Issue 1, April 16, 2012

Weather Impacts on Insects

This has been an unusual weather year, and there have been numerous concerns about its effect on insects. Although the winter was mild, the warmer temperatures had little effect on insect survival or spring emergence. Although we notice the difference between normal winter temperatures in the teens, single digits, and below zero degree F temperatures, and those this past winter with highs in the 30’s and 40’s degrees F, they are not so obvious to insects. Most insects have a base temperature of about 50 degrees F, meaning that they do not develop at temperatures below it. Being cold-blooded animals, insects are dormant and essentially non-functional at these lower temperatures. Temperatures in the 40’s result in little more development than those in the single digits.

Insects in Illinois tend to have a geographic range from about Atlanta, Georgia to the southern edge of Hudson Bay in Canada. With Illinois being in the middle of that range, it takes exceedingly warm or cold winter temperatures to have much effect on them. There are exceptions, but they are not the rule. Exceptions include Japanese beetle whose grubs die when the ground is frozen at least one foot deep for at least three weeks. Gypsy moth eggs die when exposed to three successive days of high temperatures of 20 below zero F or colder. Overwintering larvae of mimosa webworm have a high mortality in normal northern Illinois winters. Many of those that survive the winter crawl under the shingles of heated buildings. Generally, the warmer winter temperatures had little effect on insect survival over a normal winter.

Occasional days of very warm weather have little effect on wintering insects. However, the two to three weeks of warm weather that we had this spring caused many insects to emerge earlier than normal. At this time, insect emergence and development is about four weeks ahead of schedule. Several insects that normally emerge or hatch in mid-April to early May were present in late March. These include European pine sawfly, Eastern tent caterpillar, and Gypsy moth.

The cooler days with highs in the 50’s degree F that we experienced recently greatly slowed insect development and allowed the calendar to partially catch up. Typically, early springs tend to be followed by two to three week periods of abnormally cold weather. This slows or stops insect development, resulting in a more normal year of insect activity later in the growing season. These are commonly referred to as blackberry winters. According to Wikipedia, a blackberry winter is a southern and Midwestern term for a period of about 14 days when the low temperature reaches as low as 20 degree F. It normally occurs in May when the blackberries are in bloom.

Freezing temperatures cause little mortality of spring-occurring insects. Insects reduce the liquid portion of their blood and cells during winter which lowers their freezing temperature. They also produce long chain alcohols including glycerol and ethylene glycol that also lower their freezing temperature. Spring-occurring insects retain much of these features through most of the spring, making them very resistant to cold injury. Cold injury typically occurs when ice crystals form in cells, causing cell wall rupture. With these adaptations lowering the cell contents’ freezing temperature, this damage does not occur and insects survive. (Phil Nixon)

Phil Nixon

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