Insects and Winter Weather

by Karen Ripley, Forest Health Program Manager
Washington Department of Natural Resources 

I’m frequently asked whether winter weather will likely be sufficiently severe to knock back a specific insect population. Not usually. Here’s a quiz that will describe why.

1. What is “phenology”?

A:  The study of periodic plant and animal life cycle events and how these are influenced by climate.
B.  The study of the relationship between a person’s character and the shape of his or her skull
C.  The use of corrosive compounds found in tree bark to make resins, solvents, and antiseptics.
D.  The study of language parts that sound alike but have different meaning to native speakers.

Answer:  A. Phenology is a study of the timing of natural events. Native insects time their life cycle and behavior to avoid the harshest of winter conditions. Many, like tent caterpillars and tussock moths, overwinter as durable eggs protected in masses or deposited in sheltered places. Many insects overwinter in sheltered sites that stay dry and provide protection from extreme conditions. Western spruce budworm caterpillars overwinter in little silken sleeping bags called “hibernacula” constructed in bark cracks.  Mountain pine beetles stay beneath tree bark, which may also be insulated below snow that accumulates around infested trees.Phenology

2. What is “glycerol”?

A.  The form of sugar that is used directly by the body for energy.
B.  A colorless, odorless sugar alcohol that is the backbone of many fat molecules and is commonly used as a food preservative, sweetening agent and ingredient in explosives.
C.  A 1996 hit made by the British grunge/alternative rock band Bush.
D.  The act of sliding, falling and tumbling down a snowy mountainside.

Answer:  B.  Glycerol is a natural anti-freeze. When liquid water freezes it expands into a lattice. This concentrates other chemicals and may form sharp crystals that can damage cell walls and tissues. Some insects prepare their bodies to avoid freezing in winter by reducing internal water and increasing the volume and concentration of glycerol. This enables them to survive lower temperatures without freezing. This strategy is very common in the northern hemisphere where predictable weather enables extensive seasonal cold hardening. So, rather than extreme winter cold being a factor in dramatically reducing insect outbreak populations, it’s more common that insects are killed by unusual, sudden cold snap freeze events in early fall or late spring. Atypical events may catch insect populations before they have adequately prepared or as their bodies emerge from winter readiness.

3. What is “freeze tolerance”?

A.  The genetically-controlled ability to quickly drink a slurpee without the onset of rapid, debilitating headache.
B.  Lack of prejudice against winter recreation activities such as ice fishing, Christmas vacation in Minnesota, or camping in snow caves, igloos or ice motels.
C.  The ability of some insect species to survive ice formation within their tissues.
D.  The variation in freezing temperature associated with different concentrations of molecules dissolved in a liquid.

Answer:  C. Many insects can tolerate freezing. This adaptation is more common in the southern hemisphere (where climate can be highly variable with fairly short duration freezing periods) and in the arctic (where the duration of freezing periods may be very long) than in the northern temperate regions. They do it by enabling the rapid formation of ice in tissues that won’t be damaged.

4. What temperature is required to kill cold-hardened mountain pine beetles?

A.  -48 °F (This is the coldest temperature measured in Washington State.  December 30, 1968 in Mazama and Winthrop). 
B.  -40 to -31 °F consistently or for several straight days.
C.  -58 °F to +5 °F (This is the temperature range the US Centers for Disease Control and Prevention recommends setting freezers for vaccine storage). 
D.  -80 °F (This is the temperature of some storage freezers at the US Centers for Disease Control and Prevention. (Warning:  May not be true. I read it in an advice blog about techniques for sterilizing habitat items like sticks or rocks to put into an aquarium for raising frogs.))

Answer:  B.  This temperature condition used to occur frequently on forested mountain peaks of the western U.S. and in the northern Rocky Mountains in British Columbia. By regularly reducing beetle populations, this temperature feature protected lodgepole pine and whitebark pine that otherwise would seem to be of susceptible sizes and stand density. The length of the growing season also exerts an important effect on mountain pine beetle population dynamics. As the climate has warmed in recent decades, these barriers appear to have fallen, enabling rapid mountain pine beetle population growth and range expansion, causing extensive mortality of lodgepole and whitebark pine.


5. Why are woolly bear caterpillars interesting in winter?

A.  They are freeze-tolerant.
B.  They overwinter as larvae that can quickly become active during periods of mild weather or if you accidentally bring one inside with the firewood.
C.  They have colorful fur coats that forecast winter weather conditions. A narrow brown band observed in the fall predicts a harsh winter.
D.  The width of the middle brown band is associated with age of the caterpillar. So, a wider band could mean that the previous winter ended earlier allowing the caterpillar to have grown older by the time it was observed in the fall.

Answer:  A and B are correct.  C is folklore.  D may be true.woollybear narrow 2010 cropped