Bruce Potter, University of Minnesota Extension IPM Specialist, recently discussed about how best to manage pesticide resistant insects and keep currently effective pesticides effective.
According to Potter, this year spider mite injury has been spotty, relegated to droughty areas of the state, but well-timed rain showers, dew and cooler, cloudy weather can reduce pressure.
Soybean aphid infestations have been spotty in 2022, first appearing in river bottoms and along woodlots where their winter host resides.
While aphid populations remain low in many areas, recently economic threshold-level populations have been observed in and increasing number of southern and western Minnesota fields. As soybean vegetative growth slows and ceases, large numbers of soybean aphids move within and between fields.
At this point in the growing season, it is very important for folks to be scouting whole plants, particularly for smaller and harder-to-spot aphids on the lower leaves of the plant, or risk underestimating population densities.
In a recent podcast sponsored by the Extension office, audience members were polled about how they make the decision to spray for soybean aphid.
A total of 59% of respondents indicated they follow the research-derived economic threshold (of building populations of at least 250 aphids per plant on 80% of plants), 14% believe that this economic threshold is too high, 10% tank mix an insecticide when they apply a fungicide, 3% spray when custom applicators are spray, 3% when neighbors are spraying, 7% spray when their crop consultant tells them to and another 7% don’t spray.
Potter assured the audience that even though crop prices are higher than in the recent past and folks are worried about losing yield, “soybean aphids have no idea how much you are going to get for a bushel of soybeans and they won’t eat any more when crop prices are high or any less when prices are low.”
The 250 aphid-per-plant threshold is based on many years of trials across the Midwestern states (including Minnesota) and is “pretty darn conservative,” according to Potter.
When population densities reach 250 aphids per plant, they haven’t yet caused feeding injury that will result in yield losses, rather the threshold was designed with a built in 7-to-10 day window to get an insecticide on before such yield loss can occur. According to Potter, “The 250 aphid/plant economic threshold should be used through the R5 stage. Aphids have less time to reduce yield as the growing season starts to wind down and yield loss from economic threshold populations becomes less certain after the R4 stage.”
While it is understandable that folks would like to save a trip across fields by tank mixing an insecticide with their fungicide. Potter says, “The other thing that happens if you are spraying an insecticide and a fungicide without regard for how many insects are out there means you are knocking out possibly both some beneficial fungi and insect predators. And as aphids move into the field flaring aphids or spider mites if you’ve got hot, dry weather.
“When spraying fungicide, if you don’t have aphids, save a little money and keep an insecticide out of the tank.”
This may result in threshold-level aphid or spider mite populations developing where they might not have.
Unwarranted fungicide applications can also negatively affect entomopathogenic fungi that help to suppress aphid and mite populations.
Farmers also do not have an unlimited supply of insecticide active ingredients at their disposal.
Several years ago, when pyrethroid-tolerant soybean aphid populations were first observed, many Minnesotans learned the hard way that soybean aphid is capable of evolving to overcome management strategies. Reserving our insecticides for spraying threshold-level soybean aphid population densities is one way to slow the evolution of soybean aphid populations tolerant to other insecticide groups.
Newer, more selective insecticides (ex. Sivanto, Transform, Sephina) are on the market for treating them, but if folks still want to spray a pyrethroid insecticide, a tank mix with a neonicotinoid or one of the newer products is recommended as pyrethroid-tolerance is likely to be a fixed trait still present in Minnesota soybean aphids.
According to Potter, “spraying just a pyrethroid is pretty risky,”
Potter stresses the importance of season-long scouting for crop pests. Scouting for Bt-resistant corn rootworm populations is an excellent reason to scout corn this year. One can both see how well their Bt trait is/isn’t holding up and the population densities that corn crops in 2023 may encounter.