Evaluation of an Omri-Approved Spinosad Insecticide for Control of Cabbage Root Maggot in Direct-Seeded Root Crops, 2016*

Feeding by cabbage root maggot (CRM) larvae causes stunting and collapse of plants, and tunneling damage on root crops renders them unsaleable. We evaluated the OMRI-approved spinosad product, Entrust SC, as an in-furrow drench treatment at the time of seedling emergence alone or with a follow-up application one week following. These treatments were compared to an untreated check and to a furrow drench with the industry standard, chlorpyrifos, at the time of seeding. The experiment was conducted May–Jun 2016 at the University of Massachusetts Research and Education Farm in Deerfield, MA in a field with soil classified as Hadley silt loam. Soil was amended with 50 lb/acre urea (46-0-0) (N-P-K) and 20 tons/acre manure-based compost to increase attractiveness of the site to CRM flies for oviposition. Soil was harrowed twice and a roller was used to make a fine seedbed. An RCB design with four replications of each of the three treatments and an untreated check was used, with plots measuring 10 ft of bed length with three rows per bed. Weeds were controlled by hand and, due to very high pressure, flea beetles were controlled using a single application of Wrangler at 1.5 fl oz/acre on 23 Jun. ‘Hakurei’ turnips (Johnny’s Selected Seeds, ME) were direct-seeded on 10 May using a Jang TP1 precision seeder equipped with a Y-12 seed roller and a 9/14 gear setting to achieve 0.5-inch in-row and 12-inch between-row spacing. A border of untreated turnip plants was seeded around the perimeter of the experiment and between plots within the bed, to attract CRM flies. At-plant treatments were applied on 11 May using a CO₂ pressurized backpack sprayer delivering 200 gpa at 30 psi through one TeeJet TP8002E nozzle to soak the soil in the seeding band, simulating an in-furrow application. The Entrust treatments were applied on 27 May and 8 Jun in the same manner, over the sprouted turnip plants. The total number of plants per plot was recorded weekly, on 26 May, and 3, 8, 15, and 22 Jun, and provided a measure of plant stand. Marketable yield was assessed on 15 and 26 Jun by harvesting 10 plants per plot at random and making visual estimations of root damage using the following criteria: a turnip root with >25% of the surface covered with shallow tunnels or with more than one deep tunnel or other deformation was considered unmarketable. All data were analyzed with SAS 9.4 using PROC GLM and means were separated using Fisher’s LSD (α = 0.05).