SW MN IPM Stuff 2016 Issue 13

July 29, 2016

This newsletter is also available in a print-friendly pdf format: SW MN IPM Stuff 2016-13

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

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.

We have received 1596 degree-days (base 50°F) and 15.07 inches of rainfall May 1-July 29. An additional 105 degree-days (base 50°F) accumulated from April 11-30. Above average degree-day and precipitation accumulations continue. 

This is above long-term average degree-days and rainfall! There may be a period of drier weather and moderate temperatures on tap that could favor for soybean aphid population increases.


Some areas of the state have a good corn crop coming. In most cases pollination and ear size look good.

Corn rootworm

The SW MN IPM crew has been digging and rating corn roots from research plots in SW MN this week. Due to poor planning, we ended rating1000 roots for corn rootworm damage this year. We greatly appreciate the help from other projects at the SWROC.

It looks like we will have good data on trait and insecticide performance. There is significant western corn rootworm feeding damage to corn systems and lodging in these trials. These trial locations had high beetle populations last year. For those that like to plan ahead, I will have some preliminary results in the next issue or two.

Early in the season, corn heavily damaged by rootworm feeding sometimes looks nitrogen or potassium deficient...cause it is.

So far, I have not been in any fields with more than moderate northern corn rootworm beetle populations this year but then, I don't get out much.

corn earworm larva

Corn earworm larva

Corn earworm

Dan Schmitz and I looked at a corn earworm damage in a Brown County dent corn field. Earworm cannot be controlled once in the ear. The larvae are cannibalistic so you typically find just one in an ear.  Although feeding damage in ear tips can create an environment favorable to ear mold fungi, losses to field corn are usually minor. Corn earworm are a major sweet corn pest. Monitor flights of corn earworm at corn borer: https://www.vegedge.umn.edu/moth-data/cew-info


Waterhemp and volunteer corn are now above the soybean canopy. Both weeds can attract rootworm beetles when pollinating and eggs may be laid near volunteer corn by both northern and western corn rootworms.

Soybeans are R3 (beginning pod) to R5 (beginning seed) stage. This variability in soybean stage in some areas might be expressing itself in soybean aphid populations.

foliar symptoms of SDS on soybean

Foliar symptoms of SDS

Sudden death syndrome (SDS) is starting to show up in some southern Minnesota fields. The early season cool, saturated soil conditions likely helped with infection. In addition to the percentage of the field with SDS symptoms, loss from this disease is also related to the timing of plant death. Plant death after partial seed development maintains some yield.

The foliar symptoms of brown stem rot are similar but do not usually appear until the R6 (full seed stage) of soybeans. Brown stem rot will have a portion of the stem pith showing a brown discoloration, SDS does not.

Do not forget about two-spotted spider mites in areas of the state that were dry earlier. Brad Muller reports still finding some mites in western MN after recent rains and good soil moisture. I talked with a couple others that were in the process of checking on mites in the eastern part of the state. A few two-spotted spider mites are present in most soybean fields every year. A few mites are not a reason to panic! If easily found, they may influence soybean aphid insecticide choices.

Note: Syngenta's Agri-Mek SC is now labeled for spider mite control in soybeans. It needs to be applied with an NIS adjuvant.

Soybean aphid watch 2016

In general, populations are increasing across MN but are not at economic threshold levels yet. There are a few small pockets in SW MN where aphids have reached threshold but these are rare. At this point, there are no areas of the state with widespread yield threatening soybean aphid populations. This will probably start to change over the next week.

Don't underestimate aphid populations! As I mentioned last week, even though there is still new growth developing aphids are scattered through the canopy this year. In some cases most of the aphid infestation is not near the top of the plant.

Some of you have observed that there are few aphids in late planted replants and pea beans this year. One hypothesis, and it's only a hypothesis, that might explain the lack of aphids on young beans relates to aphid movement from buckthorn. Because of warm weather early this spring, aphid movement to soybean probably occurred early this year, in some areas before many soybeans were emerged. Observations indicate initial soybean aphid populations in soybean were scattered and low. Predation and parasitism on these scattered aphids was very high in most cases. Even small fields near buckthorn were slow to build and supply winged aphids for other fields, including the late planted soybeans that are usually preferentially colonized. Movement of winged aphids produced early this season seems to be mainly to un-infested plants within the same field.

Steve Commerford made an observation in soybeans that had been replanted because of hail that might support the above hypothesis. He noticed that small beans in fields that were replanted into a remaining stand had numerous aphids. Those in fields that were worked and replanted did not.

We are just now starting to see the beginning of the annual widespread dispersal event, also delayed because of later than typical average soybean development. Populations within and between can be expected to change rapidly.

soybean leave with aphids

Pyrethroid insecticide resistant?

For those of you in the area with pyrethroid resistant aphid populations during 2015, it unfortunately appears the problem may not have gone away.

At the SWROC, three days after treatment with a normally effective rate of a pyrethroid insecticide (1.6 oz of Warrior II in this case), scattered areas of aphids survived and are reproducing. These areas may represent genetic clones from individual colonizing females.

This was not a replicated experiment but the results are similar to those in 2015. Large areas of dead aphids and other smaller areas of live aphids, even near the top of the plant.

We are working on trying to better understand the scope of this problem. Check your work 3-4 days after any insecticide treatment and check multiple areas of the field.

A brief public service message

Just because someone sends an electronic picture of a plant heavily infested with aphids, or a corn leaf with spots on it,  does not mean there is an economically threatening problem in your field; maybe not even the one where the picture was taken. Sales marketing has always been aggressive. The internet has made it stronger and faster. Caveat emptor.

Happy trails,

Bruce Potter