SW MN IPM Stuff 2019 Issue 11

08/02/2019

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 iconIPM STUFF 2019-11

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 July 29, we are only 3 to 4 days behind on growing season degree-days (1441 degree-days compared to a 1514 degree-day long-term average from May 1.) However, locally,  corn was not planted until mid to late May. At 14.01 inches of precipitation since May 1, we are now 2.94 inches above long-term average. Of course, on your farm, your mileage may vary. 

Most corn has tasseled and, regardless of temperature or maturity, should reach physiological maturity about 60 days from tassel. I am starting to see some brown silks in early fields.  

Southern Minnesota counties saw significant green snap (some up to 50% with snap below the ear). 

Soybeans for the most part are behind where they typically are at this calendar date. Depending on planting date and maturity group, soybeans are at R2 (full flower) to late R4 (Full pod stage: a ¾ inch long pod at one of the four uppermost nodes on the main stem with a fully developed leaf). At the top of the plant, the terminal cluster of flowers opens during late R4 and, lower in the canopy, flowering ceases on branches during early R5 (Beginning seed stage: A ⅛ inch long seed in a pod at one of the four uppermost nodes on the main stem with a fully developed leaf). After this time, soybeans can no longer add pods. Yield will come from retained pods and seeds/pod and by seed size.

Corn

European corn borer (ECB)

ECB degree-day development models can be found at VegEdge Degree-day Models & Forecasts. Scouting for univoltine stage corn borer should be wrapping up now. Scouting for 2nd generation scouting should be starting in southern Minnesota (Figure 1). Focus on later silking non-Bt hybrids first. ECB control from an insecticide application is more difficult with 2nd than 1st generation.

degree day model map for MN and surrounding statesFigure 1. Degree-day prediction for 2nd generation multivoltine corn borer and timing scouting. Source: A. Hansen, U of MN VegEdge.

I received an email and photo from SE MN about ECB larvae infesting soybean stems. Corn borer was also found in non-Bt corn in the area. Although corn is the preferred host for ECB, they can infest other plants and have been reported from diverse plants such as beans, peppers, potatoes and soybeans.

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 are looking for statewide infestation levels and are collecting larvae to check for parasitism and percent single generation biotype (univoltine).

Corn Rootworm

damaged corn leafFigure 2. Corn rootworm beetles will feed on corn leaf tissue when silks are not available. Not a disease and not corn blotch leaf miner.

At the SWROC, western corn rootworm are more numerous than expected. Root lodging and one or more beetles/plant are present in long-term corn planted to non-Bt corn (rootworm study areas). These areas were not planted until June but as mentioned in a previous issue, egg hatch was late this year. Evidently, the hatching larvae were able to avoid the flooded soils and were effective in finding and colonizing small corn root systems.

Beetles have been feeding on leaf tissue while waiting for late-planted corn to silk. Western corn rootworm beetle emergence is not yet complete.

It is time to start scouting for beetles if you have not already done so. Thank you to those who are contributing sticky trap scouting data.

Aphids

I am starting to see the first winged bird-cherry oat aphids moving to corn from senescing small grains. Very few individuals and not a concern as of yet.

Disease

Symptoms of bacterial leaf streak are showing up in SC Minnesota corn. This is a bacterial disease and will not be controlled with fungicides.

Common rust is present at low severities in many fields.

We have not yet seen gray leaf spot or northern corn leaf blight in susceptible sentinel varieties at Rosemount, Waseca, or Lamberton. These plots are in rotated corn.

Van Larsen mentioned finding low levels of Physoderma in Wabasha and Filmore Counties in SE Minnesota.

Soybean

Soybean aphid watch

In many fields, soybean development and aphid development are both at least a week behind a more typical year (whatever that is). Soybean aphids survive and reproduce very well on early reproductive stage soybeans. As soybean vegetative growth slows and ceases between late R4 and early R5, soybean aphids often produce migratory winged forms. Some of these flights are long distance. At this point in the growing season, later planted or maturing soybeans will be increasingly attractive hosts as winged aphids move about the landscape.

