SW MN IPM Stuff 2020 Issue 10

Volume 23 number 10   6/26/2020

This newsletter is also available in a print-friendly pdf format: PDF iconSW MN IPM Stuff 2020-10

This newsletter and the advice herein are free. You usually get what you pay for.


For the most part, the crop in SW (and much of Minnesota) looks pretty good. It could always be better, but so far, this one looks pretty good.

Crop weather

Rainfall, air and soil temperatures, degree-days, soil moistures, and other current and historical weather data for the University of Minnesota Southwest Research and Outreach Center (SWROC), a little spot about two miles west of Lamberton, MN, can be found at SWROC Weather.    

Wind has limited timely herbicide applications for many crops and created issues with off target movement.


Early-planted corn in the Lamberton area is 9-10 collars. Corn rootworm larvae are active and their injury to the nodal roots of corn is visible.


Early planted soybeans are V5 and R2. Pay attention to herbicide labels.

There is not a single cause for the puckered soybeans out there.  

Soybeans began to yellow significantly after recent rains.

If you have not already started, you should start checking fields that tend to see early aphid populations (indicator fields) for soybean aphids soon.


I received a report of aphid populations flaring after a synthetic pyrethroid insecticide application. In previous years, there have been reports from WC Minnesota of pea aphid populations exploding after insecticide applications for alfalfa weevils and potato leafhoppers. This time it is from the SC part of Minnesota. Do not make sequential applications of the same insecticide group. 

PEST ALERT: True Armyworm

MN counties map marking 6 counties in central MN and 1 in northwest MNFigure 1. Counties with reports of true armyworm infestations at or near economic threshold levels.

Reports of true armyworm infestations continue to come in (Figure 1).  At this time the infestation is not as widespread as in 2018. Most reports have been in wheat, pasture roadsides and other grasses.

Armyworms cannot overwinter in Minnesota and migrate into the state each spring. Lush grasses are preferred egg laying sites for the armyworm moth and lodged areas of small grains or the grass field borders of corn and soybeans should receive special attention when scouting. When they have defoliated an area, larvae will move from field borders or between fields. They feed on grasses seldom damage broadleaf crops.

The risk of armyworm infestation in corn is increased when a live grass cover crop or grass weeds are present when the egg-laying moths are active. Only the Agrisure Viptera (Vip3A) Bt trait is labeled to protect corn from armyworm. See the Handy Bt trait table 2020 for more information.

several armyworm larvae next to US nickel coin and human fingertipFigure 2. Armyworm larvae. Overall color can range from olive to nearly black.

Most of these infestations are likely from moths arriving on the systems that brought rain to Minnesota May 24-26, 2020. The black light trap at the SWROC picked up a flight at that time. However, along with persistent southerly winds there have been persistent captures of low numbers of armyworm moths across Minnesota this spring. In addition to multiple armyworm moth migrations into Minnesota, an extended egg-laying period, egg and larva exposure to variable microclimates, and availability of food can lead to finding armyworms of different sizes in the field.

The larvae are most active at night or cloudy days. Preliminary scouting in small grains or grass borders can be done with a sweep net. To assess populations in small grain crops, shake the canopy and look for the larvae on the ground. Be sure to check under residue.

The economic threshold for small grains is 4-5 larvae / square foot but treat if the larvae begin clipping heads.

The threshold in whorl stage corn is 25% plants with 2 larva/plant or 75% plants with 1/plant. Infestations in larger corn are rare; minimize defoliation above the ear if they occur. Before making and insecticide application, be sure that larvae are still present.

You may want to treat a swath ahead of an infestation that is migrating.

For more information on biology, scouting thresholds see true armyworm in corn and true armyworm in small grains.

Poor herbicide weed control is not always due to poor application techniques or herbicide resistance.

common stalk borer inside giant ragweed stemFigure 3. Common stalk borer in giant ragweed stem.

I recently investigated a report of poor control of giant ragweed in corn. There was more than one reason for some weeds surviving pre-emerge and post-emerge herbicide applications but the main culprits were insects.

Many of the larger giant ragweed stems had a common stalk borer (Figure 3) inside. Some common stalk borer larvae had started to leave the ragweed for corn. A few survivors had European corn borer, or close relative, (Figure 4) inside the stem. Looks like they found a way to survive in a field of Bt corn. Presumably, the tunnels produced by the insects could interfere with herbicide movement within the plant.

corn borer inside giant ragweed stemFigure 4. Corn borer or relative inside a giant ragweed stem.

Happy trails,

Bruce Potter