SW MN IPM Stuff 2019 Issue 14

Volume 22 number 14

08/22/2019

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

A print-friendly version of this newsletter is available to download here: PDF iconSW MN IPM STUFF 2019-14

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 

As of August 19, we remain about three to four days behind on growing season degree-days (1840 degree-days compared to the 1927 degree-day long-term average from May 1). 

A Sunday morning storm dropped 1.35 inches of rain here and the winds lodged soybeans and in some areas snapped more corn. At 15.64 inches of precipitation since May 1, we are now 2.4 inches above long-term average. Some areas received considerably more or less because on your farm, your mileage may vary.  

Field corn ranges from blister to early dough and some early-planted fields look good. There are also some fields with compaction, fertility and green snap issues. Most fields will benefit from and some will need a late frost. 

Depending on planting date and maturity group, most soybeans are late R4- late R5 (full pod to beginning seed stage) stage some early planted soybeans are pushing R6 (full seed).

Crop Pests

Corn

European Corn Borer (ECB)

Corn borer management for 2019 is over and it is time to start thinking about 2020. If you have fields without above ground Bt traits that you would be willing to let us examine for fall survey please contact me at bpotter@umn.edu. We will be looking for ECB infestation levels statewide and collecting larvae to check for parasitism and percent single generation (univoltine) biotype. 

Thanks to those who have already volunteered fields!

Corn Rootworm

yellow and black but on corn leafFigure 1. Gravid female western corn rootworm

Field observations and yellow sticky trap captures are finding some unexpectedly large populations of beetles (Figure 1) in some fields. 

Western corn rootworm beetle populations in non-rotated corn are variable.

Northern corn rootworm populations may be increasing and I expect this will be a focus when cooperative sticky trap captures are analyzed.

Soybean

Soybean aphid watch

First the good news.

Much of Minnesota still has very few aphids and depending on management philosophy,  missed their associated costs. This scouting season should be over or winding down in most fields over the next week or so but... 

Now the bad news.

Unless disease collapses populations or environmental cues trigger a move to buckthorn, this looks like it could be a year where very large soybean aphid populations develop in some R5-R6 soybean fields. 

winged soybean aphid and soybean aphids on soybean leafFigure 2. Winged aphid arriving on a previously infested late R4 growth stage soybean leaf.

Some areas are experiencing late-building aphid issues. While few aphids are left in the upper canopy of R5 stage soybeans, in some fields, populations of those aphids left behind in the lower canopy are building. There appear to be some issues in areas of SW MN, and likely elsewhere where large numbers of winged aphids continue to move into late-planted soybeans.

This reflects what we are seeing in a soybean planting date study at the SWROC where mid-June and July planted soybeans are now being colonized by additional winged aphids (Figure 2). In both cases, populations have the potential to increase rapidly.

Insecticide control is a component of this study, so at the end of the year, we should know whether an insecticide application was the correct decision. 

In the meantime, here are some suggestions for those who might be struggling with late-season decisions.

Within a geographic area, some scouting of fields through the R5 stage is advised. If populations are increasing, using the 250 aphid/plant threshold until you reach R6 could help you avoid an unpleasant surprise in R6.  

Don’t base your decision on the edge of the field where aphids are often more numerous. 

On R5 stage soybeans, remember to look for small aphids low in the canopy - this is where most of the action on R5 and R6 stage soybeans usually happens.

Are the aphids healthy? Locally, lady beetles have been scarce in soybeans early this season. However, I have started to see the first signs of fungal disease and the first mummies from parasitic wasps in aphids at the SWROC. With cool, wet conditions fungal diseases can rapidly eliminate an aphid infestation. 

Consider crop stage. How long will it be before you begin to see yellow pods (R6.5) in the field and the possibility of aphid-induced yield loss is reduced? If very late-planted, do the soybeans have a reasonable yield potential and chance of avoiding frost? What about pre-harvest interval (PHI)?

Yield loss occurs much later than 250 soybean aphids/plant. If and when economic threshold levels are reached, larger, later stage soybeans suffer economic yield loss less frequently than when the threshold is reached earlier in the season. Is there still enough growing season before the late R6 soybean stage to accumulate more than 5,000 aphid-days? Good growing conditions can also allow soybean to tolerate more aphid pressure (aphid-days). Usually, this means you have more time to make a treatment decision.

Those darn defoliators

Over the past week, I’ve had conversations with several folks about green cloverworm populations in the NW part of the state. As soybeans progress through the R6.5 stage (a yellow pod on the plant), economic benefit from insecticide becomes doubtful. Additionally, pre-harvest intervals of insecticides become a factor. Meanwhile, in many soybean fields to the south, predators and disease are causing a collapse of defoliating caterpillars. Green cloverworm seem particularly susceptible. 

Black, nearly liquified larvae killed by virus disease (Figure 3) and chalky, white larvae killed by a fungal disease (Figure 4) are signs that caterpillars may be becoming scarce. 

The potential for natural enemies to control insect pest problems for us is one reason that economic thresholds can be more economical than early or insurance insecticide applications.

blackened, dead green cloverwormFigure 3. Green cloverworm killed by a viral disease.

Soybean disease stuff

I am starting to see areas where the yellowing of upper foliage is occuring. I have not had a chance to check which combination of nutrient, root disease, SCN and Phomopsis is responsible.  

White mold is present in some areas of the state; expect symptoms to peak over the next week or so. Frogeye leafspot symptoms are showing up in more fields but severities, in most cases, are low.

dead green cloverworm with a white fungal-like coatingFigure 4. The remains of a green cloverworm larvae that was killed by a fungal disease

Soyban Gall Midge

Since the last issue, Cottonwood has been added to the list of infested MN counties (Figure 5). 

Minnesota map marking counties in WC and SW MNFigure 5. Minnesota counties where soybean gall midge has been detected in 2019.

Instead of trying to squeeze in a late summer field day, we will try to focus on gall midge in fall and winter meetings. We will be able to discuss the results of management research in MN and other states by early winter. 

Fields with significant damage from this insect still appear limited to Rock County. 

Determining the extent of infestations of this insect is one of the first steps in management.

Please let me know if you suspect this insect in your Minnesota fields!

Winding down

Insects are starting to take a break from the 2019 SW MN crop. Thistle caterpillar/painted lady butterflies will be migrating out of Minnesota. Soon, other insect species will soon cease to be a crop production issue as feeding stops and they prepare to overwinter or the adults migrate out of Minnesota. Even the summer students are starting to migrate back to classes. 

Happy trails, 

Bruce Potter