SW MN IPM STUFF 2017 Issue 8

7/7/2017

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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 http://swroc.cfans.umn.edu/weather

We have picked up some heat recently, and, as often happens in Minnesota summers, rainfall has become variable with some areas needing a shot of rain. For the past few days, moisture stress was visible on corn growing on coarse textured or compacted soils.

On June 30, soil moisture was slightly below historical average. As of July 7th, 1010 degree-days (Base 50°F) have been accumulated. This is about 2-3 days behind average. There have been 8.68” of precipitation compared to a 7.83” average over the same period. The hot dry recent weather might help reduce any armyworm problems mentioned last week. We are starting to find a few small armyworm larvae from the June 22nd flight. It will help minimize, at least temporarily, foliar fungal diseases. 

Harvest of canning peas is underway and the first local sweet corn tassels showed up on the 4th.

SW MN small grain cereal crops have a few more wheat stem maggot than usual. Other than susceptible varieties in variety trials, diseases have been relatively light at the SWROC. Winter rye and wheat are mature or nearly so and harvest of both winter and spring planted crops should be early this year.

When you have too many bees

And the hive gets crowded…a queen and many of the workers leave the hive to start a new colony and a new queen and some workers stay behind. These swarms are a natural phenomenon, regulated by chemical communication between bees, as bee populations in the hive increase during the spring.

Sometimes more than one new queen survives and the hive may produce more than one swarm. The queen(s) remaining behind needs to mate before producing eggs. Fortunately, each spring, drones (males) are produced as the hive’s population increases. Although they do not gather nectar, these drones do a bit of work in the hive when needed contrary to some bad press.

After they leave the hive, these swarms rest temporarily close to the hive they left until scout workers find a location for a new home. We found a swarm at the SWROC on June 30th. This swarm probably absconded (left) from a hive pollinating an organic radish seed production field next door. I found their resting location interesting. The swarms are not exceptionally aggressive, but the workers are protecting the queen in the center of the mass of bees. I pushed the issue as I took this quick video: https://youtu.be/UYjwaF58k0w.

I imagine Jim Fowler had similar thoughts on Wild Kingdom; right after Marlin Perkins talked him into dangling an antelope carcass in front of crocodiles, leopards and other cuddly creatures

Beekeepers try to minimize swarming because swarming reduces colony size and therefore a hive’s potential honey production.

Alfalfa

Potato leafhopper populations are increasing in many fields.

Black bean aphid on burdockFigure 1. Black bean aphid on burdock. The white bits along the nymphs' abdomens are a waxy exudate

Edible bean

I’d be watching potato leafhoppers closely. I am seeing some large populations of black bean aphid on burdock. As their name implies, this dark aphid species also feeds on bean.

Wheat

I had a report and some pictures of very heavy aphid populations in a WC MN wheat field today. This field was still milk stage. I don’t have many details as of yet. Typically, treatment of aphids after heading does not pay.

Corn

Early planted corn is up to 11 collars. Most of the crop is looking good very good with good color now. Early planted, short season hybrids will start to tassel soon. 

I have not observed any emerged corn rootworm beetles yet. I typically see the first beetles about the same time the earliest field corn starts to tassel. We will be digging corn roots to evaluate damage in the corn rootworm experiments at the SWROC once we see beetles emerging. 

First generation corn borer moth activity has been very low statewide (https://www.vegedge.umn.edu/moth-data/ecb-info). I have not found any shotholing from corn borers at the SWROC in corn without a Bt corn borer trait. Univoltine flight should be starting soon.

Corn diseases have been light to this point. We have seen a small amount of common smut. Areas that received hail should be expecting Goss’s blight on susceptible hybrids. Hot, dry weather will help limit the development of this bacterial disease.

potato leafhopper nymph on soybeanFigure 2. Potato leafhopper nymph on soybean.

Soybean

Most soybeans are starting to look good and many early-planted, narrow rows are canopied or nearly so. At the SWROC, early-planted soybeans have V7 (seven trifoliate nodes) and are at R2 (full bloom). Soybeans are most sensitive to defoliation and other stresses during reproductive stages, particularly pod fill.

Potato leafhopper are very abundant in some soybean fields. Potato leafhopper do not generally impact soybean yields in Minnesota, particularly once soybeans pass the vegetative stage. The wedge-shaped potato leafhopper nymphs are similar in color to soybean aphid but much more active.

a very colorful thistle caterpillarFigure 3. A very colorful thistle caterpillar. Photo: M. Bruyette

Defoliators

This is a very good year for thistle caterpillars. I have not heard of any populations of larvae that are close to yield-threatening. However, in a couple of weeks, we should be seeing an abundance of painted lady butterflies. There will be another generation of 

larvae this summer that may cause some issues in soybeans, sunflowers and hopefully, Canada thistle. Economic crop losses are rare. Seeing large numbers of the spikey, sometimes colorful, painted lady larvae reminds me of a 1970’s punk rock concert.

oblique banded leafroller on soybeanFigure 4. Oblique banded leafroller on soybean

Another larva has been observed rolling soybean leaves, similar to thistle caterpillars. These small, smooth, green larvae are oblique banded leafrollers. The adult is a moth belonging to the family Tortricidae. The larvae feed on several plants in addition to soybean, most in the rose family. While I have not seen any populations large enough to hurt soybean yield, oblique banded leaf rollers can be pests of apple and some other fruits.

green cloverworm and damageFigure 5. Green cloverworm and damage

Green cloverworm larvae are present as well. Usually the following generation is the one that causes concern.

