SW MN IPM Stuff 2019 Issue 6

Volume 22 number 6


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 2019-6

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.

Early-planted corn is 6 collars and soybeans V3. Crop development is well behind this in many fields. Barring weather anomalies, corn growth is very rapid after the five-leaf stage and, in the earliest planted fields, rows are starting to close. As soybeans reach the V3 stage, iron deficiency chlorosis (IDC) is starting to show up. If IDC appears in areas that it hasn’t been an issue before, it could be related to your variety, a temporary (hopefully) herbicide interaction,  and/or soybean cyst nematode (SCN).

Things are hectic this time of year. Regardless of the pesticide, avoid drift to sensitive crops.

Caenurgina looper mothFigure 1. Caenurgina looper moth

Things that go bump in the night

Drab, tan or gray, moderate-sized (1.25-1.5 inch wingspan) moths are very common in field edges and grassy areas now (Figure 1). Unlike most moths, both of these species are active during both daylight and nighttime.

Most of these appear to be forage loopers. However, there are two very similar members of the moth genus Caenurgina; the forage looper C. erechtea, and the clover looper, C. crassula. While there are typically some differences in wing patterns, the moths are so similar that a genitalia examination is required to make a species ID... and who has time for that.

Caenurgina looper larvaFigure 2. Caenurgina looper larva

The pin-striped caterpillars of both species feed on legumes and various grasses and there are reports of forage looper feeding on giant ragweed. The larvae (Figure 2) can be found in alfalfa and soybeans but by themselves are seldom abundant enough to cause economic defoliation.


Black cutworm

Dave Pfarr sent an image of one of the black cutworm larvae that were causing problems in a peaty, late-planted Nicollet county field. The larvae were nearly mature and matched development predicted by degree-days.

In spite of the number of moths picked up by pheromone traps this spring, we appear to have avoided any widespread corn injury from this insect. This may be related to the significant proportion of corn planted to Bt traits effective against black cutworm. Cold nights may have interfered with mating. Eggs or larvae or eggs may have survived poorly in water-logged low-lying areas. Finally, the late start of tillage in 2019 left large acreages of cropland attractive for egg laying and, as a result, egg laying was not concentrated in a limited number of fields. Whatever the reason, limited crop damage is almost always a good thing. Black cutworm management based on well-timed scouting can save input costs.

True armyworm

It is time to start scouting. Pay attention to areas where grasses have lodged on the edge of corn or small grain fields. Prime armyworm habitat also includes areas in corn where dense growth of grass weeds or grass cover crops occurred. Extension educator Claire LeCanne was finding armyworm in a cover crop experiment in SC MN, and, while not at concerning levels,  they have been easy to find in grassy areas at the SWROC. For more armyworm information see: Armyworm in corn

If you notice wilting corn, hop vine borer and common stalk borer have been active and may be the cause. These tend to be isolated problems and most often associated with field edges. Insecticide control is poor once the larvae burrow into corn.  

European corn borer (ECB)

Black light trap captures of ECB moths remain low. Based on degree-day models, scouting for the first generation multi-voltine corn borer flight should be starting soon in parts of the MN River Valley and SC MN. ECB development will lag behind in other, cooler areas of Minnesota. ECB degree-day development models can be found at VegEdge Degree-day Models & Forecasts. Focus on non-Bt hybrids and don’t waste time scouting 1st generation ECB in corn less than 15-inch extended leaf height.


painted lady butterflyFigure 3. Painted lady

Thistle caterpillar - When good butterflies go bad 

Each spring, “painted lady” butterfly adults migrate into Minnesota from the Southwest. The painted lady is a strong migrant and has a nearly worldwide distribution, which leads to another common name, “the cosmopolitan”.

Adults prefer to feed on nectar from flowers of members of the aster family. The larvae feed on thistles, thus the moniker “the thistle caterpillar”, and other members of the aster family. They also feed on members of the mallow and legume family, including soybeans. The larvae have the behavior of webbing leaves together with silk.

