SW MN IPM Stuff 2021 Issue 7



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

At the SWROC, we had had 3.09 inches of rainfall since April 10. Not a lot but better than some areas. We have already experienced ten days above 90ºF this June. Last week we had 120ºF soil temps at ½ inch and maximum two-inch depth soil temps at or near 90ºF for several days. Looks like we will get a brief respite from the heat, but precipitation (liquid form in moderate amounts) is needed.


Most corn is now V6 and above and depending on planting date row spacing, and moisture, rows are now closed in some fields. This canopy will help moderate soil temperature, but the crop is now using more water. Portions of some fields are starting to roll by mid-morning. In areas that went into the spring with good subsoil moisture we still have some time.

Corn rootworm eggs have started hatching. We floated larvae from some continuous corn at the SWROC on June 15 and 16. They were not hard to find. Nearly all were first-stage larvae, so the hatch is in its early stages here. We also started seeing fireflies late last week. In this part of the world, the degree-day model for rootworm egg hatch and the appearance of fireflies as predictors still agree.

Dry soils will compound the effect of root feeding by corn rootworms and can reduce the effectiveness of at-plant, soil-applied insecticides. Corn rootworm resistance to proteins may reduce the effectiveness of Bt hybrids. This could be interesting times for rootworms, particularly in fields that have been in long-term corn.


The earliest emerging soybeans are now V6 and R1.

Some minor symptoms of Rhizoctonia are starting to show up. As expected with the hot dry weather in most fields, other diseases are hard to find at this time.

Two spotted spider mites 

Watch edges of fields next to mowed perennial vegetation (e.g., alfalfa, roadsides) for damage symptoms. I have not yet noticed any infestations this year, but June’s weather should have favored mite reproduction. Several extremely cold April and May temperature events might have impacted overwintering mites but their effect on 2021 spider population is unknown. Because of mites’ reproductive potential, just a few survivors are enough to eventually produce a problem.

Herbicide injury

Seeing drift and carryover problems this spring. Continued dry weather would increase the chances for carryover into 2022.

Yellow beans

Over the past week, portions of soybean fields have been turning yellow. There are several possible causes that may be exacerbated by hot, dry weather: herbicide, IDC/salt, soybean cyst nematode (SCN), and all the above.

Do not rule out SCN (Figure 1). In southern MN, the bodies of the small, lemon shaped SCN females are beginning to appear outside soybean roots now. Long-term, continued use of SCN resistant varieties has selected for SCN populations able to infest and reproduce on SCN resistant soybeans in some fields. Because of this selection pressure, SCN PI 88788, and in some cased Peking, resistance may no longer work well in some fields. To complicate diagnosis of resistance, some soybean varieties sold as SCN resistant are not very resistant.

soybean field with wilted soybean plants
Figure 1. Severe damage to 2021 SW MN soybean field caused by SCN.

Soybean gall midge

Adults are emerging in SW MN now and we are finding the first small larvae in soybean stems. Over the next few weeks symptoms will become more visible and these symptoms and plant wilting and death will continue to increase over the summer. Soybean gall midge might be an even tougher adversary than I had hoped. During a rare bout of optimism, I suspected that one possible upside to this spring’s hot, dry weather would be reduced damage from soybean gall midge, but that is starting to look less likely.

While foliar insecticides may provide some control of adults, research results have been inconsistent. Because of the potential for spider mites this year, insurance insecticide applications are not without risk.

See https://youtu.be/Bt9DkgOQVpw for early season scouting tips. Please contact me if you find soybean gall midge in one of your Minnesota soybean fields.

Soybean aphid watch

We are finally starting to find some soybean aphids at the SWROC. Populations are very low at this time and have likely been negatively impacted by hot, dry weather. We are finding a few plants where aphids populations have been tended by ants and aphids tend to be easier to find in soybeans sheltered by windbreaks and in corn.

As always, I like to hear your soybean aphid populations.


Alfalfa weevil

Alfalfa weevil populations were high in some areas this spring and the weevils did not necessarily follow developmental model predictions. Things are winding down although some larvae can still be found. Adult weevils are emerging form cocoons, and their numbers are increasing in some fields. These summer adults seldom pose a great risk to the crop as they will do a small amount of feeding before aestivating for the remainder of the summer. There may still be a field or two where weevil feeding is keeping regrowth stopped.

Scouting should focus on potato leafhoppers and plant bugs now. Aphid populations may increase if weather cools, especially in fields that have been treated with an insecticide earlier.

Small grains

Aster leafhopper nymphs are abundant in some small grain fields now. Mike Flint sent pictures last week and we found numerous nymphs at the SWROC this week. Although aster leafhoppers can transmit the aster yellows phytoplasma, the crop in the southern part of the state is likely far enough along to minimize risk.


An old friend, Dave Willis mentioned that some NW MN grasshopper populations are being treated. These populations started building two years ago. Treatment threshold is 50 or more nymphs/square yard in crop border and 30 or more nymphs within the field. The threshold drops to 20/square yard and 8/square yard for adults in borders or within field, respectively. Count them as they emerge from a 3 square foot area in front of you.

Small hoppers are more susceptible to insecticides. 

In southern MN, grasses, alfalfa, or soybean are attractive to egg-laying adults in late summer and fall.

Points of interest