For those of us who don’t ice fish or ski, the best part of winter in northern New York is the lack of mosquitoes. Thick blankets of snow muffle the whizzing fervor of our least welcome trail partners, but not all insects go dormant in the winter months. The family of insects known as ground beetles contains species that remain active throughout the year.
The term “ground beetle” connotes a geophilic creature. Although this description is true for many of the 40,000 ground beetle species, it doesn’t do justice to the vast spectrum of niches occupied by these adaptable arthropods. For example, ground beetles in the genus Calosoma, are renowned for their climbing abilities. Crawling through the canopies of Adirondack trees and shrubs, Calosoma beetles hunt tent caterpillars and the larvae of gypsy moths. Their habit has earned them the moniker “caterpillar hunters.” For these arboreal assassins, hunting season extends from April to November.
In New York, remaining active in November is an impressive feat for an insect. Beetles have traditionally been considered poikilotherms, meaning that their body temperature is variable and is influenced by their environment. Few insects express the behavioral and physiological skills to operate with such a low body temperature. One group of ground beetles that can arguably surpass the cold weather survival skills of the caterpillar hunters is the genus Agonum. They remain active through the winters of Europe, Canada, and the Adirondack region. How do they stay alive in such cold months?
Winter-active ground beetles are classified as either freeze-tolerant, because they can endure extremely cold temperatures (-40oF to -70 oF), or freeze-avoidant, because they move to warmer locations to avoid extreme cold. The Agonum genus contains many freeze-avoidant species. Whereas Calosoma beetles thrive in the treetops, Agonum species occupy a subnivean climate. Deep layers of snow provide insulation from subzero temperatures. Humidity is also somewhat higher in the snowpack, which helps the beetles avoid desiccation. Larvae burrow through the snow looking for food, which may include small invertebrates like springtails, or the larvae of other insects that have gone dormant.
Temperatures within the snow are warm enough to keep the beetles alive, but also cold enough to lower their metabolisms. If the larvae are unlucky in their search for food, they can persist for 30 days without a meal. When the cold winter months give way to spring, the ground beetles move out of the snow to resume feeding.
As opposed to freeze-avoidant insects, freeze-tolerant species do not rely on seeking warmer habitats. Beetles capable of tolerating extreme cold can do so because of several biochemical tools. One example of a helpful chemical is glycerol, which can be found in the tissues and body cavities of cold-tolerant ground beetles. Glycerol is a polyhydric alcohol found in the antifreeze liquids that drivers once put in their cars, before it was replaced by ethylene glycol. It helps prevent ice crystal from forming inside the insects.
Another group of chemicals that keep insects free of ice are the antifreeze proteins. These proteins lower the freezing point of water, in the presence of ice, without affecting the melting point of the water. Although they were first found in Antarctic fish, they have since been found in beetles that are naturally found in the Adirondacks. Fighting the freeze is helpful for insects because water expands when it freezes. If water froze inside an insect, it could damage the tissues of the insect due to the expansion of the jagged ice. Furthermore, insects need their body fluids to remain liquid, because they have an open circulatory system, which relies on the diffusion of essential nutrients through the body cavity. If the body cavity freezes solid, this exchange of nutrients is impaired.
Surprisingly, some freeze-avoidant species also have antifreeze proteins. One would suppose that if they seek out warmer habitats, then they wouldn’t need the molecular protection. What is the point of having proteins that they shouldn’t need to use? Perhaps nature doesn’t always let the beetles choose; a beetle caught outside of the snow might need to rely on proteins that would be unnecessary under ideal conditions. Another possible explanation is that the proteins serve multiple uses, and they are helping the beetles in some other way.
Understanding the ground beetles’ behavioral mysteries might help humans conserve these valuable arthropods. Winter-active ground beetles prey upon the insects that afflict rye production, such as moths and flies. Farmers who grow crops in greenhouses or high tunnels may get a visit from ground beetles, because they hunt for aphids. If the overwintering behaviors of ground beetles can be better understood, maybe agricultural systems could become more productive.
While most insects are in a dormant state, some remarkable beetles are capable of living an active life. Their combination of chemical protection and behavioral adaptation allows them to exploit niches unavailable to other insects, and by doing so, they protect our crops. If you have a chance to explore the snow during your next winter hike, maybe you can see them in action.
Ground beetle found in Essex, NY, in March, 2020. Photo provided by Shane Foye.