Maggots on the Move

By Marissa Schuh, IPM extension educator

As we come out of a chilly few weeks, insects are warming up too.  While the cold has slowed all insects, our cold-tolerant early-season pests are getting ready to emerge. Degree day models for seedcorn maggot and cabbage maggot indicate that the flight period for these pests is arriving across Minnesota.  Seed corn maggot is a pest of larger seeded crops, like beans, peas, and sweet corn.  Cabbage maggot targets the roots of recently transplanted brassica crops, as well as the roots of radishes and turnips. 

Looking at the forecast, seed corn maggot populations will start peaking in some parts of Minnesota in the next week. Cabbage maggot emergence will start in Southern Minnesota in the next week.

Insect development is driven by heat.  Degree day models are a tool used to estimate the development of different insect pests based on current and predicted temperatures.  Models for these pests indicate that seed corn maggot populations are increasing in the southern third of the state over the next week, and that cabbage maggot emergence will begin in the next week as well.

Why care about degree day models at all? Both seed corn maggot and cabbage maggot attack young plants, and because they burrow into the plant, they are hard to treat.  For both these pests, our control methods are preventive in nature.  In the case of cabbage maggot, tools for management are more limited than ever.

Seed corn maggot feeding on a bean seed resulted in a stunted and distorted seedling. Photo: Howard F. Schwartz, Colorado State University, Bugwood.org

As we head into a period of potentially high pressure, think about how cover crops and weeds are managed in the field you are planting into.  Both seed corn and cabbage maggot are attracted to decaying organic matter.  The goal is to give the decaying plant matter 3-4 weeks to break down before you plant in that space. 

Plant large, healthy transplants in the case of cabbage maggot.  For seed corn maggot, handle seeds carefully to keep the seed coat intact.

The biggest thing that can be done is use row cover to exclude the adult flies, who can’t lay eggs on plants that are underneath netting.  The degree day models are useful in figuring out when to deploy the row cover to exclude the most flies.

If sporadic plants with stunting and nutrient deficiency are seen in a brassica field, pull them ou to check for cabbage maggot. Photo: Marissa Schuh.

If you would like to use a pesticide to control these pests, remember that the pesticide treatments are preventative, and there isn’t anything that can be used to get rid of these maggots once they are burrowing into plants. For more information on treating these pests, see the Midwest Vegetable Production Guide.