Given the recent plunge to single digits and sub-zero temperatures, multiple questions came in regarding the affect on insects. Specifically, can we expect these temperatures to kill off insect pests?
While winter climate can influence which pests we may or may not encounter in our region, in general the insects we do encounter (native or naturalized species) are largely unaffected by these low temperatures. There are a number of adaptations which enable insect populations to remain largely unaffected. Migration and 'supercooling' are two ways insects survive winter.
Migration can involve geographical relocation or simply migration deeper in the soil or into the wood of the tree. Japanese beetles, for instance, overwinter as grubs in the soil. In winter they will migrate deeper in the soil in order to stay beneath the frost layer. When the average depth of the frost remains in the top few feet, grub migration enables a large portion of the population to complete their life cycle and emerge the following summer as adult beetles.
An interesting adaptation found in many insects is the ability to ‘Supercool’ themselves. There are a few ways in which insects can do this, but generally speaking they can produce chemicals which act like antifreeze inside their bodies.
Being cold-blooded, an insect's internal temperature is largely influenced by outside temperatures. Supercooling enables insects to avoid ice crystal formation inside their bodies, thereby minimizing and preventing the cell destruction and subsequent death.
An emerald ash borer will overwinter in the larval or pre-pupal stage. With the ability to supercool body fluids, paired with an overwinter location beneath the bark, they are protected from otherwise lethal temperatures. Prolonged temperatures in the ballpark of negative 30F can begin to cause larger EAB population die-off, but we do not typically see that occur to any great extent in the Upper Midwest.
Early or late frosts can “catch insects off guard”, causing freeze kill. Early frosts in fall hit before an insect has time to migrate to a protected location or produce the supercooling chemicals in preparation for winter. Late frosts in the spring can have the same lethal consequences on insects who have emerged, ready for spring activity.
While severe winter conditions can reduce some insect population numbers to an extent, the population numbers are more heavily influenced by the weather and climatic conditions during the seasons when the insects are active. From the time of insect emergence through mating, seasonal weather will have a more significant impact on pest populations. Abnormal and extended dry or wet periods at the right time (or wrong time from the insect's perspective), can have a noticeable impact on an insect species.
As a G.O.T.C. Recognized Facilitator & Master Instructor, I.S.A. Board Certified Master Arborist, and T.C.I.A. Certified Treecare Safety Professional, Curt has spent over 30 years dedicated to the study and care of trees.