SW MN IPM STUFF 2018 Issue 10

Volume 21 number 10

8/9/2018

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 2018-10

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

The 2018 growing season remains above average in precipitation. For the period May 1 to August 8, we are 18.92” vs. the 11.71” historic average or 7.2 inches above average! The April snow is not included in this moisture tally but it also helped slow things down too… a tough spring here.

Crop development continues to be rapid. It is still a warmer than average growing season, and we are now 182 GDDs (a week) ahead of long-average in corn growing degree-days with 1856 GDDs vs. 1674 GDDs.

Early planted, shorter maturity corn is starting to dent.

Soybeans are mostly R5 with replants later in development. Looks like another week of good weather for crops in SW MN and ideal weather for soybean aphids.

Slow drying grain and weeds emerging through canopies have delayed small grain harvest in some fields.

On-line resources for the identification of stuff

Sometimes I need to identify insects, plants, and mushrooms that may or may not be crop pests. In case you feel the urge, here are a few of the resources I use.

Insects

Mushrooms (Please do not eat one based off an on-line ID)

            Mushroom world http://www.mushroom.world/mushrooms/list (many ways to search)

            USDA forest mushroom guide https://www.fs.fed.us/nrs/pubs/gtr/gtr_nrs79.pdf

Flowering Plants

            Minnesota wildflowers https://www.minnesotawildflowers.info/ (worth the $5)

            Wildflowers of the US  https://uswildflowers.com/ (sections for multiple states)

            USDA/NRCS Plants database https://plants.sc.egov.usda.gov/java/ (distribution   maps, invasive species, noxious weed lists)

Some things to consider right about now

Corn

  • Observations in corn at the SWROC appear to show a relationship of bird cherry–oat aphid populations with corn that experienced earlier stress from excess water. Nitrogen stress is more visible in areas with higher aphid population densities, and there are likely differences in other sap constituents too. This is not easy to document without any good controls but it might be worth a future experiment. In any event, watch fields with high aphid populations for stalk rot later.
  • In drier areas, check two-spotted spider mite populations, particularly where a fungicide and insecticide was applied earlier. I have not heard of any problems in SW MN, but it will still be worth a look. From the pickup, the symptoms of a spider mite infestation can be confused with drought.
  • We might get a short break once corn begins to dent. Then, as corn nears harvest, a look at stalk rots and notes of other crop pest issues for future crops.
  • Some areas of southern MN are seeing an unusual amount of Physoderma. Al Van Grow has observed fields where breakage at lower stalk nodes has already begun. Late-planted corn in high rainfall areas are reporting the most problems. A corn disease that deserves a closer look.
  • Northern corn leaf blight took advantage of cool wet weather and susceptible dent and sweetcorn cultivars.     
  • Stephan Melson has discovered a new challenge in the use of yellow sticky traps for adult corn rootworm adult monitoring. It seems that raccoons are knocking over the corn plants holding the traps. Based on the amount of rain and flooded fields in his area, the coons may perhaps, be mistaking the bright yellow squares for Mae West life vests.
  • We will conduct a statewide survey for European corn borer and several corn diseases again this fall. The goal is to look at the current year’s potential losses and future risks.                                

Soybean

  • Scout for soybean aphids and defoliating insects. Note diseases.
  • Symptoms of bacterial blight, Phytophthora, white mold are present in some fields. SDS symptoms should start showing up soon.
  • Areas with symptoms of anthracnose stem blight are showing up in some soybeans this year. They are associated with SCN in one case.
  • A new generation of SCN females is beginning to emerge from soybean roots in the Lamberton area. Next week could be a good time to scout.
  • Some of the symptoms observed with stem canker (Diaporthae) have been unusual this year. Circular areas with plants killed before R3 (Figure 1) and malformations in the stem lesions (Figure 2).
  • Two-spotted spider mites observed in drier areas of NW MN. Over on the eastern front, “Deep throat” has been seeing non-economic levels in some fields to the east as well. Something to watch for in insecticide treated fields.
  • I am seeing a large number of green cloverworm moths at the SWROC now. Will see how well their egg and larval survival goes.

Soybean plants killed by stem canker

 Unusual tissue development in stem canker lesion

A Midgework orange?

Ryan Anderson provided some Rock County samples of soybeans that were infested with small, pale to bright orange gall midge (Diptera:Cecidomyiidae) larvae (Figure 3). These matched the description of the insects causing the recent kerfuffle in IA, NE, and SD. Symptoms include: soybean plants brittle near the base, dark lesions on the stem, hollowed pith, larvae under the stem epidermis, and ultimately plant death. In previous years, these midge infestations were associated with injury from hail and disease. Recently, the relationship of infestations to previous stem injury became less clear and this includes the Rock County infestation. Are these insects able to infest plants directly?

