SW MN IPM Stuff 2020 Issue 11

Volume 23 number 11 -  7/01/2020

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

This newsletter is also available in a print-friendly pdf format: PDF iconSW MN IPM Stuff 2020-11

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.   

The SWROC missed the excessive rainfall that some have experienced.

Soybeans

bright orange soybean gall midge larvae on a soybean stemFigure 1. Soybean gall midge larvae. Photo: Noah Stavnes, UMN

After 15 days without a capture in emergence cages, overwintering generation soybean gall midge adults were captured at a Rock County site on 6/30. A large percentage of larvae from earlier emerging adults are now orange third instars (Figure 1). When we examined soil samples from this year’s soybeans, we found mature larvae that had started to leave plants but had not yet pupated.

These later emerging adults indicate a likely overlap in overwintering and 1st generations. It also illustrates the potential difficulties in timing an insecticide application for adult control.

Signs of soybean gall midge infested stems (Figure 2) are easy to find in infested fields. In Rock Co. we’re starting to see gall midge infested plants wilt and die.

soybean stems with brown, dried out bases and a single soybean gall midge larva on one stemFigure 2. Lower stem symptoms of soybean gall midge infestation. A late instar larva ready to leave the plant can be seen in the plant on the left. Photo: Noah Stavnes, UMN

While soybean gall midge and their damage to soybean have made a return appearance in extreme Southwest Minnesota, it is not necessarily true for other areas infested in 2019. Although overwintering larvae were found in soil samples, no adults have been captured in emergence cages and larvae have been found in soybeans (including volunteer soybeans) at a WC MN study site.

I am seeing quite a few green cloverworm moths in southern Minnesota now. While these could be recent immigrants, it is more likely that they are the offspring of moths that migrated into Minnesota earlier this spring. Although these insects were abundant in 2019, green cloverworms often succumb to diseases and parasites. Wait until economic threshold levels (20% defoliation) are reached.

Alfalfa

I sent the student interns out to check on the potato leafhopper populations at the SWROC. The organic alfalfa here usually provides a good perspective on the amount of damage these insects can cause in any year. They came back with bags full of potato leafhopper adults and nymphs; well over 1/sweep.

Potato leafhopper reproductive rates have been phenomenal this spring. If you haven’t checked your alfalfa lately, it might be worth the effort.

Some alfalfa producers have gone to an automatic insecticide spray each cutting program. While this approach simplifies management decisions, overreliance/overuse of insecticides can cause other problems. Reports of pea aphid population explosions after synthetic pyrethroid insecticide applications have become more common in recent years. There are two possible reasons, removal of predators controlling the aphids or insect resistance, perhaps both. If you apply an insecticide it pays to check your work.

Armyworm Update

MN counties map with marking several counties in central and soutehrn MN and one in NW MNFigure 4. Counties with reports of true armyworm infestations at or near economic threshold levels.

Reports of armyworm damage and insecticide treatment continue to come in (Figure 4). The shaded counties do not indicate the relative number of problem fields. There are likely some infestations in counties that are not shaded as well. 

Over the past week, I’ve been receiving pictures of fuzzy white objects in corn and wheat fields. Andy Linder sent a great shot of the phenomenon from an oats field. The deceased armyworm is covered with cocoons of a parasitic wasp (Figure 5). 

Most of the damage is caused by large larvae. The heat will push the development of larvae and we should start to see feeding by the larvae slow and pupation start soon.

armyworm covered in fussy wasp cocoonsFigure 5. Cocoons of parasitic wasps on armyworm. Photo: Andy Linder

Make sure armyworms are still actively feeding before you treat a field. This morning, I received a picture of armyworm pupae from Keith Pillatzki in Yellow Medicine County. Armyworm pupae are found below the soil surface.

Have a great Independence Day!

Happy trails,

Bruce Potter