SW MN IPM Stuff 2020 Issue 13

Volume 23 number 13   7/16/2020

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Greetings,

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.

Severe weather, including hail and tornados, continues to cause damage around Minnesota during the 2020 growing season. On July 8, a strong tornado hit west central Minnesota. On July 11, hail and wind ravaged a crop with tremendous potential in parts of central and south central Minnesota and southwest Minnesota. 

Hail can increase infections from some pathogens, but they tend to be bacterial rather than fungal. Fungicide applications to cure the effects of hail are unlikely to be productive from a yield perspective. For more information on management of hail damaged crops see: A July hailstorm in Minnesota 

The more moderate temperatures in the forecast will be beneficial for pollinating corn. Some pathogens, if present, will be favored by the cooler weather. The cooler weather will reduce stress on soybean aphids.

Based on the number of inquiries, the red headed flea beetle, Systena frontalis, is abundant in some corn and soybean fields. These insects are commonly observed every year. There are at least two generations/year with the beetles currently found in fields being the second. The larvae live in the soil, presumably feeding on roots or organic matter.

red headed flea beetle on a leafFigure 1. Red headed flea beetle

The adults (Figure 1) cause pin hole feeding in soybean and other broadleaf crops. They create elongated scars by feeding on the epidermis of corn. Most of the feeding on corn tends to be in the lower canopy. Red headed flea beetles tend to be more abundant near field borders.

Although they can cause issues in some crops, they are unlikely to cause economic damage to corn or soybeans. Consider them as any other defoliator and use defoliation based economic thresholds.

Monarch butterflies are abundant now. They have become a minor road hazard in some areas. Good to see them though.

Corn

corn root with damage from western corn rootworm. Roots are black indicating damage.Figure 2. Western corn rootworm damaged corn root

Corn ranges from pollinating through near tassel emergence. 

Rootworm damage in some continuous corn fields will be impressive this year (Figure 2).

As small grain crops mature, winged cereal aphids will move to other grasses, including corn. If you have decided to apply a fungicide and insecticide combination, pay attention to aphids…in dry areas, spider mites.

Soybeans

In addition to the flea beetles, there are other defoliators in soybeans. I have seen green cloverworms, thistle caterpillars, and loopers at low levels out there. Jon Bork sent photos indicating 1st generation bean leaf beetles are doing well. Although they are very rare in SW Minnesota, I am sure I’ll see Japanese beetles when I visit the Rosemount Research and Outreach Center this week. Lump together the defoliation from the various species present. Use 20% defoliation of the plant’s total leaf area as an economic threshold. Do not just count defoliation on the worst leaves.

Orange and white soybean gall midge larvae feeding on the stem of soybeanFigure 3. Soybean stems infested by soybean gall midge at two different times.

I have been watching the recent flight of painted lady butterflies but the potential for economic damage from their offspring is reduced with the large soybean canopies in most fields.

The second flight of soybean gall midge adults are emerging. Damage (Figure 3) from the 1st generation larvae (Figure 4) is visible in heavily infested fields in Rock County. Some of the insecticide treatments I am studying look promising against 1st generation. I am waiting to see if any can hold up to the 2nd and 3rd generations.

Last week we found larvae that appear to be soybean gall midge on sweet clover in Rock County. Symptoms were similar to those on soybean. The larvae will be sent in for DNA confirmation but similar infestations have been reported from Nebraska. Lots left to learn about this insect.

Please let me know if you come across any suspected soybean gall midge infestations.

Edge of a soybean field with plants wilted indicating damage from soybean gall midgeFigure 4. Soybean gall midge caused the dead and dying plants on the edge of this soybean field.

Soybean aphid watch 2020

I have received a few reports of increasing aphid populations in central (Nate Drewitz) and east central MN (Jim Gill, Dave Nicolai). The SC MN indicator fields I have checked are similar with close to 100% infestation rates but, after a quick start, storms and continued abundant moisture have slowed increases of aphids/plant. The aphid populations at the SWROC are gradually increasing but not yet at 50% plants infested.

I start paying closer attention to fields once I find more than 50% of the plants are infested with at least one aphid. It’s not threshold but average numbers of aphids/plant can increase more rapidly once most of the plants have been colonized. The recent cooler weather will temporarily take some of the stress off of aphid populations. Aphids tend to survive and reproduce well on plants from the R2 – R4 stage. By moving to the new vegetation developing lower in the canopy they can avoid heat and other environmental stresses. Do not forget to look for aphids in the lower canopy, especially.

I took a look at rainfall from mid-June to now (Figure 5), the period when aphids should have moved off buckthorn and been using soybeans as a host. Lab research in Illinois has shown that similar to some other aphid species, soybean aphids do best on plants that are moderately moisture stressed and less well on plants exposed to abundant moisture or drought. There are areas in central, east central and southeast Minnesota that might see aphid populations reach treatable populations in some fields first. In the driest areas, parts of western MN for example, aphids may leave if plants go under drought stress... watch for spider mites if drought develops and persists. 

Soil moisture is only one of the factors that can influence early season soybean aphid populations. Areas with early-planted soybeans, soils testing lower in potassium and nearby buckthorn often see aphids first.

Fields with hail damage or drown-outs may see aphid populations late in the season if soybean maturity is delayed or soybeans planted late.

I’ll be curious to see how the 2020 season plays out. It is time to increase your scouting efforts. Let me know how close my predictions are.

map of MN, WI, ND and SD showing precipitation totals for each countyFigure 5. 30-day rainfall amounts June 14-July 14

Alfalfa

Potato leafhoppers continue to be an issue. A simple threshold for alfalfa is based on the number of leafhoppers/pendulum sweep and height of alfalfa. Watch the pre-harvest interval when considering insecticide applications for taller alfalfa.

Adults will move to adjacent soybeans or other hosts when hay is cut and back to alfalfa as regrowth develops.

Potato leafhopper economic threshold
Alfalfa height
3" 6" 8-11" >12"
leafhoppers/sweep 0.2 0.5 1.0 2.0

Potato leafhoppers are much less likely to affect soybean yields compared to potato, alfalfa and edible beans.

Mark Hoeckel reported high pea aphid populations in some west central MN fields that had not been previously sprayed. I am waiting for the verdict on a south central Minnesota field that was bioassayed for pyrethroid resistance. 

Sulphur butterfly (alfalfa caterpillar) larvae are abundant in some southern MN alfalfa fields. While the larval feeding is not likely to be economic, we could see an abundance of yellow butterflies soon.

Happy trails,

Bruce Potter