Some Minnesotans are experiencing a bit of weather whiplash as some areas that experienced a historically severe drought in 2021 are currently experiencing very wet weather. In addition to delaying spring planting, the weather also impacts when crop pests will emerge or arrive.

On May 11, 2022, Dr. Dennis Todey, director of the USDA Midwest Climate Hub; Bruce Potter, University of Minnesota Extension IPM specialist; Drs. Jeff Coulter, UMN Extension corn agronomist, and Seth Naeve, UMN Extension soybean agronomist; joined UMN Extension educators David Nicolai, Anthony Hanson and Jared Goplen for a wide-ranging discussion of how the wet 2022 spring weather and forecast for the rest of the growing season will affect spring field operations, summer crop growth and development and the arrival and emergence of crop pests. This was the first episode of the 2022 Strategic Farming: Field Notes program in this series.

La Niña to Blame for Wet, Cool Spring

La Niña is a periodic climate pattern caused by cool surface temperatures in the Pacific Ocean near South America. During La Niña conditions, warmer surface water is blown westward toward Indonesia, allowing cold water from deeper in the Pacific Ocean to rise toward the surface along the coast of South America and more of the Pacific Ocean along the Equator. La Niña conditions in the spring can cause cooler temperatures and more precipitation in the Northern Plains states. This spring, the northernmost quarter of Minnesota has received more rain than normal and temperatures have averaged 4-10 degrees cooler than normal over the last 30 days. Colder temperatures and cloudiness have also delayed soil warming and drying, leading to reduced field work days.

Going forward, La Niña conditions increase the chances for warmer, drier summer weather like that forecasted for Minnesota in June-August 2022.

Large Weather Systems Bring Pests to Minnesota

Black cutworm is one of the migratory crop pests that Potter monitors in his network of pheromone traps. Black cutworm larvae are unable to survive the harsh Minnesota winter, instead spending their winter in warmer climates to our south. This is the case for many of the pests of Minnesota’s agronomic crops including armyworm, aster leafhopper, potato leafhopper and cereal aphids.

Weather systems that bring warmer, more moist air north can also be an important means of transporting pests to Minnesota. The location within Minnesota where these insects land is dictated by a combination of where low-pressure systems track over the state and where rain actually falls, as long-distance migrating insects tend to be “rained out” on the backside of thunderstorms before making their way into fields. Black cutworms have traveled to Minnesota this way in 2022, with significant captures detected through UMN Extension’s cooperative pheromone trapping network after a weather system moved through the state at the very end of April.

The impact of weather conditions on crop pests does not end once they are rained from clouds. Because insects are cold-blooded animals, temperature has an impact on pest development, with cooler temperatures slowing pest development and warmer temperatures speeding pest development. The relationship between insect development and temperature can be exploited by entomologists. In the case of black cutworm, they use current and historic weather data to calculate the accumulated degree days when eggs will hatch and larvae get large enough to negatively affect crops. This knowledge can help to better target scouting efforts and, if warranted, insecticide treatment.

Survival of insects that survive as eggs, larvae or pupae in crop residue or soil are affected by air temperatures and snowfall. Snow creates a more hospitable environment for insect pests than bare ground. Where there was adequate snow cover, insects like alfalfa weevil and bean leaf beetle have a better chance of surviving the winter. Alfalfa weevil, a pest capable of surviving the Minnesota winter, is just began to emerge in some southern Minnesota fields last week (week of May 9) and are now active in both southern and central Minnesota.

Crop specialists are likely paying attention to seed corn maggot emergence as well as this pest can be more impactful when planting coincides with fly emergence. Degree-day models can be used to estimate the timing of seed corn maggot egg laying in your area as people might want to avoid having planting coincide with peak seed corn maggot adult emergence. Without a delay, eggs that these pests lay will be hatching and larvae would be actively feeding at the same time that corn or soybean plants are germinating and emerging. Fields that have had manure applied or have higher organic matter content are at particular risk of injury due to this pest and should consider a seed treatment insecticide if planting close to fly emergence.

Spring Weather Can Affect Disease Pressure Later in Season

In years when soybean planting is followed by significant rain and cooler temperatures, diseases such as Pythium seed and seedling disease and sudden death syndrome (SDS) can be more prevalent. Although both BSR and SDS do not typically exhibit external above-ground symptoms until crops reach the pod-fill growth stages, infection is influenced by post-planting soil conditions. There is little one can do to manage these diseases after the crop is planted, but seed treatment fungicides labeled for Pythium and SDS are available.

How Much Do Planting Delays Contribute to Lost Corn Yield Potential?

The question that is on most crop producers’ minds as planting is further delayed is, “just how much is yield potential reduced when planting after the optimum date?” Recent analysis of corn planting date data collected from 2009 through 2016 by Coulter finds that on average Minnesota corn yield potential remains unaffected through May 12; when planting May 13-19 corn yield potential is 97-98% of maximum, and when planting May 20-25 yield potential is 94-96% of max. It is recommended that those who need to delay corn planting to between May 22-28 should consider switching to a hybrid that is 5-7 relative maturity units earlier than a full season hybrid.  

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