Insecticides for Control of Cabbage Root Maggot in Spring Cabbage, 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. Anthranilic diamide and spinosyn insecticides were evaluated relative to the industry standard, chlorpyrifos, a chlorinated organophosphate for which resistance is known to have developed in some populations. 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/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. A RCB design was used with 4 replications of 5 insecticide treatments and a de-ionized water check, with 60 cabbage plants per replicate plot. “Farao” cabbage seeds (Johnny’s Selected Seeds, ME) were sown in 128-cell flats in the greenhouse on 28 Mar and were fertilized with 200 ppm 20-10-20 (N-P-K) (Peter’s Professional Peat-Lite Special) on 24 Apr and 30 Apr. Seedlings were transplanted by hand on 2 May into beds 6 ft apart in double rows 18 inches apart and with 12 inch in-row spacing. On 1 May, Coragen, Verimark, and Entrust-A treatments were applied as tray drenches in the greenhouse by soaking seedling trays in a reservoir containing the appropriate per plant rate of material for 4 h, after which any remaining liquid was poured over the top of the tray. The untreated check, Lorsban, Entrust-A and Entrust-B banded at-plant drenches were applied on 6 May 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. The Entrust-A and -B treatments included a second banded application on 22 May using the same protocol. Weekly, on 19 May, 27 May and 2 June, 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. On 7 Jul, 10 plants per plot were harvested and head diameter and marketable weight was recorded. All data were analyzed using analysis of variance and means were compared 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 05 May, 3 days after transplant, when adult flies were caught on yellow sticky cards placed in the field. Pressure from CRM and FB was high in this field throughout the experiment. All treatments except for the Entrust-B treatment significantly increased crop vigor, decreased root damage, and increased yield relative to the untreated check ( Table 1 ). Verimark significantly increased plant vigor and marketable yield relative to all other experimental treatments ( Table 1 ).

Author notes