SW MN IPM Stuff 2020 Issue 15

Volume 23 number 14   8/14/2020

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It’s the top of the eighth inning for corn and soybeans.

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.

Some of the drier areas where spider mites have been a concern have received needed rain that may help slow mite population increases and help fungal disease development. Unfortunately, high winds lodged some corn fields from Canby east to Sacred Heart. It looks like more severe weather could be on tap for today and tonight. Be prepared to start taking lodged corn early.


Most SW MN fields are R3 stage.

Disease pressure has been low in the fields I have been in throughout SW MN. Physoderma brown spot can be found on susceptible hybrids. I have observed some Goss’s, even in drier areas, but so far nothing severe.

Watch for tar spot and let Dean Malvick (dmalvick@umn.edu) know if you come across a field with suspicious symptoms. 

Western corn rootworm populations have started to decline at the SWROC.


Most soybeans are early to mid-R5 stage with some fields planted to shorter-season varieties in R6.

Soybean Aphid Watch

Soybean aphids continue to build in some areas but fields at or near threshold remain spotty. The aphid research area at the SWROC passed the 250/plant average economic threshold on 8/10 and we applied insecticide treatments on 8/11 to mid-R5 stage soybeans. At this time, and this location, lambda cyhalothrin (e.g. Warrior) treatments are struggling but bifenthrin seems to have worked. Unless populations increase rapidly, I doubt we’ll detect yield losses at this site.

Yes, this is a tough time of year to make aphid treatment decisions. Yield loss will not occur from 250 aphids/plant, even on early reproductive stage soybeans, much less late R5. Remember, the 250 aphids/plant threshold is an action threshold that allows time to treat the field well before yield loss occurs. The threshold runs through R5 but is no longer valid after soybeans reach the R6 stage.

The probability of an economic return from an insecticide treatment declines as soybeans develop through the R5 and R6 growth stages. However, very large populations (1000s/plant) of aphids can sometimes take yield in R6 stage soybeans. The reason the threshold it is not higher in R5 is to protect against missing any populations that explode during late R5 to early R6. Populations less than 250 by R6 are highly unlikely to develop large enough, fast enough to cause economic loss. After pods begin to yellow walk away.

Scouting and treating aphids after mid-R5 is sometimes based on the grower’s perception of risk and stamina of those doing the scouting. For help gauging how long there is left until R6 and more importantly R7 when most yield has been fixed see: Number of Days Between Soybean Growth Stages.

There are observations that can indicate reduced risk from aphid yield loss. This year, lady beetles and other predators have been relatively rare in the soybean fields I’ve been in but where they are abundant they can slow or stop aphid population growth. At Lamberton, I am starting to see gray to brown, fuzzy aphids infected with entomopathogenic fungi, most likely Pandora. This week we found the first “aphid mummies”, aphids killed by parasitoid wasps. For the remainder of the season, aphids may begin moving to buckthorn as soybeans age and cool weather sets in.

caterpillar remains on a soybean leaf - caterpillar body is covered in a white fungusFigure 1. A caterpillar killed by a fungal disease

From the eastern front, I received several photos taken by “Deep Throat”. The photos showed the white remains of caterpillars killed by fungal disease (Figure 1), probably Beauvaria, as well as diseased leafhoppers and grasshoppers.

Disease usually keeps Minnesota green cloverworm populations low in most years.

Soybean gall midge symptoms are becoming easier to find. A large number of adults emerged over the past week so I expect damage to increase over the next month. It is important that we better understand the distribution of this insect in Minnesota. Please contact me if you find this insect. In areas where white mold occurs, the non-pest white mold gall midge has been observed. The following provides guidance on telling the two species apart: How to distinguish soybean gall midge from white mold gall midge on MN Crop News

Spider mites – Cooler temperatures will slow reproduction. Cool, wet (including heavy dews) conditions favor the development of the fungus, Neozygites, which helps control mite populations in most years.

For 2021 portions of West Central Minnesota into South Dakota will need to keep bean leaf beetles in mind. Recently, some fields saw 1st generation adults defoliating soybeans and insecticides were triggered when pod feeding started.

Sudden death syndrome (SDS) symptoms are beginning to appear, particularly where soils were compacted.

Happy trails,

Bruce Potter