By Anthony Hanson, Regional Extension Educator - Field Crops Integrated Pest Management
At least up to the start of November, we’ve had a mild fall in west-central Minnesota so far and across much of the state. However, the cold of winter will come eventually, and with that, many of the insects that have been living in our crop fields will be feeling the winter chill too. Aside from a few insects like honeybees that generate heat during the winter, an insect’s body temperature will be about the same as the temperature of whatever environment they are in. During fall and winter, there can still be some interesting activity in the fields that effects what we see next year.
Some common pest insects we see each year actually cannot survive typical Minnesota winters and instead rely on migration. Potato leaf hopper is one of these species that instead overwinters near the southern Gulf states and returns each spring on southerly winds. Those that remain in the state instead of migrating south in in the fall usually will not survive past early December as they cannot survive prolonged periods of below-freezing temperatures. Winter does help reset these populations each year even though leafhoppers will return.
While a cold winter doesn’t keep these migrating insects out all year, it can help keep some of our insect populations low. However, others can survive our winters by tolerating the cold, finding shelter, or both.
Many insects try to avoid lethal ice forming within their body by lowering the point when water freezes in their body fluids with antifreeze compounds, such as glycerol. Soybean aphid is a pretty hardy insect, especially for one not native to Minnesota, that does not freeze until around -30°F. They overwinter as eggs on buckthorn, but are otherwise directly exposed to the air through all of the winter as long as the plant isn’t buried in snow.
Others still try to avoid freezing, but also take advantage of shelter out in the field. Alfalfa weevil has had a strong resurgence in the last couple years in our area. This could be due to potential insecticide resistance or strains that have an extended emergence period; folks with U of M Extension are working on adapting pest management plans for upcoming years. In the meantime though, winter won’t automatically solve our alfalfa weevil problems. Adults that developed early last summer remain dormant until the following spring, so they will be overwintering in alfalfa fields or in nearby sheltered grassy areas. Any snow caught here will act as insulation. This protects alfalfa stands from winterkill, but the tradeoff is that weevil survival will likely be higher. There may be lower survival in years with little snow cover, but some weevils will still survive to the next spring. Be sure to scout fields in May and June to see how populations develop that year.
Other insects will take advantage on not only the snow cover, but also the insulating effect of the soil. Various white grub species such as June beetles overwinter as larvae in the soil. Here they get not only a temperature buffering effect from snow cover, but the soil itself. Even just 4-8 inches underground, the actual minimum soil temperature may only range from just near freezing to between 23 to 14°F depending on the year.
Of course, some insects that stay in the state don’t remain in the field. The multi-colored Asian lady beetle is very well known for finding its way into old farmhouses to wait out the winter, unlike our native lady beetles. In nature, they will find trees or other protected areas to aggregate, but warm buildings are often found instead. This behavior makes the insect a pest when it gets into homes even though they are beneficial for eating soybean aphids. This species isn’t native to Minnesota, but wasn’t introduced to control soybean aphid. Instead, it wasn’t until we had high soybean aphid populations when they first came to Minnesota that this non-native beetle became much more prevalent.
We’ll see how cold this winter gets and what to possibly expect next spring for insect populations.