At this time, populations continue to be generally low with early planted soybeans most likely to harbor aphid populations now. However, over the past week, populations in some early-planted and early-infested fields have increased. Most areas of Minnesota that typically have aphid problems are still well below threshold and in our tours throughout SW MN chasing the soybean gall midge it is hard to find aphids in many areas where soybean planting was delayed.

Don’t assume all fields have similar aphid populations and don’t be lulled into a false sense of security. There is time for things to change when aphids start moving from early-infested fields. Jason Fussy reports some central MN soybean aphid populations are at or above 250 aphids/plant economic threshold (ET). Over the past few years, this area has consistently seen some of the earliest occurring threshold populations.

winged soybean aphidEarlier this week, I was in several fields near the  MN River valley fields where aphids are a chronic problem. While these aphids were much more abundant there than other areas I have visited, the infestations tended to be concentrated in borders and portions of fields. Part of the economic or action threshold requires most plants (80% or more) to be infested with one or more aphids but these fields were not quite there. Start paying close attention to fields once 50% or more of the plants have aphids. This can happen quickly when aphids migrate into a field.

The second part of the threshold is an average 250 aphids/plant. While they may soon reach threshold per plant populations, these fields still were well below threshold. These fields were planted earlier than most and approaching late R4. It will be interesting to see if aphids continue to increase or leave.

This has a chance to be a very low aphid year. The drier weather in the forecast will favor aphids. Will there be enough winged aphids produced to be able to colonize large areas of Minnesota? I hope not but... it is time to start paying attention.

Those darn defoliators

Green cloverworm and thistle caterpillar populations continue to cause concern but remain sub economic in most cases. The dark colored green cloverworm moths are very numerous in some west central MN fields.

Frenchy Bellicot reported finding green cloverworm that caused defoliation levels near threshold in southern WC MN. Like most fields, a wide range of larva sizes are present. When assessing defoliation make sure insects are still present and actively feeding. Significant loss of photosynthetic area, wind damage and herbicide injury can influence the action threshold somewhat, particularly after vegetative growth has ceased.

Soybean gall midge

A second generation of larvae are now present in the Rock County soybean study fields and adults are still emerging. Pay close attention to wilting or lodged soybeans on field edges including under the canopy. Check for white to orange midge larvae under the epidermis. 

Until today, the infestations appeared confined to Rock County, possibly due to the extremely late planting in much of the surrounding counties. While visiting a green cloverworm infested field near Montevideo today, I blundered into gall midge infested plants in a couple fields. In addition to the Yellow Medicine County find, Jim Traetow and Chuck Wilmes bird-dogged an infestation in Faribault Co. and the IPM student interns were able to put Jackson Co. on the map. 

There is a new U of M extension fact sheet on soybean gall midge. This will need to be updated once entomologists have a chance to interpret 2019 research results. You can view the document at Soybean gall midge in MN soybean or download a PDF version at: Soybean gall midge in MN soybean PDF.

Let me know if you suspect this insect in your fields.

Disease

Septoria brown spot is present in lower canopy of some fields.  

Frogeye leafspot levels are very low and currently far below 2018 levels.

In some SW MN fields, bacterial blight is at unusually high levels in the upper canopy. This bacterial pathogen cannot be controlled with fungicide. Loss of leaf area, if severe, may influence insect defoliation ratings.

Alfalfa

Potato leafhopper populations often start to decline in mid-August. 

Some fields have populations of plant bugs that have the potential to cause yield loss. The action threshold is 3-5 tarnished plant bugs/sweep or 2-3 alfalfa plant bugs/sweep. Thanks for the heads-up Nick.

Wheat, Oats, Barley

Not much left to do but harvest in southern MN.

Happy trails,
Bruce Potter