SW MN IPM STUFF 2018 Issue 9

Volume 21 number 9

8/3/2018

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

A printer-friendly pdf version of this newsletter is available to download here: SW MN IPM Stuff 2018-09

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

Most corn is now blister to milk. 

Soybeans are R4 – R5 with replants later in development.

Some things to consider right about now

ALFALFA

  • Scout for potato leafhopper and plant bugs. Leafhopper populations are currently high in alfalfa and soybeans. Numbers tend to decline after mid-August.

CORN  

  • Rootworm beetle scouting using whole plant counts or yellow sticky traps. You may have already been able to make a decision on those fields with high populations. We are now at peak adult populations. These populations will soon decline and may have already started to do so in some fields.
  • Bacterial leaf streak symptoms were severe in some sweet corn and dent corn hybrids at the Southern Research and Outreach Center, Waseca. 
  • Start to walk away from this year’s pest management at dent stage corn.

SOYBEAN

  • Evaluate fields for fungal diseases for future year’s management.
  • Looking for SCN females on soybean roots is probably more effective at this time; soil samples for eggs will yield maximum numbers at the end of the season. Be consistent on crop, timing, and locations of the samples in fields you plan to sample over several years.
  • Scout for soybean aphids and defoliating insects.
  • If you notice any small, yellow to orange maggots (soybean gall midge) in the stems of wilting soybeans, please let me know.
  • Septoria brown spot in the lower canopy and bacterial blight are the most prevalent soybean diseases in most fields. These diseases do usually cause yield loss. I have received a few pictures of symptoms resembling frogeye leaf spot. The lesions were very scattered in those images.

When you are unsure of a disease diagnosis, obtaining a laboratory confirmation is wise. You can find the instructions for taking and submitting samples to the U of M Plant Disease Clinic at https://pdc.umn.edu/

Soybean aphid watch 2018

The summer migration continues. Fields with economic threshold (ET) level infestations (80% or more plants with aphids, average of 250 aphids/plant, and increasing populations) have now been observed over a wider area of Minnesota. In many fields, populations are increasing rapidly as winged aphids continue to colonize new fields. In other fields, populations have started to temporarily decline as large numbers of winged aphids leave.

Populations are still very low or not increasing in many fields. Small, sheltered fields and late-planted soybeans have usually been the first fields reaching thresholds as economic infestations develop in an area. Predator and parasite populations are helping in many fields. Large soybean fields, away from wooded areas, have been slower to develop.

Some early-treated fields have reached economic thresholds for a second time.  

Remember to check herbicide performance.

Do not assume that winged aphids cannot re-infest an insecticide treated field.  Watch for spider mites in treated fields.

As you look at soybeans in some areas that have become droughty or have already had an insecticide application, do not forget to look for increasing spider mite populations.

We sprayed two insecticide trials at the SWROC on August 2. If things look interesting, I will set aside some time for informal tours of research plots later this month.

Thanks to those of you that have been helping keep me up to date with this year’s aphid populations! I probably owe you a beverage.

Late-season soybean aphid information

Dense soybean canopies, early morning dews, aphids inconveniently scattered throughout the canopy, and other factors complicate late-season aphid scouting. 

It is very easy to underestimate soybean aphid populations within the canopy of large soybean plants. Pull soybean plants so you can examine the whole plant.

Calibrate your eye by closely counting a few leaflets of varying aphid densities. Learn what 10, 20, 100, etc. aphids look like on different leaf sizes. You do not need to count individual aphid. Instead, estimate the number per plant with the goal of deciding whether the field is averaging well below, close to, or at/above a 250/plant average. Re-calibrate your estimating skills occasionally. Once mastered, this skill allows you to estimate aphid populations on the plants you pull and quickly move through the field.

  • Shaking the plant before estimating aphid populations will help remove water droplets and dead aphids- seems to makes things easier for me anyhow.
  • Include in your estimate, the small, pale “white dwarf” aphids found in the lower canopy of early R5 stage soybeans.
  • Note disease, parasites, and predators.
  • Do not hesitate to enlist help if you have vision problems or otherwise cannot effectively scout fields.
  • Unless you are doing soybean aphid research, lighten up. For practical purposes, there is not much difference between 200 and 300. There is a difference between 50 soybean aphids and 250 or more though.

Time, weather, and other logistical constraints can force some compromises in scouting and timing of insecticide applications. There is no problem with this as long as the compromises are reasonable. For treatment scheduling purposes, you can estimate when fields will hit economic threshold by doubling the observed population every 2 to 5 days. Ground truth your calculations.

Try to enjoy yourself out there.  The 2018 aphid season will soon be history.

Happy trails,

Bruce