Efficacy of Thiamethoxam Seed Treatments to Control Cabbage Root Maggot in Broccoli, 2014 *

Because the feeding by cabbage root maggot (CRM) larvae causes stunting and collapse of cultivated brassicas, stands of spring transplants may be reduced and damage to root crops may occur in the spring and fall. Seed treatments offer a simpler way for growers to apply insecticides compared to soil drenches which require much time and water to apply. We evaluated efficacy of different formulations and rates of thiamethoxam seed treatments compared to imidacloprid applied as a soil drench banded over the row post-transplant. The experiment was conducted 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 urea (46-0-0) (N-P-K) and was further amended with 20 tons/acre of a manure-based compost to increase attractiveness of the site to CRM flies for oviposition. A RCB design was used with 4 replications of 5 insecticide treatments and an untreated check, with 60 broccoli plants per replicate plot. ‘Preakness’ broccoli seeds (Syngenta, Greensboro, NC) arrived pre-treated with FarMore F300 fungicide blend plus the insecticide except for the untreated check which was treated with FarMore300 only. Seeds were sown in 128-cell flats on 18 Apr and were maintained under greenhouse conditions for four wks. Seedlings were fertilized with 200ppm 20-10-20 (N-P-K) (Peter’s Professional Peat-Lite Special) on 30 Apr and 7 May. Seedlings were transplanted by hand on 12 May into beds 6 ft apart with 2 rows set at 18 inches apart and with 12 inch in-row spacing. On 13 May, the imidacloprid was applied using a CO ₂ pressurized backpack sprayer delivering 200 gpa at 30 psi through one TeeJet TP8006 flat fan nozzle to drench a 6 inch band of soil. Change in plant vigor caused by damage from flea beetle (FB) and CRM was rated weekly, on a whole-plot basis by visually estimating percent vigor (0–100%). This visual estimate took into account measures of vigor such as the number of plants killed by CRM, plant stunting, wilting, discoloration, and feeding damage caused by FB, and provided a relative comparison between treatment plots and a theoretical, healthy plot, with no reductions in vigor and no missing, killed plants. Additionally, 10 plants per plot were chosen randomly and were rated individually for percent vigor (0–100%) using the same indices, where: 0% = a dead plant; 50% = a plant that is half as vigorous (based on size, color, leaf area free of FB damage, etc.) as a healthy plant; and 100% = a plant that is healthy, large, and has no FB damage. These 10 plants were then destructively sampled and the number of CRM larvae present in root and stem tissue was recorded. Root damage was rated using the following ordered categorical scale: 1 = strong root development and no damage to main or secondary roots; 2 = slight damage but secondary roots still present; 3 = moderate root damage, i.e. not many secondary roots and secondary roots discolored; 4 = severe damage, i.e. almost no roots present; and 5 = plant completely dead. These ratings were performed on 22 May (10 days after transplant (DAT)) 30 May (18 DAT), and 6 Jun (25 DAT). Leaf damage caused by FB was analyzed independently on 6 Jun by visually estimating the percentage of leaf area with feeding damage. All data were analyzed using PROC GLM in SAS Version 9.4 and means were separated using Fisher’s LSD (α = 0.05).

Average monthly high and low temperatures (°F) were 86.4/33.1, 87.8/46.4, and 88.7/53.5, in May, Jun, and Jul, respectively. Total rainfall (inches) was 1.31, 0.32, and 0.02 for these months, respectively. The first sign of CRM flight was observed on 5 May, 7 days before transplant, when adult flies were caught on yellow sticky cards in the field. Pressure from CRM and FB was high throughout the experiment. Broccoli plants grew slowly and did not form heads, likely due to high pest pressure and dry, hot conditions. No treatments significantly reduced root damage at the first two ratings, but by 6 Jun (25 DAT) the imidacloprid drench provided significant reductions in root damage relative to the untreated check. All treatments significantly increased plant vigor within plots up to 18 DAT, but by 25 DAT, 49 days after the treated seeds were sown in the greenhouse, only plants in the high rate of F1400-C and the imidacloprid treatments had a significantly higher vigor rating than the untreated check. All treatments significantly reduced damage from FB through 25 DAT.

Author notes