SW MN IPM Stuff 2019 Issue 10

Volume 22 number 10

7/29/2019

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: PDF iconIPM STUFF 2019-10

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 SWROC Weather.

Soybean

Last Friday, an agriculturalist called. He was concerned about some of the insecticide recommendations in an area where insecticide resistant soybean aphids caused problems in 2018. Finding small numbers of thistle caterpillars was triggering a recommendation for the addition of a reduced rate of a pyrethroid insecticide with soybean foliar fungicide applications. The agriculturalist’s concern was justified and I thought might be worth a few comments. They may be of interest to some who have not yet committed to an insecticide/fungicide application.

Soybean aphids are present and most, if not all, MN soybean aphid populations are currently low. Meanwhile, green cloverworm, thistle caterpillar, loopers, Japanese beetle, flea beetles and other leaf feeding insects are present in soybeans. When insects and their defoliation threaten yield, insecticides can be valuable tools, helping to manage insect populations below levels that cause economic loss.

Scouting, when used only as a reason to find insects to spray and applying insurance insecticides can create more problems than they prevent. On the other hand, thorough and well-timed scouting can give you the confidence that you are making an economically wise choice or that doing nothing is the right decision.

Often, insect pests and crop damage are more prevalent on field edges. Avoid making whole –field decisions based on field borders or isolated areas only. Many insect species prefer to feed on new growth. Don’t base an insect management decision on the worst leaves you come across. Average the defoliation of leaves from the upper, middle, and lower canopy. Leaves that are senescing no longer contribute to photosynthesis and should not be included. Because it is so easy to overestimate defoliation, use a defoliation assessment key until you are comfortable making on-the-fly estimates.

A rescue insecticide application to reproductive stage soybean is recommended when whole-plant defoliation reaches 20%. This amount of leaf feeding will make a field look ragged but is well ahead of when yield loss occurs. Remember, soybeans are tough and good at compensating for lost stand or leaf area. Make sure insects are still present and actively feeding. Killing insects after they have pupated or been killed by beneficial insects does not make good economic sense but, unfortunately, is a commonly used management strategy. Choose the labeled insecticide and rate for the species that is most abundant or hardest to control.

There is no responsible or common sense reason to add an insecticide to a fungicide application for below action threshold populations of defoliating insects. Particularly suspect are the recommendations of “just in case” reduced rate insecticides. As you make insecticide decisions, consider the possible effects on synthetic pyrethroid (e.g. Warrior, bifenthrin) insecticide resistant soybean aphid and two-spotted spider mite.

It’s cheap, it’s easy, so… Why not? Because insurance insecticide applications are seldom economical and often create expensive and hard to manage problems.

  • Poor insecticide timing increases the likelihood that the field can be re-infested. Contrary to many marketing programs, insecticide residual is not long-lasting. Most soybeans are still producing new nodes and leaves at the top of the plant and will until the R5 (beginning seed) stage. This new growth will be unprotected and even those insecticides that are absorbed and translocated within the plant do not protect many leaves.
  • Broad-spectrum insecticides remove beneficial insects. Any surviving pests can rapidly increase in fields with few predators and parasites. On the other hand, some pests are needed to attract beneficial insects to the field and hold them there. By minimizing naturally occurring controls, you can inadvertently flare populations of non-target pests such as spider mites, or leave the field wide open for immigrating pests.
  • Applying an insecticide, especially at a low rate is a good way to find an insecticide resistant population. Remember those synthetic pyrethroid insecticide resistant soybean aphids? Poorly timed, low rate insecticides are a good way to make the problem worse in your field and the fields of other farmers. Applying an insecticide always selects for resistant individuals, even when populations are low. Use your insecticide applications wisely.

Set yourself up for success! Use scouting, thresholds and common sense.

Happy trails,

Bruce Potter