SW MN IPM Stuff 2016 Issue 10

Volume 19, Issue 10 - July 1, 2016

This newsletter is also available in a print-friendly pdf format: SW MN IPM Stuff 2016-10

This newsletter and the advice herin are free. You usually 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 Southwest Research and Outreach Center (SWROC), a little spot about two miles west of Lamberton, MN can be found at the SWROC weather webpage.

We have received 945 degree-days (base 50°F) and 8.15 inches of rainfall May 1 - June 16. While degree-day accumulations are 100 more than long-term average for this time of year, we are now only slightly above average in precipitation for the period. There were an additional 105 degree-days (base 50°F) accumulated from April 11-30. This is reflected in the growth of the crop. The largest corn on the SWROC is averaging V12 and most beans are flowering and at R1-2. As corn and soybeans enter reproductive stages, any stress will become increasingly detrimental to yield.

I was able to visit areas to the north that I had not been through for a while. While most of the crop in MN is in good shape, there is a large area in SW, SC and C that has had issues with flooded soils.

Replants of hail and drown-outs in Brown and Redwood Counties are nearly complete and soybeans are starting to emerge in the fields replanted last week. Driving through parts of Martin County last night, folks were busy re-planting drowned out spots.

Now comes the part of summer that is often a bit nerve racking for western Minnesota farmers. Will the rains continue on a timely basis? Parts of West Central and Central MN are in need of rain now and moisture stress is starting to show up in areas of SW MN with gravel or sandy subsoils. In spite of all the rain at the SWROC, corn was beginning to roll during the heat of day where gravel and soil compaction are involved.

 

ipm10a

Figure 1. Spider mite damage to field edge of soybean in 2012

Ken Fransky reported grasshopper nymphs were easy to find in one of the drier areas of the state but not yet at economic levels. Yes, grasshoppers come to mind in dry weather, but typically they need several dry years to build populations. Alfalfa fields are often good indicators of grasshopper populations in an area.Remember that poor root systems from compaction, disease or insects can increase moisture stress. 

Roadside haymaking is in full swing. Mowing of road ditches forces grasshoppers into the edges of crop fields. Spider mites also move out of their over wintering areas as ditch hay is made. Watch for symptoms of spider mite feeding on the edges of corn and soybeans in areas with moisture stress. Over the past few years we have been seeing spider mites under cooler and wetter conditions than typical.

ipm10b

Figure 2. Potato leafhopper adult. Not the bright green color and wedge shape. Very active insects and may need to be counted as they leave the net.

Sad News

When I logged into the UW Extension Ag Weather website, I noticed a message stating that due to a lack of funding, the corn borer, alfalfa weevil and other insect degree-day maps are no longer functioning properly. Hopefully this useful resource can be continued in some form.

Alfalfa

We are seeing economic populations of potato leafhoppers in some SWROC alfalfa.

ipm10c

Figure 3. Potato leafhopper nymphs. These are also active and move sideways when disturbed. Cute but destructive little critters.

The treatment threshold for potato leafhoppers is based on leafhoppers/sweep and alfalfa height.Remember to scout edible beans and other sensitive crops too. Potato leafhoppers are often found in soybean where damage is rare and usually limited to seedlings.

Small grains

Insect and disease management in the southern part of the state is over for the year. Pre-harvest intervals for pesticides are at play now.

ipm10d

Figure 4. Bacterial streak infections on wheat

Stripe rust is present in U of MN variety trials spring wheat without fungicide treatment. Incidence and severity is nowhere near those seen on susceptible winter wheats. 

Fusarium head blight is present but at this point incidence and severity appear low.

Oat crown rust was severe on many varieties in the trial as well. The dark spores of the teliospore stage have started to form. The abundant rainfall and wet weather provided the environment for severe bacterial streak on some varieties in the trial. Fungicides are not effective on this bacterial disease. 

Corn

For updates on European corn borer and corn earworm moth flights, see black light and pheromone trap data at U of MN VegEdge. Based on degree-day accumulations, the flight of univoltine corn borer strain should be underway in SW and parts of WC MN.

We are picking up a few armyworm and black cutworm moths in the light trap now. These are most likely the offspring of moths arriving here earlier this spring. Neither species has caused any widespread damage this growing season.

ipm10e

Figure 5. Early instar larva. Note head capsule and three pairs of thoracic legs. This one looked lonely when we floated it out of a corn root system.

We have examined root systems and floated larvae from only a few plants at two SW MN corn rootworm research sites. Those root systems checked have very feeding and few larvae. These sites were predominately western corn rootworm in 2015. I suspect the heavy rains and flooded soils had an impact on larval survival, but rootworms have a way of surprising you. Low rootworm populations are bad for entomological research but good for farmers. It would be better news for farmers if drowning a corn crop wasn't the reason for rootworm reduction. Dr. Ken Ostlie has seen higher rootworm pressure and corn rootworm beetles beginning to emerge in research sites located in drier parts of the state.

Soybeans

IDC is an issue in many fields this year. Soybean cyst nematode are associated with some of these.

Soybean aphids are becoming easier to find in many fields. Unlike last year at this time, infestations are very spotty and aphid populations are low here. Predators have been finding and feeding on many of the small colonies as they start to develop. Using the 250 aphid/plant economic threshold will give your insect allies a chance to prevent soybean aphid problems this year.

ipm10f

Figure 6. Bio-control and the aphid response

We are in the middle of a run of weather favorable for soybean aphid reproduction. In case the lady beetles and other predators and parasites of aphids can't keep up, it might be wise to start checking a few early infested fields to track how quickly aphid populations increase. After the 4th of July holiday is soon enough if you have not already started. Replanted soybeans might present an aphid scouting and management challenge later this season.

One exception to the low soybean aphid population is an experiment at the SWROC where we simulated volunteer soybeans. These soybean plants were infested early. Although the soybeans are shaded and tall, aphids are doing well under the corn canopy. Most plants are now infested and many plants have over 200 aphids. This is definitely not representative of aphid populations in any fields in the area.

IPM student interns took the above photo while sampling aphid and predator populations. On the leaf petiole to the right dark, shriveled aphids, probably killed by aphid midge larvae, can be seen. If you look close, you may see a few tan mummies of aphids killed by parasitoid wasps. The ants near the flower on the bottom have been tending aphids for the sugary honeydew waste they produce and have been moving the aphids from predators. These ants are about to be frustrated. A couple winged adult aphids can be seen at the top. The stress from crowding and predators has caused this colony of aphids to produce a generation that will have wings and leave.

Many of these insect interactions are entertaining to observe. Your opinions may differ, but it seems that there is just as much action as a Twins game and much more than any soccer match. 

Events

Internet search exercise: Why are many insects that feed on milkweed orange?

We have several species of Asclepias, more commonly known as milkweeds, blooming now. The adults of many species that feed on milkweed are orange in color, the monarch butterfly being one example. What's with the orange color?

ipm10g

Milkweed beetle on common milkweed

ipm10h

Milkweed longhorn beetle on common milkweed

ipm10i

Milkweed bug on swamp milkweed. This species of milkweed is found in moist soils

ipm10j

Not orange, but I find them on common milkweed often. Earwigs are abnormally abundant in SW MN this year.

ipm10k

No insects are visible in this photo but the flowers of butterfly milkweed are orange and the plant is a host for many of the same insects as common milkweed.

Happy Trails,

Bruce Potter