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Nix “Insurance Applications” of Insecticide with Fungicide on Soybeans

Applying Insecticide When Defoliating Insects Are Below Threshold Levels Can Spur Insecticide Resistance.

Applying a fungicide in tandem with an insecticide seems to be a way to kill the proverbial two birds with one stone. Don’t do it, though, if defoliating insects are below established infestation thresholds, says Bruce Potter, a University of Minnesota Integrated Pest Management specialist at the Southwest Research and Outreach Center at Lamberton, Minnesota. He adds this may conflict with pest management advice given in a popular television program, but the comments by Potter below may make farmers more money. 

I had a conversation with an agriculturalist in late July. He was concerned about some of the insecticide recommendations in an area where synthetic pyrethroid 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 interests to some who have not yet committed to an insecticide/fungicide application. 

Soybean aphids are present and most, if not all, Minnesota soybean aphid populations are currently low. They may, or may not, stay that way for the rest of the growing season. 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. 

Well, It’s Cheap, It’s Easy, So Why Not?  

Well, 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 selects for resistant individuals, even when insect populations are low. By allowing only the more insecticide tolerant individuals to survive and removing natural controls, a poorly timed, low rate insecticide application is a good way to make the problem worse in your field and the fields of other farmers.

What about those soybean aphids that are resistant to synthetic pyrethroid insecticides? Soybean aphids reproduce asexually during the summer, very rapidly at times. This means an insecticide resistant aphid will produce multiple generations of resistant offspring. Choose your insecticide applications wisely. 

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