Apparently, painted lady larvae have been common in Nebraska and Iowa soybeans this year. Since the previous issue of IPM Stuff, there have been a few more reports of Minnesotan 1st generation thistle caterpillars, again from the SC part of the state. Stephan Melson and Dave Wirth have both sent images of larvae and damage. These reports have been from areas of southern MN where some soybeans were planted early, emerged, and large enough for the immigrating butterflies to lay eggs.

thistle caterpillarFigure 4. Thistle caterpillar

There will be at least one more Minnesota generation of thistle caterpillars this summer. During 2017, the 2nd generation caused widespread concern in MN. Based on this year’s populations in MN and states to our south, the second generation may provide some scouting entertainment later this summer.

While butterflies, including painted ladies, are generally considered an enjoyable part of the environment, they may be viewed less favorably when their offspring start chewing holes in crop leaves. There are several action thresholds for thistle caterpillar control. All of these assume that larvae are still present and will be actively feeding. Pre-bloom soybeans can tolerate more defoliation than soybeans in the reproductive and pod fill stages. Use 30% defoliation on smaller pre-bloom soybeans. On larger pod fill soybeans, it is still probably best to use the 20% defoliation guideline rather than numbers of larvae. That said, numerous larvae/plant might cause me to pull the trigger at a bit less defoliation.

Defoliation damage from multiple insect species can be combined when making a treatment decision based on defoliation. On the other hand, do not treat insects causing damage other than leaf feeding, soybean aphids for example, at a lower threshold just because you are finding a few thistle caterpillars.

Do not base a treatment decision on the worst plant or area of the field. The University of Nebraska developed a nice illustration on how to assess insect defoliation in soybeans (Figure 5.). It helps prevent focusing on the worst plants and leaves. Once you get yourself calibrated you can determine potential problem fields very quickly.

Some things to consider when deciding on an insecticide treatment for thistle caterpillar in soybean:

  1. Size of the larvae – Mature larvae are approximately 1.25 inches long. Most of the feeding is done by the large larvae; however, if most of the larvae are full grown, or if you see chrysalis in the field, damage may be ending. Make sure insects are still present in the rolled and webbed leaves before you apply an insecticide.

  2. The thistle caterpillar’s preference for the upper canopy and webbed leaves makes them very visible. Fear produces a tendency to overestimate defoliation.

  3. Area affected – If the larvae are mainly on field edges or in a particular area of the field, you do not need to treat the whole field.

  4. Larvae are usually concentrated on the upper part of the plant and not that difficult to control. Use a labeled insecticide at a labeled rate.

  5. Very small soybeans are more easily defoliated by low numbers of larvae/plant.

defoliation chartFigure 5. A guide for assessing soybean defoliation. Don't focus on the worst leaves. Used with permission from University of Nebraska Extension.

Soybean gall midge

The first 2019 MN report of a soybean gall midge adult was on June 20th! Three adults were captured in an emergence cage that was placed in a Rock County field that had a 2019 soybean infestation. This does mean the insect survived the winter. It does not mean there will be an economic infestation. More information will follow.

damage on soybean leaves from slugsFigure 6. Slug feeding on soybean. Close inspection reveals areas where the mouthparts of the slimy mollusks have rasped only partially through the leaf tissue. Feeding on the hypocotyl will kill soybean.


Don’t assume that holes in soybean leaves are caused by an insect. I am finding slug feeding injury in no-till soybeans at the SWROC (Figure 6). At this point the defoliation I have viewed is not economic. Insecticides will not work on the mollusks and can eliminate ground beetles and other predators.

Four legged insect mimics

Cutworms do not cut soybeans at a uniform height or attack single row portions of a field. Certain stages of small soybeans are a delicacy for deer and rabbits. Deer usually leave visible tracks while rabbits, unless they are the size of Harvey, do not.



Alaine Bellicot reported non-economic cutworm damage in some WC MN sugar beets. As larvae near pupation, black cutworm damage should soon be winding down.

White grubs

Caleb Hanson came across significant white grub damage to a West Central MN soybean field. Most white grubs spend three years in the soil as larvae. Most damage occurs in the second year.

Based on 2017 reports, some other areas of WC Minnesota could see some problem fields in 2020.


Scout armyworm in winter rye and lodged grassy areas adjacent to spring cereal crops. Scout aphids before heading. Assess foliar disease before and again at heading.

Happy trails,

Bruce Potter