MSU Extension

In Michigan, there are numerous insect pests that feed on cabbage. For example, there are sporadic pests such as root maggots, thrips, aphids, flea beetles and cabbage loopers, and annual pests such as the imported cabbage worm and diamondback moth. Management strategies usually focus on the lepidopteran pests (loopers, imported cabbage worms and diamondback moths) as they are consistently present and have multiple generations per season.

Major insect pests (click on photo see a larger image).

Natural enemies can contribute to pest control; however, broad-spectrum insecticides negatively impact beneficial insects. The use of selective insecticides can contribute to an increase in biological control efficiency; therefore, these two methods as recommended by Michigan State University Extension can be used simultaneously in an integrated pest management program to strengthen pest suppression.

The Michigan State University Vegetable Entomology Program tested four insecticide treatments and an untreated check (Table 1) for caterpillar control in cabbage. Baythroid XL was chosen as a disruptive product that was expected to negatively impact beneficial insects, and unlike other treatments, Baythroid XL was applied every two weeks throughout the growing season. The remaining treatments were designed to be rotations between low to moderately disruptive products that would be applied based on thresholds. However, low insect pressure resulted in only one of the treatments needing a second application during the season. Thresholds, measured in larval units, were calculated using the system presented in the 2010 Ohio Vegetable Production Guide (see page 115). Insecticides were applied using a single-nozzle hand-held boom (40 gallons per acre and 30 psi). All treatments were applied with a surfactant called Silwet L-77 at 0.25 percent v/v.

Table 1. Treatment list with impacts on beneficial insects, rates and application dates for a cabbage trial conducted at the MSU Horticulture and Teaching Research Center, Holt, Mich., summer 2012.

*all treatments were applied in conjunction with Silwet L-77 at 0.25 percent v/v

All treatments significantly lowered caterpillar seasonal mean numbers compared to the untreated control (Figure 1). Baythroid XL and Coragen resulted in the fewest number of caterpillars, while Avaunt was significantly outperformed by all the other insecticide treatments. Not surprising, the mean number of caterpillars was inversely related to the number of insecticide applications made during the season. The low caterpillar pressure required only one application for the Avaunt treatment, while Coragen and Intrepid/Coragen treatments needed only two applications.

Figure 1. Impact of insecticide treatments on caterpillar numbers in a cabbage insecticide field-trial conducted by the MSU Vegetable Entomology Laboratory. All products were applied in conjunction with the surfactant Silwet L-77 at 0.25 percent v/v. Bars with the same letter are not significantly different (α= 0.05).

The seasonal mean number of beneficial insects also differed significantly between treatments (Figure 2). The untreated control and Avaunt treatment resulted in significantly more beneficial insects compared to all other treatments. The treatments applied only twice (Intrepid/Coragen and Coragen treatments) had significantly more beneficial insects compared to the Baythroid XL treatment, which entailed five applications.

Figure 2. Impact of insecticide treatments on beneficial insect numbers in a cabbage insecticide field-trial conducted by the MSU vegetable entomology laboratory. All products were applied in conjunction with the surfactant Silwet L-77 at 0.25% v/v. Bars with the same letter are not significantly different (α= 0.05).

There were no differences between treatments for either the number of or weight of marketable cabbage. This could be due to the low caterpillar pressure, but it is also possible that increase in beneficial insects in untreated plots was able to keep pest pressure below economic threshold.

For more information on cole crop insect management please visit the MSU Vegetable Entomology’s Cole Crops page.

Dr. Szendrei's work is funded in part by MSU's AgBioResearch.