SW MN IPM Stuff 2016 Issue 16
August 19, 2016
This newsletter is also available in a print-friendly pdf format: SW MN IPM Stuff 2016-16
This newsletter and the advice herin are free. You usually get what you pay for.
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 https://swroc.cfans.umn.edu/weather.
We have received 2031 degree-days (base 50°F) and 17.70 inches of rainfall May 1-August 19. An additional 105 degree-days accumulated from April 11-30.
As of August 19, we were 115 degree-days (5 or more days) above long-term average in degree-day accumulations. We were 3.74 inches above average in precipitation for the same period. We received additional rainfall and some hail on August 18. There were lodged sweet corn and soybeans in areas with the higher winds.
As is typical, the corn crop has responded to the warm weather and most fields are dough to dent stage. Most soybeans are R5 with some early planted and shorter season soybeans in R6.
Soybean Aphid Watch 2016
I apologize if I created any confusion on my last newsletter. Although yield loss can still occur later, use the 250/plant threshold only until the soybean R6 stage begins. Many fields are at or near that point.
Soybean aphids and other insect pests are able to reduce soybean yield until the R6.5 stage (yellow pods begin) stage. You want to pay some attention to soybean insect (and identify weed and diseases) issues until then. However, this year's aphid scouting efforts should increasingly focus on fields with less mature beans.
Some things to consider with late-season aphid insecticide applications.
Aphids will eventually leave soybean for buckthorn. Longer nights, cool weather and maturing soybeans combine to begin the movement from soybean to buckthorn. A calendar date is not useful in predicting the timing; aphid movement from soybean to buckthorn has ranged from mid-August to mid-September at the SWROC. This weekend's cool forecast may start the process, particularly in northern MN. Don't treat soybean aphid populations that are declining.
Pathogens, parasites and predators may be more helpful. Cooler, wetter conditions favor the most common soybean fungal diseases of soybean aphid. There is often heavy dews and persistently wet soybean canopies this time of year. Beneficial insects have had all summer to build populations on aphids in soybean and other plant environments and may be able to move into aphid infested fields. For example, Harmonia (a.k.a. Asian lady beetle) populations are just now becoming abundant.
Base your scouting and decisions on soybean maturity. Be vigilant but don't over-react. Aphids need time and numbers to reduce yield in a soybean field. Low aphid populations this time of year may not be able to spend enough time in early-maturing fields to be a problem.
You may want to treat extremely high soybean aphid populations (1000s of aphids /plant) as late as early R6 stage soybeans (beans fill the pod cavity at one of the upper four nodes) to avoid yield reduction. However, end-of-season treatment anxiety typically occurs as a result of missing a soybean aphid problem earlier in the season. To ensure that you are not faced with a dilemma on whether or not to treat soybean aphids close to harvest - use the 250 aphid/plant threshold until R6. See scouting aphids for more details on scouting and soybean reproductive stages.
The Pre-harvest Interval (PHI) of insecticides needs to be considered. Always read the label or make sure someone has read it for you. PHIs range from 18 to 30 days for most commonly used soybean aphid insecticides and they influence which, if any, insecticide(s) can be used this late in the season. There are soybean fields within a month of combining.
A reminder: Please report any problems with insecticide control of aphids you observe: http://blog-crop-news.extension.umn.edu/2016/08/assessing-and-reporting-potential-cases.html.
A rumor circulating through parts of the SW MN area was passed on to me. Seems like some folks think that the sudden absence of soybean aphids from some fields is due to a reach-back effect of neonicitinoid seed treatments applied this spring. Seed treatment reach-back? Obviously, pest insects/insecticides and weeds/herbicides are not the same thing, nor do the systems behave the same. Sounds like someone's already trying too hard to sell crop inputs for next spring or hoping they didn't waste input dollars this spring. There is a pretty simple biological explanation. Migration of winged aphids, predation and disease are the causes for aphid population collapses.
Goss's blight and wilt is taking a yield hit out of some susceptible and moderately susceptible hybrids, particularly in fields where hail occurred. This includes some of the corn at the SWROC. Yield loss will vary by the amount of leaf area killed. Losses will be less when defoliation occurs on more mature plants.
Note which fields have problems and use that information in making seed decisions. If you have had a Goss's problem on the farm or area, select hybrids with resistance to this bacterial disease. I would avoid using corn stover from heavily infested fields for bedding. Stover infected with the bacteria can transport the disease to new fields and cause severe disease, even in rotated corn.
Late season soybean diseases are starting to appear. Symptoms of sudden death syndrome (SDS) are present and may continue to increase and as soybeans reach the full seed stage (R6) brown stem rot symptoms will start showing up.
Soybeans turning yellow before maturity may be an indicator excess water, poor nodulation or low nitrogen availability, soybean cyst nematode (SCN), and the Diaporthae/Phomopsis "top dieback" complex. Often, more than one cause is at play. Start the diagnostic process by looking at roots for SCN.
Don't forget to check later pollinating corn for rootworm beetles.
Late-planted fields and replant or areas where standing water delayed maturity are attracting large numbers of northern and western corn rootworm beetles now!
These areas, and those with late-pollinating weeds (ragweed, waterhemp), are at risk for large rootworm egg populations.
Green cloverworm is causing minor defoliation in many fields. The erratic flying, dark moths are present in some fields.
In fields that I have been in, most larvae are mature or pupated and many are diseased. Remember that soybeans can tolerate 25% defoliation during pod fill without yield loss. The treatment threshold is 20% defoliation of the whole canopy, not just the worst leaves. SW Minnesota has been spared the entertainment of Japanese beetles experienced in other areas.