Some soybean growers in EC and SE MN may be seeing Japanese beetles and the distinctive defoliation they cause. At this point, I am not aware of any significant populations of this invasive insect in SW MN but suppose it is only a matter of time.

There will be additional species of defoliators appearing as the season progresses. Soybeans respond to defoliation, regardless of the source, the same way – a loss of photosynthetic area. The difference between leaf loss from insects or disease compared to herbicide injury or hail is that insects and disease can continue to defoliate over a longer period. Lump the defoliation from multiple species and causes together when assessing the potential for yield loss.

Japanese beetles and damage to soybeanFigure 6. Japanese beetles and damage to soybean. These are southern IA beetles. Photo: C. Sinn

Fortunately, soybeans have spare leaf area and can stand considerable defoliation. An insecticide treatment during soybean reproductive stages may be economic if defoliation reaches 20% and insects are still abundant and actively feeding.

Base defoliation estimates of the entire canopy. Average the percent leaf loss on upper, middle and lower canopy leaves. Since insect populations are often highest on field edges, make sure you sample plants through the field.

This defoliation guide is based on feeding patterns of several species and may be useful to help you estimate defoliation: http://www.extension.umn.edu/Agriculture/soybean/pest/visual-guide-for-estimation-of-soybean-defoliation/

Soybean aphid watch 2017

If one looked hard enough and long enough, most soybean fields with trifoliate leaves probably have a few aphids. Natural enemies are helping slow growth, and the indicator populations I am watching are doubling at about once every week. This could change as/if the percentage of plants infested with soybean aphid increases.

dark soybean aphid has been parasitized by a parasitic waspFigure 7. The dark soybean aphid has been parasitized by a parasitic wasp.

Parasitic wasps have been playing a larger role in aphid control the past few years. The black aphid mummies caused by of Aphilinus sp. parasitism are starting to show up now. Other wasp genera create tan soybean aphid mummies.

Before applying an insecticide, think about disrupting efforts of the beneficial insects out there working on the behalf of soybean farmers…actually the beneficial insects are very self-centered but famers benefit nonetheless. 

Herbicide injury

Sadly, I am hearing about more cases of dicamba injury to soybean. There are some drift issues with other chemistries as well.

dicamba injury to susceptible soybean in the SWROC herbicide trialsFigure 8. Dicamba injury to SUSCEPTIBLE soybean in the SWROC herbicide trials. This injury is relatively minor and may or may not impact yield.

Unless they contain the Extend trait, soybeans are extremely sensitive to dicamba. Very small amounts of dicamba as drift or contamination from spray tanks or booms can produce symptoms.

In some cases, the reason for off target damage is obvious. In others, at least initially, the reasons for the damage are less clear. Unlike some other herbicides, dicamba injury does not show up until new leaves appear. 

For information on drift complaints, see the Minnesota Crop eNews: suspect-pesticide-drift-what-to-do.

Hopefully, there are few of these problems out there...

Monsanto has a hotline for dicamba concerns at 1-844-RRXTEND

New soybean scouting technique under development?

green cloverworm, thistle caterpillar, potato leafhopper and possible imported longhorn weevil on fron of a four-wheelerFigure 9. Green cloverworm, thistle caterpillar, potato leafhopper and possible imported longhorn weevil detected with novel soybean scouting technique. Photo: D. Throat

I received a picture from “Deep Throat” over on the eastern front. He was using the front of his four wheeler to determine the insect fauna in soybean fields. While his method does have some merit as a detection tool, a lot of work remains. He still needs to calibrate his method to account for several variables including vehicle speed, canopy height and moisture and wind direction.

Of course an entirely new set of calibrations would need to be done for 2-wheel scouting vehicles and pant legs. The latter has already had some preliminary work done; if your plant legs are sticky coming out of a soybean field you are likely past the economic threshold for soybean aphid.

Pest Management Field Day at the Southwest Research & Outreach Center August 9

We have a pest management field day planned for August 9th from 9:30 AM to 12:30 PM. The event will cover soybean insects, SCN, corn insects, and weed management.  After the tours and a lunch, there will be the opportunity to discuss future research and education needs. This field day is sponsored in part by Minnesota Soybean.

Other UM SWROC Events

Additional Extension events you might be interested in include: Field school for ag professionals (St. Paul Campus)

Happy trails,

Bruce