Field Notes session talks corn rootworm challenges and solutions

Angie Peltier, UMN Extension educator, Fei Yang, UMN Extension corn entomologist and Bruce Potter, UMN Extension IPM specialist

The following information was provided during a 2023 Strategic Farming: Field Notes session. Read further to learn more about this free program that takes place each Wednesday morning throughout the growing season.

Corn rootworm milestones

At this time of the year, rootworm management should be focused on how to manage rootworms in the next corn crop. Beetles are currently mating and laying eggs and the eggs must go through a period of rest (also known as a diapause) over the winter. Larvae hatch next year in early June, feed for a few weeks and then pupate. Beetles emerge in mid-July. Both egg hatch and beetle emergence happen over an extended period of time.

Two corn rootworm species differ in ability to resist management strategies

Two species of corn rootworm occur in Minnesota – western and northern (Figure 1). While both feed on corn roots, they have evolved to overcome different management strategies. Some populations of northern corn rootworms have evolved to have an extended diapause period, with some percentage of larvae hatching in the following year (the ‘normal’ diapause period), some hatching after experiencing two winters and some hatching after experiencing three or four winters. Those with an extended diapause may therefore not emerge when a non-host crop such as soybean is being grown, but rather can continue their diapause until corn is again planted in a field; this led to the loss of crop rotation as a surefire way to limit northern corn rootworms pressure in some corn fields.

 

Figure 1. Northern corn rootworms (left) and male (top right) and female (bottom right) western corn rootworm beetles. Photos: Bruce Potter, UMN Extension

Western corn rootworm populations have also evolved to resist the plant-incorporated protectant referred to as Bt. Bt stands for Bacillus thuringiensis, the scientific name of a soil bacterium that produces toxin proteins that can kill insects that ingest them. In this case the genes that provide the instructions for cells to manufacture the toxin are engineered to be expressed in corn cells. If a sensitive corn rootworm eats corn root tissue expressing the Bt toxin, it dies. Unfortunately, many Minnesota western corn rootworm populations have evolved to overcome multiple Bt traits. Bt-resistant populations of northern corn rootworm have been found in North Dakota, and resistant are populations are likely occur in Minnesota. However, Bt-resistant northern corn rootworm populations are not yet common and so time will tell whether resistance to Bt traits will become more widespread.

• Click here for more information about extended diapause northern corn rootworm populations.
• Click here to find out which Bt traits are in your corn hybrid and which pests they target.

Preserving effective rootworm management strategies

Reserving management strategies for only when they are needed is a good way to slow populations shifting from being sensitive to resistant. There is no need to use an in-furrow insecticide or hybrids with rootworm Bt traits if one has a low rootworm population density to manage.

Long-term continuous corn can lead to very high rootworm populations. These large populations can help speed resistance when the same management practice is used over and over. In addition, management resistance often remains hidden when rootworm populations are low.

Scouting for corn rootworms

To get a field-wide picture of the number of rootworms that are present to cause production challenges in future corn years, don’t scout the outer rows of a field, as this is where rootworms tend to move in and out of a field, but may not be where they will lay their eggs. There are two scouting strategies: go into the field and do rootworm beetle counts on whole plants on two plants at multiple locations in the field, paying special attention to ears by pulling silks back and inspecting them and tassels, the other is to set up 6 sticky traps per transect inside the field at some distance from one another, changing and counting the number of beetles on each trap weekly for four weeks.

• Click here for additional information about scouting for corn rootworms.

Questions fielded by this week’s guests

Questions answered included how many volunteer corn plants in a soybean crop are needed to lure corn rootworms into soybean fields to lay eggs; have you heard of any crops or cover crops that can attract predators of corn rootworms; what is the best timing to treat the crop for corn rootworm beetles; and what is the best way to preserve the efficacy of Bt?

Do you want to attend Field Notes yet this growing season? This program runs from 8:00-8:30 a.m. on Wednesdays through the 2023 growing season. Topics will be announced the week of the program and may include issues related to soil fertility, agronomics, pest management, equipment, and more.

Learn more and register

Can’t make the live session? No problem. The discussion-based series will be posted immediately following the webinar to your favorite podcast-streaming service to listen at your convenience. Listen here online.

Thanks to the Minnesota Soybean Research and Promotion Council and the Minnesota Corn Research and Promotion Council for their support of this program.