Brassica Bugs

 Marissa Schuh, IPM Extension Educator, mschuh@umn.edu

As early-planted cole crops continue to chug through the growing season, all the insect pests that love to eat them are also out.  Let’s review who we’re seeing now and if we can do anything about them.

Cabbage Maggot

Cabbage maggot feed on the roots, weakening and often killing plants. Photo: Marissa Schuh, UMN Extension.

Can anything be done now? No.

You’re busy, so let’s cut to the chase: once you see cabbage maggot feeding in the roots, there isn’t much to be done.  Make a note to brush up on cabbage maggot control options next winter and try something different next spring.

Flea Beetles

Flea beetle feeding causes numerous, small holes in leaves. Photo: Whitney Cranshaw, Colorado State University, Bugwood.org

Can anything be done now? Yes.

This is a pest that we do have options for, though the window for much of the feeding has passed.  The larger the crops are, the more resistant they are to feeding, though they do often feed on what we are hoping to sell.

• If 10-20% of the leaf area of plants are damaged, consider spraying

• Kaolin clay can provide a physical barrier to feeding (but doesn’t always work)

• For conventional pesticide options, see the Midwest Vegetable Production Guide.

• On the organic side, product performance is mixed.  Pyrethrin and Spinosad products can work.  I like the summary table found on page 47 of the Cornell Organic Production and IPM Guide for Cole Crops, which includes on how dozens of products have performed in University trials.

There are also cultural controls that could be implemented in the future years.

Diamondback moth

Diamondback moths are small, smooth caterpillars that create many small holes in leaves. Photo: Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org

Can anything be done now? Yes.

The rest of the season will be spent dealing with diamondback moth, as well as imported cabbageworm and cabbage looper.  We have scouting-based thresholds available that help us know if spraying is economical.

Scouting Thresholds for Caterpillar Pest in Broccoli and Cauliflower 

From Michigan State University’s Caterpillar Pests in Cole Crops.

• Plants in Seedbed Stage

• Imported Cabbageworm, Cabbage Looper - 10% of plants with larva

• Diamondback moth- 10% of plants with larvae

• Plants in Transplant through First Curd Stage

• Imported Cabbageworm, Cabbage Looper - 40% of plants with larvae

• Diamondback moth- 20% of plants with larvae

• Plant in Fist Curd to Final Harvest

• Imported Cabbageworm, Cabbage Looper - 10% of plants with larvae

• Diamondback moth- 10% of plants with larvae

Scouting Thresholds for Caterpillar Pests in Cabbage

From University of Minnesota

• Plants in Transplant through First Cupping Stage

• Imported Cabbageworm, Cabbage Looper - 10% of plants with larvae

• Diamondback moth- 10% of plants with larvae

• Plants in Transplant through First Curd Stage

• Imported Cabbageworm, Cabbage Looper - 30% plants infested with 1 or more medium-large imported cabbageworm larvae and/or 1 or more cabbage looper eggs or larvae.

• 50% plants infested with 5 or more larvae each.

• Plant in Cupping to Harvest Stage

• Imported Cabbageworm, Cabbage Looper - 10% plants infested with 1 or more medium-large imported cabbage worm larvae and cabbage looper eggs or larvae.

• Diamondback moth- 10% plants infested with 1 or more larvae each.

There are many products effective on these caterpillars.  Both organic and conventional products can be found in the Midwest Vegetable Production Guide.