As winter nears, I look forward to my “long winter’s nap” where I catch up on reading, writing, various research projects, and processing photography. It’s generally a bit slower pace of life and I ready for it as the days grow shorter and chillier.
For many insects their “long winter’s nap” is called diapause, a period of suspended development. This is the most common overwintering mode (Leather et al. 1993). This period is marked with a preparatory time followed with a time when the insect does not feed. The end of diapause is not an immediate happening. It is an extended period of time coupled with particular environmental events, one being the return of a certain photoperiod. In the course of an insect life, it typically will enter diapause as it progresses from egg to adult. The life form — egg, larva, pupa, or adult – the insect overwinters in depends on the particular species.
What cues do the insects use to determine the onset of winter?
While diapause is genetically controlled there are environmental factors stimulating the beginning and ending of this time period. Three important environmental cues are photoperiod, temperature, and nutritional needs. Photoperiod is the more reliable cue and is defined as the insects’ response to day-length; the corresponding change in temperature interacts with this. Insects also get cues from their food plants; the senescence (aging and deterioration) of their nutritional sources signals impending harsh conditions. Diapause is also synchronized so emergence occurs when chances of finding a mate are high. Isn’t nature incredible?
How do they tolerate the cold northern winters?
Insects have different strategies of cold hardiness that allow them to survive at low temperatures. While some can tolerate freezing, most insects in our climate use the biochemical strategy of supercooling or freeze avoidance and many of them overwinter in an immobile state as larvae or pupae (Leather et al. 1992). Supercooling is when water cools below the freezing point yet does not freeze. This process is initiated in the fall. The insect stops feeding and clears the digestive system, this removes water and naturally increases soluables. Next an increase in polyols (an alcohol) and sugars occur (glycerol is the most common sugar) which act like antifreeze and are collectively known as cryoprotectants. Overwintering larvae can supercool to temps ranging from -68F to -86F!
The Goldenrod gall fly (Eurosta solidaginis) produces 3 cryoprotectants with polyols having the highest concentration (Baust and Lee 1981). These flies spend 11 months of their life in this gall as a larva.
The fire-colored beetle (Dendroides canadensis) is a beetle that has the ability to switch from freeze tolerant to intolerant, depending on the weather conditions of that particular year. While the red bark beetle (Cucujus clavipes) is also known to use both strategies, Dendroides canadensis was the first insect discovered to have this ability (Horwath and Duman 1984).
What stage in their life cycle are they?
Whether an insect overwinters as an egg, nymph or larva, or pupa is important to their survival strategy. They must control their development and reproduction in order to optimize their survival throughout the cold winters.
The recurring theme of ecological specificity applies to insect overwintering as well. The stage used by the insect is dependent upon its taxon and life cycle. It is unfortunate we don’t know more about the overwintering strategies of many of our known insects. I suppose this is expected when as only 20% of insects are described and an even lower percentage of those have full known biologies.
Eggs are the most cold hardy but they are not impervious to the rigors of winter’s cold. Praying mantis (Mantis religiosa) overwinters as eggs yet in a 6-year study their mortality ranged from 15-86% depending on the temperature fluctuations (Salt and James 1947).
Moths and butterflies tend to overwinter in a particular stage depending on the family taxa. While one can find overwintering forms in eggs, larvae, pupae, and adults, in moths 55% are pupae and in butterflies 56% are pupae (Leather et al. 1993).
Overwintering adults can be found alone or in aggregations with other insects. Think of those pesky Boxelder bugs and Asian beetles that swarm our homes as the thermometer dips! Others you’ll find overwintering as adults are Snow fleas (Hypogastrura nivicola) and Winter crane flies (Trichocera sp) . On a sunny, warm winter day, these can be found on top of the snow. Be sure to take your camera and post your photos on the TPE Facebook page!
Most aquatic insects overwinter as larvae or nymphs under the ice. They continue feeding and growing until spring comes and they are ready to emerge. Common Green Darner (Anax junius) takes 2 summers at the larval stage before emerging as an adult. These larvae are aquatic.