Other members of this family of flies include the beneficial aphid midge, a predator on aphid species, and the Hessian fly, a pest of wheat.

For several years, I have occasionally noticed similar larvae associated with hail-injured soybean stems but did not notice any yield limiting problems and paid little attention to them. I recently visited a McLeod County field where Karl Nesse found small, orange maggots associated with white mold (Figure 4).  After looking at the larvae from both locations, I am not sure they are the same species. 

I don’t get out much, but it does not appear that Minnesota is seeing the same number of problems as neighboring states.

I will be sending the larvae from both locations away for identification. More later.

Gall midge larvae and damage    Midge larvae

Soybean aphid watch 2018

Economic threatening infestations have become increasingly widespread. Amazing how this has progressed from a small area in central Minnesota, I view these aphid outbreaks similar to the momentum of a snowball rolling downhill - gaining mass and velocity along the way. Something to consider if predictive models for soybean aphid outbreaks are developed.

At this point in an aphid outbreak, many of us are starting to get a little edgy. It is advisable to delay insecticide application as long as practical where alate aphids are active. I am hoping that, like most years, winged (alate) aphid movement should begin to slow next week as more fields reach the mid-R5 stage. Late planting and replanted areas in southern MN complicates things a bit. Fields with aphid issues left untreated and late soybean maturity in southern MN may extend the period of long-distance alate aphid movement this year.

At this time of year, within a geographic area, soybean aphid infestations are greater in the later planted/maturing soybean fields. Aphids arriving late season can cue in on the edges provided by unclosed rows. In a SWROC date of planting study, the planting dates after mid- May reached threshold while earlier planting dates did not.  A Martin County consultant sent a list of dates and maturities showing a similar trend.

Do not give up too soon. Reports from consultants in WC Minnesota indicate that some mid R5 stage soybean fields have recently seen increased aphid populations. In some cases, upper leaves were colonized by immigrating females and in other cases, small lower canopy populations have begun to rapidly increase. Aphid populations can increase rapidly in late R5 to early R6. Use the average 250/plant threshold until R6.

Water damaged fields present another dilemma. Will aphid control provide an economic benefit? Uneven soybean growth will lead to uneven aphid populations. If you want to protect yield, base your decisions on economic injury level and economic threshold.

Pyrethroid insecticide resistance continues to be a problem in some areas. There have been several reports of poor control with synthetic pyrethroid insecticides from WC Minnesota (the Koch lab is bio assaying some of these). Additionally, pyrethroids did not perform well in a 2018 SWROC insecticide study.

While using a pyrethroid insecticide may be a risky decision in central and Western Minnesota, they may perform well in other areas. A consultant reported adequate control with lamda-cyhalothrin and with bifenthrin in Dakota County. Your mileage may vary.

Scout fields 4 - 5 days after treatment to evaluate control regardless of product used.

Reinfestation by winged aphids is an issue in this year’s SWROC insecticide study, plots treated with chlorpyrifos (e.g. Lorsban Advanced), a short residual insecticide, were re-colonized by winged aphids within four days.  Check fields treated more than 10 days ago with chlorpyrifos (and other products)!  You do not need to spend a lot of time at it unless you find increasing aphid populations.

Although you cannot know which, if any, insecticides immigrating aphids were exposed to, it is still best to avoid retreating a previously treated field with the same insecticide.

If you are wondering why you have aphids in an insecticide treated field there are several possible reasons. 

Poor coverage: 1) Aphids are well controlled in the upper part of the plant of the plant but not low in the canopy. Typically caused by wrong nozzles, pressure, water volume, boom height (including aerial) or by spraying during high winds or inversions. 2) Areas near field obstacles or skips in field with aphids. The application missed these areas. 

Re-infestation: Small aphid nymphs, often near winged adults, or aphids all at the same stage of development (Figure 5). Aphids present on the uppermost leaves, seldom as low as mid-canopy of larger soybean plants. Some products (e.g. dimethoate, chlorpyrifos) have very short residuals. For many insecticides (e.g. pyrethroids), leaves that develop after application will be free of insecticide. Low insecticide use rates and poor application techniques minimize the effective life of insecticides on soybean plants.

Pyrethroid insecticide resistance: Several aphid development stages are present throughout the canopy with ongoing reproduction (Figure 6). If alate adults are present, there will be large nymphs and wingless adults present. Particularly in heavy infestations, it is possible for a plant to have the winged aphids coming and going. In the cases I have seen, control varies by groups of plants or small areas of the field. It is likely that the areas of poor control are due to the clonal offspring of an insecticide resistant female. These resistant aphids are the biotype that will continue to colonize plants in the field.

Two alate aphids colonizing

Leaf from research plot with a pyrethroid insecticide failure

Happy trails,

Bruce