SW MN IPM Stuff 2019 Issue 15
Volume 22 number 15
This newsletter and the advice herein are free. You usually get what you pay for.
A print-friendly version of this newsletter is available to download here: SW MN IPM Stuff 2019-15
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 SWROC Weather.
As of August 26, we are now about five days behind on growing season degree-days (1938 degree-days compared to the 2057 degree-day long-term average from May 1).
At 15.89 inches of precipitation since May 1, we are now 1.8 inches above long-term average. Some areas received considerably more or less because on your farm, your mileage may vary.
There ares some dry pockets starting to show up in some parts of SW MN.
Like much of Southern MN, we could not get into the field early this spring. Early-silking corn has started to dent. Yield potential significantly declines with late planting dates in an SWROC study (Figure 1).
Depending on planting date and maturity group, most soybeans are R5 (beginning seed stage) to pushing R6 (full seed). Soybeans that were planted early are progressing normally but later planted soybeans have some issues. Based on recent conversations with some of you, there is the potential that, in some cases, upper pods have slowed or stopped development. Warm weather will help the soybean crop.
European Corn Borer (ECB)
We will be looking for ECB infestation levels statewide and collecting larvae to check for parasitism and percent single generation (univoltine) biotype. If you have non-Bt fields that you would be willing to let us examine, please contact me!
Thanks to those who have already volunteered fields!
Soybean aphid watch
It is late in the season and, as always, questions arise on aphid treatment. As typical, some of the fields that were treated early have been reinfested and are now at or approaching economic threshold (80% or more plants infested, average of 250 aphids or more/plant, aphid populations are increasing, and soybeans are less than R6 stage).
What makes 2019 different? Soybean (and aphid) development is well behind that of a normal year. Aphid populations are very spotty throughout the state and between and within fields. Recent aphid population increases have been slowed by unusually cool weather (Optimum temperature for reproduction is ~82ºF and we have been averaging much cooler than that).
Quit worrying about soybeans fields that have reached R6 stage if you have not reached an economic threshold population. What about those fields where populations are still increasing? The probability for an economic benefit declines as soybeans mature because more yield has already been fixed and because there is less time to accumulate aphid pressure before maturity.
For those who like to calculate what-if scenarios, I updated the downloadable Aphid day calculator spreadsheet. It lets you look at aphid-day (measure of aphid pressure over time) accumulations at varied reproductive rates, starting aphid populations, and etc.. It is not an economic threshold calculator!
Those darn defoliators
Green cloverworm are still active in some fields and, unlike the painted lady/thistle caterpillar, green cloverworm moths and caterpillars can continue to be active as long as there is still green soybean foliage present. However, before you decide to treat, make sure that larvae are still present and the infestation is through the field. Estimate defoliation on all of the green leaves on the plant, not just the upper leaves where insect feeding is usually concentrated.
As soybeans mature into the R6 stage, potential yield impact declines. As soybeans reach the late R6.5 stage and particularly once you see signs that the field has started to turn, unless you are seeing extreme insect numbers and leaf area loss, I would walk away. You might even consider speeding up when driving by these fields just to help discourage making any late management decisions. Pre-harvest intervals for insecticides start to become limiting as well.
Soybean gall midge
Redwood has been added to the list of infested MN counties (Figure 2). Two infested plants were found while assessing soybean aphids at the SWROC. If plants break at the base, they are worth a second look for soybean gall midge.
I suspect this insect is more widespread than the current distribution map shows because symptoms are hard to notice in fields with low levels of infestation.
At research sites in Rock County, 2nd generation adult emergence has stopped, at least temporarily with 1st-3rd stage larvae present.
I am receiving reports from additional counties where an orange gall midge larvae are found in fields with white mold infections. I have not added them to the map as of yet. We have adults and larvae from some of these and they will be used to confirm if these are the same or a different species. Please let me know if you suspect this insect in your Minnesota fields!