And, there’s always the exceptions! Some insects require 2 or more years to reach maturity. One example is the Osmoderma beetles; they spend three years in the larval stage in rotting wood before pupating in fall and emerging the following year. A sidebar interesting fact is there are only 3 beetles of this genus living in the U.S. (Galloway, MLBS website)
The forked fungus beetle (Bolitotherus cornutus) can overwinter as an adult or a larva depending on whether they “hatched” in spring or fall. The larva overwinter in their host fungus but we don’t know where the adults will overwinter. Perhaps in the dead or dying tree where the fungus is growing? Maybe in the soil near the roots of the tree? Maybe in the fungus?
The Virginia Ctenucha (Ctenucha virginica) moth overwinters as a caterpillar (larva) in the leaf litter of grasses, sedges, and iris. It has 2 generations in a year so one can find the caterpillar in the dark stage for the cooler months and in the light stage for the warmer months. BugGuide page has great photos of the differing caterpillar colors. Check out your prairie and see if you can find this one!
Not only do insects have to change their chemistry (supercooling) and know the most strategic life cycle stage in order to survive, they also need to add coloration to their winter survival “to do” list. Darker colors not only hide them from predators, it absorbs heat. Most of the insects overwintering as eggs are black. Geometridae moths pupae start out green and darken as they overwinter in the soil. The Eastern tent caterpillar moth (Malacosoma americana) lay their eggs in masses on tree limbs. They are white when laid and then turn dark and blend into the tree limb.
Where do they overwinter?
While we know very little about most insect overwinter habits, we can generally say some of the places they overwinter include leaf litter, under dead plants or grasses, crevices of tree bark, stems of plants, tree stumps, galls, old rodent burrows, and under just about anything. Bumblebees like to choose overwintering spots under another object; it could be under a rock, a fallen tree, a deck, or a porch.
Soil and snow are good insulators. Many moths and butterflies overwinter as pupae at the soil surface. Moths such as Spotted cutworm (Xestia c-nigrum) overwintering at the soil surface have much lower lethal temperatures than insects such as the Japanese beetle (Popillia japonica) whose larvae can burrow 4-8” into the soil.
These insects using the soil surface, leaf litter, and under dead plants have the added benefit from the insulating effects of snow during the winter but this places them at greater risk of mortality with early spring management practices.
How do they choose an overwintering site?
Topography plays a key role in many aspects of ecology. It is often the explanation to the variations found in the general tenets and paradigms of ecological restoration because it can create micro-local situations. What might appear to be similar habitat can result in quite different weather patterns. These micro-climates affect how insects exploit overwintering habitats.
Like all aspects of life strategies and survival, there are costs associated with overwintering. The immobility predisposes them to unpredictable events and predation. Dessication and precise timing of emergence pose threats as well. Overwintering has its threats and the mortality rate depends on the severity of the weather. The Codling moth (Cydia pomonella) has an 80% mortality rate in a typical winter.
How do insects know when it’s time to emerge after overwintering?
Photoperiods and temperature are again the most important cues. Synchronizing this is important. Being out of harmony with the environment and the growth of their food plants or hosts (if one is a parasitoid) can be disastrous.
There are so many unknowns when learning about insects. This lack of knowledge can be an opportunity. Whether you are one who gets out into nature during the cold, snowy, winter months or you prefer to stay warm inside, you can contribute. While snowshoeing, look up and around and see if you can spot an egg mass or a gall housing a larva. Insects are more cryptic and harder to find in their overwintering mode. Take a picture and post it on TPE Facebook page being sure to note the plant and the date the photo is taken. If you prefer to do inside research, pick out an insect or two and dig through the literature and note your findings. However you choose to do the research, you can be certain that your contributions are important!
Baust, J.G. and R.E. Lee. 1981. Divergent mechanisms of frost hardiness in two populations of gall fly, Eurosta solidaginis. Journal of Insect Physiology 27: 485-490.
Galloway, Hazel. “Hermit Flower Beetle.” Mountain Lake Biological Station, University of Virginia. Accessed 04 Oct 2016. http://www.mlbs.virginia.edu/organism/hermitflowerbeetle
Howath, K.L. and J.G. Duman. 1984. Yearly variations in the overwintering mechanisms of the cold-hardy beetle Dendroides canadensis. Physiological Zoology 57: 40-45.
Leather, S.R., K.F.A. Walters, and J.S. Bale. 1993. The Ecology of Insect Overwintering. New York: Cambridge University Press.
Salt, R.W. and H.G. James. 1947. Low temperature as a factor in the mortality of eggs of Mantis religiosa. Canadian Entomologist 79: 33-37.