As winters continue to trend milder due to climate change, will the threat of overwintering insects, weeds and diseases increase?
If this is a question you’ve been asking, you’ve come to the right place. Here’s how above-average winter temperatures may impact next year’s crop (plus some tips on how to manage these threats accordingly).
Mild winters and insect pressure
In some locations, cold winters can prevent some insect species from establishing or migrating from southern states.
Freezing temperatures can help reduce insect populations for the subsequent growing season. But mild winters in which the soil doesn’t freeze have little to no effect on species that overwinter underground, such as seedcorn maggot, Western corn rootworm and wireworms.
Minimum winter air temperature can help forecast insect freeze mortality, but that system does not work for every insect, as many overwinter in the soil or near the surface, like bean leaf beetle, where they are still protected under residue and/or in loose soil.
Other pests lay eggs that can survive the winter, such as soybean aphid. Air temperature is a good predictor of soybean aphid populations since these pests overwinter as eggs on buckthorn buds that provide minimal protection from the cold (unless insulated with snow cover). Since soybean aphid eggs will freeze between -25 and -35 F (most freeze around -29 F), mild winters result in better winter survival.
However, warmer winter temperatures do not automatically indicate heavy insect populations in the next season. Spring weather conditions must also be accounted for, so in-season scouting is always recommended to get the full picture of insect pressure.
Mild winters and disease pressure
Similar to their effect on overwintering insects, mild winters also allow pathogens to get a head-start on infecting crops. If they’re not killed or suppressed by cold temperatures, diseases can overwinter on winter annual weeds and volunteer plant residue left in the field.
For example, during mild winters, Cercospora leaf blight that overwinters on crop residue can be exacerbated by spring rains.
Additionally, warmer soils are more conducive to overwintering diseases such as Phytophthora root and stem rot.
Research also shows that when fewer than 10 days during December, January and February reach temperatures under 17 F, early infections of frogeye leaf spot have occurred.¹
Mild winters and weed pressure
If you experience mild winter conditions, pay close attention to timing on burndown and herbicide applications. Since milder winter temperatures give winter annual weeds and some early-season weeds a jump-start on emergence, you will likely need to start burndown operations earlier.
Early scouting is key to get an accurate picture of overwintering weed populations in your field.
Not only do mild winters impact winter weed populations, but unusually warm and dry months leading up to planting season (like the conditions in spring 2024) can lead to earlier green-up of winter annual weeds and early-season annual weeds, necessitating earlier residual herbicide applications.²
However, these conditions are tricky for applying herbicides, because while warm spring temperatures signal to weeds to begin growth, they are often still too cool for optimal post emerge herbicide effectiveness.
When temperatures are below 60 F, absorption of herbicides like glyphosate is lower compared with applications at higher temperatures, and they act much slower to kill weeds.³
As with everything in crop production, managing weed pressure is a balancing act, and every season calls for slightly different management tactics. In short, close monitoring and frequent scouting are your friends.
Don’t go it alone — experts are available to help you make management decisions. Reach out to your seed retailer, a nearby extension office agent, or a seed company professional like your regional BASF representative.
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Endnotes
- Dorrance, Anne, et al. “Overwintering of Pathogens and Insects - What do Winter Temperatures Tell Us About Next Season?” Ohio State University Extension, Mar. 2020, agcrops.osu.edu/newsletter/corn-newsletter/2020-03/overwintering-pathogens-and-insects-what-do-winter-temperatures. Accessed 14 Nov. 2024.
- Anderson, Meaghan. “Weeds Took Advantage of a Mild Winter.” Integrated Crop Management, Iowa State University, 12 Mar. 2024, crops.extension.iastate.edu/cropnews/2024/03/weeds-took-advantage-mild-winter. Accessed 14 Nov. 2024.
- Jhala, Amit, and Jenny Rees. “Low Temperature and Frost May Affect Efficacy of Burndown Herbicides.” CropWatch, 22 July 2019, cropwatch.unl.edu/2017/low-temperature-and-frost-may-affect-efficacy-burndown-herbicides. Accessed 14 Nov. 2024.
