SW MN IPM Stuff 2019 Issue 3

Volume 22 number 3


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 2019-03

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.

This spring of 2019 in SW Minnesota and elsewhere has not been easy for fieldwork. Corn and soybean planting is well behind average. Around here, the earliest planted fields of corn are up enough to row and there is corn that has not yet been planted. There will be some stand issues due to field prep, wet soils, poor control of seed depth, and over-enthusiastic planting speed.  Recent rain may help moderate some, but not all, of these issues. Uneven emergence will make diagnosing stand loss caused by insects and disease more difficult.

I’m more than a little uncomfortable talking about scouting crop pests this spring. Folks have been busting it whenever they get a few rain-free days in a row. Like many of you, planting has been slow and very few of my corn experiments are in the ground yet. As far as scouting goes, it might be a few days before anyone will be able to walk or drive fields around here without a mud bath.

Immigration problems

This spring’s southerly winds have been bringing crop pests back to Minnesota for the summer.

Black cutworm

This is a very active year for black cutworm flights into southern MN. If cutworm reproduction and survival are good, there can be fields with small crops and large cutworm…advantage cutworm.

A crop consultant friend in SD emailed a question earlier this week. His suspicions were correct - the economic thresholds for black cutworm in corn are much higher for fields where corn seed is still in the bag. Anyhow, the most recent black cutworm report can be found at: BCW Report #6 

True armyworm

Black light trap and pheromone traps have been picking up an influx of true armyworm moths the past week. Not a threatening migration at this time but we will continue to monitor flights of this insect over the next few weeks. Populations of true armyworm larvae are often higher in cool, wet seasons.

aster leafhopperAster leafhopper adult

Aster leafhopper

The southerly winds brought a very large flight of aster leafhoppers to some SW MN winter rye fields, including those at the SWROC. If they have fed on an infected plant before migrating, the leafhoppers can be capable of transmitting the aster yellows MLO to susceptible crops and other plants. Although wheat plants may be infected with aster yellows, some of the vegetable crops and members of the aster family are much more susceptible. At this point, aster leafhoppers are at much lower levels than the massive migration that occurred a few years ago.

English grain aphid

We had low numbers of these insects in winter cereal crops earlier but large number of winged reinforcements arrived at the same time as the aster leafhoppers. Others species of cereal aphids might be more efficient vectors but english grain aphids can also transmit barley yellow dwarf virus to cereal crops.

Alfalfa insects

Alfalfa growth has been fairly slow and so has the growth of insect populations. For thresholds and scouting information on alfalfa insects see: Alfalfa insect scouting and action thresholds.

Alfalfa weevil numbers are low in the SWROC alfalfa I checked this week. Adults and early instar larvae are present now.

pea aphidPea aphid

Pea aphids
are present but numbers are low in the sweep net samples I have taken this spring.

In previous years, poor pyrethroid insecticide performance has been reported from a few west central MN and eastern SD fields. The resistance issues, if real, are likely related to insurance insecticide applications for alfalfa weevil and potato leafhoppers. Might pay to scout and use economic thresholds to reduce financial and management risk!  

I’d appreciate hearing about any pesticide resistance issues you might come across.

More wheels on the Bt wagon develop a wobble

European corn borer

Historically, the Bt toxins have been tremendously effective against the European corn borer (ECB). In 2018, some populations of ECB in Nova Scotia Canada were confirmed resistant to the Cry1F (Herculex I) Bt toxin. Resistance issues were limited to Bt hybrids with the single Cry 1F Bt toxin.

At this point there is no reason for MN corn producers to be overly concerned in 2019. The good news is that the ECB resistance is only in single Bt trait hybrid. Single trait Bt hybrids are/were being phased out. Another piece of good news for MN corn is that so far, unlike ours, the Bt-resistant Canadian ECB have been all been E strain. Our borers, both single and multi-voltine, are Z strain, responding to a different pheromone.

Northern corn rootworm

Cry3Bb1 and Cry 34/35 Ab1 Bt toxin resistant western corn rootworm populations are known for some time. A recently published paper in the Journal of Economic Entomology documents incomplete resistance of some North Dakotan northern corn rootworm populations to these two Bt toxins. In one case, there was increased survival on a Cry3Bb1 + Cry 34/35Ab1 pyramid compared to known susceptible northern corn rootworms.

If the Bt resistance and extended diapause trait are both maintained in northern corn rootworm populations, management of northern corn rootworm is more problematic. By the way, the authors also confirmed Bt resistance of some western corn rootworm populations to Bt, including the pyramid.

Calles-Torez, V., J. J. Knodel,  M. A. Boetel, B.W. French, B.W. Fuller and J. K. Ransom. 2019. Field-Evolved Resistance of Northern and Western Corn Rootworm (Coleoptera: Chrysomelidae) Populations to Corn Hybrids Expressing Single and Pyramided Cry3Bb1 and Cry34/35Ab1 Bt Proteins in North Dakota. J. Econ. Entom.

The handy Bt trait table has been updated to reflect these recent corn borer and rootworm findings.

These recent finds indicate the importance stewarding important pest management tools. Vigilance in scouting fields of Bt corn and survey efforts for corn borer and corn rootworms in MN are becoming increasingly important.

Future problems

It’s only the 1st inning and the game has a long way to go. I don’t expect the following predictions to be any more accurate than a weather forecast and neither should the reader.

This delayed planting season might cause some issues with pest management down the road. In areas where corn was planted over a long period, the differences in corn maturities can cause concentrations of corn borer and corn rootworm in particular fields. Later than normal maturing corn may see more pressure than usual from corn earworm and perhaps even fall armyworm.

On the positive side - this cool, wet weather is probably not doing the soybean aphids on buckthorn any good. Delayed soybean planting may limit the availability of hosts when the aphids need to move from buckthorn.


At least one slug survived the winter at the SWROC.

Watch corn and especially soybeans for slug issues in no till and other high residue situations. Slugs are very infrequent pests in MN field crops but Jim Boersma came out of retirement to email a reminder that slugs have been known to mess up some southern MN fields in the past.

The wet late summer and fall of 2018 fall should have been good for slug mating. Hopefully, the winter killed a large portion of the adults and any infestations starting from the more cold-tolerant eggs will be slow developing.

Wet weather favors plant pathogens. For soybeans that are planted, wet soils mean that there is a higher risk of Pythium and some Fusariums, including SDS, but so far, the VC stage volunteer plants I have looked at look seem healthy.

If cool weather continues, it may favor some foliar diseases like northern corn leaf blight and white mold but those problems are a way off yet.

Have a good Memorial weekend!

Happy trails,

Bruce Potter