On February 15, much of southern Minnesota saw low temperatures below -20 ° F in the morning and much of the northern part of the state lows near or below -30 ° F, preceded by similar sub-zero temperatures earlier in the month . In general, this level of cold is problematic for farmers, from problems with killing alfalfa in winter to watering that freezes for pet owners. However, the cold can be a welcome event for pest control, especially if fewer winters hit these annual lows in recent years.
Cold winters prevent many potential pest insects from settling here, or require species that cannot survive our winters, such as potato leaf funnels or black cutworm, to migrate from the southern states. Extreme cold can also set back species that are established here.
Cold tolerance for insects
Most insects adapt to the temperature of their environment and are therefore “cold-blooded”. Unlike warm-blooded animals, the wind chill doesn’t really affect insects, but the air temperature does. Even so, many insects can survive temperatures well below freezing due to their own antifreeze, such as glycerin.
Similar to the weather forecast, the pest forecast can give a general overview of future insect populations. The minimum winter temperature (previously around February 15 of this year) can help predict frost mortality (see Figure 2).
Where insects hibernate, this also affects mortality. Some insects, such as the corn rootworm, overwinter in leaves or soil where they are protected from extreme temperatures, especially when there is heavy snow cover. Most areas in Minnesota so far had a snow cover between 4 and 8 inches for the coldest parts of this winter, so these insects would be protected with both snow and ground cover.
Some invasive species, like the emerald ashbore, which can hibernate under tree bark as larvae, can be affected by the cold temperatures we saw in February. Because of their cold tolerance combined with the temperature buffer in a tree, many larvae can survive intact this winter, especially in southern Minnesota. Pests that are directly exposed to air temperature, such as soybean aphids, would be most affected by the cold snap.
Since soybean aphids hibernate as eggs on sea buckthorn buds, there is little protection from the cold unless they are under an insulating blanket of snow. Eggs will freeze between -25 and -35 ° F (McCornack et al. 2005), temperatures reached in parts of the state in early February. Due to dehydration and cold spells in late autumn, some egg cell mortality can also occur above freezing point.
Outlook for 2021
Cold temperature mortality can be moderately high in parts of Minnesota (Fig. 2). Egg mortality can be close to 50% in Northern Minnesota and close to 75% in some areas. In addition, the persistently low temperatures just above lethal temperatures may result in higher mortality rates than projected on the map (e.g., several nights below -20 ° F). However, this type of prolonged stress is much harder to use to predict pest risk than using the coldest night of the year as a ballpark. The February cold snap will not eliminate soybean aphids from Minnesota, but it could reduce populations in some areas in the early season.
In 2019, we also had a polar vortex that caused even colder temperatures across the state, and projected mortality rates were also 90% higher in much of northern Minnesota. In the growing season that followed, soybean aphids weren’t a major problem across the state, but they were still present in fields and causing treatable damage in some areas.
Remember, however, that the saying “correlation” doesn’t always mean causality. Other factors could have contributed to this in 2019, as winter mortality is only part of the puzzle when predicting whether an insect pest will be a problem. In the case of soybean aphids, winged aphids are highly mobile and can colonize new areas by field-to-field flight or long-distance travel with air currents that can vary from year to year. There are overwintering populations across much of the upper Midwest where temperatures were not as lethal.
In 2021, this cold snap may help reduce aphid populations to some extent. However, don’t expect winter control of soybean aphids to be similar to that in 2019. Many insect populations can grow rapidly, so even a small percentage of the survivors can still cause problems for soybean growers under the right conditions. In short, there may be a little less risk of soybean aphid problems this year due to the cold, but problem areas can easily emerge across the state.
Start scouting early in the year so you are always up to date on the populations of soybean aphids that can develop in your area. For more information, please visit the Extension’s website for soybean aphids at https://bit.ly/2ZquCQl.
For more information about wintering insects, see “Tough Buggers: Insect Strategies To Survive Winter In Minnesota” at https://bit.ly/3s49Q5n.