of other soil insect pests.

This research was supported by an industry gift of products, seed, and research funding.