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Strategic Farming: Let's talk crops! session discussed the effects of the 2021 drought on insect pests

By Angie Peltier, UMN Extension crops educator, and Phyllis Bongard, UMN Extension educational content development and communications specialist

Twospotted spider mite damage in soybean.
Photo: Bruce Potter
Insects and other arthropods that feed on crops are cold-blooded animals whose growth, development and population dynamics are largely driven by their environmental conditions. On March 2, 2022, Bruce Potter, University of Minnesota Extension IPM specialist, Drs. Ian MacRae, UMN Extension entomology specialist, and Seth Naeve, UMN Extension soybean agronomist, joined UMN Extension educator Angie Peltier for a wide-ranging discussion of how the 2021 drought influenced pest pressure and how 2021 population dynamics may influence 2022 pest pressure. This was the ninth episode of the 2022 Strategic Farming: Let’s talk crops! webinars in this series.

To watch this episode:

Changes in crops due to drought stress

We can plainly see when our corn and soybean crops respond to drought stress by rolling or flipping their leaves to reduce moisture loss. These above-ground indications of water stress tend to occur each day when the crop’s water usage rate outpaces its ability to take up sufficient soil moisture. 

What we haven’t been able to see is those more subtle adjustments for which there is no visual cue that likely began long before leaves flip or roll. When a soybean crop is unable to keep up with water demand, the crop will begin to change its cellular composition by changing the solute content within cells to begin to take more water into the leaves through a process called osmosis. 

After some time with insufficient water availability, loss of yield potential occurs after the crop slows the rate of leaf expansion and shuts off photosynthesis beginning earlier in the day for each additional day that drought conditions persist. The longer the period of time that this sort of stress occurs, the more severe the effects on plant growth and development, yield and quality.

How insects are affected by drought stress and weather more broadly

Over many years entomologists have worked to better understand insect growth and development to determine when scouting for a particular insect pest should begin and end during a growing season and how quickly populations can grow to treatment-threshold level infestations. Unlike mammals, insects are cold-blooded animals meaning that all of their physiological processes are driven by air temperature. The warmer the air temperature, generally the faster the physiological processes in insects will run. This can mean both shorter generation times and, depending on the species, more egg-laying, both of which can result in the potential for threshold-level population densities building faster than in a ‘typical’ growing year.

Insect population densities can also be affected by how dry the previous fall was. The dry 2020 fall likely helped to set the stage for grasshopper populations to reach treatment thresholds throughout western Minnesota in 2021.

Of additional concern during drought years is that insecticide efficacy can be negatively affected. If insecticide applications take place during very hot temperatures and periods of low relative humidity, some of the active ingredient never makes its way onto the leaves, but is rather lost to volatilization.

The weather year-round can also impact the insect pests that plague Minnesota crops. Some insects are more cold tolerant than others and have a variety of behaviors or physiological characteristics that help them avoid cold weather. Soybean aphids, for example, can overwinter on small buckthorn under the snow, while soybean aphids exposed to harsh winter air may not fare as well. Some insects cannot survive our freezing winter temperatures. Examples include armyworms, potato leafhoppers, and small grain aphids, such as the English grain aphid. Populations of these, and other migratory insects, winter in the southern U.S, making their way north to Minnesota in the spring.

What about the loss of chlorpyrifos?

The tolerance for chlorpyrifos on food and feed crops has been revoked as of February 28, 2022, meaning that food and feed crops in commerce can no longer have detectable chlorpyrifos residue. This means that while this pesticide active ingredient would have been legal to apply to the crop grown in 2021, the 2021 crops onto which it was legally applied now have residues that are no longer allowable. The US Food and Drug Administration has recently issued guidance for how buyers and processors can handle commodities that find themselves in this situation. There is currently a lawsuit that has been filed by industry groups asking the court to stay the chlorpyrifos ruling, and it seems likely that little will change without another court ruling.

There are still insecticides and miticides labeled for use that are highly effective against the corn and soybean pests that Minnesota farmers routinely manage. Where people may struggle a bit and should make a plan in advance of needing to make an application is in managing pests of sugarbeet.

Fielding audience questions

Potter, MacRae and Naeve answered many audience questions about how multiple stressors can work together, how to adjust treatment thresholds for multiple simultaneously occurring pest populations, what might have been the cause of the alfalfa weevil problem so many people struggled with in 2021, insect population dynamics under drought stress and what crop producers can expect of pests going into the 2022 cropping season, and any pest to worry about more when growing soybean after soybean.

Thanks to the Minnesota Soybean Research & Promotion Council and the Minnesota Corn Research & Promotion Council for their generous support of this program!

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