Utah Pests News Winter 2007-08

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Insects:  Where are They Now? 

 
 

The spring cankerworm spends its winter as a pupa in the soil until late March.

 
 

Katydids survive the winter as eggs on branch stems.

 
 

The rose stem girdler overwinters as a larva in the pith of its host plant.

Maybe you’ve wondered where insects go in the winter. It’s a great question and a huge obstacle for insects to overcome every year. Some insects can’t survive the winter (especially in northern Utah) and simply have to overwinter to warmer climates every year. A great example of this is the monarch butterfly. They overwinter in Mexico, and adults travel north to the U.S. and Canada every summer. Monarch butterflies migrate back to Mexico every fall; individuals have been known to fly almost 2,000 miles in four months. Some insects try to find harborage in human structures where there is heat. Boxelder bugs are a good example of insects that become a nuisance in the fall as they seek winter shelter.

In most cases, insects have adapted to cold temperatures by evolving a cold-hardy life stage. The optimal overwintering life stage is variable between species, but could be an egg (e.g., aphids, some mosquitoes), a larva or nymph (e.g., white grubs, borers), a pupa (e.g., lace wings, most butterflies and moths) or an adult (ants, lady beetles, most true bugs). Often insects will bury themselves in soil, leaf litter, under rocks or other debris to offer some protection from the elements. Others will stay on or near the plant they will feed on the following year.

Perhaps the real question is HOW do insects survive the winter? Some have adapted to humans and seek warmth of buildings; often these insects are perceived as pests. But most insects go through diapause, which is a quasi-dormant period often triggered by cold temperatures and photoperiod. Wikipedia defines diapause as "a neurohormonally mediated, dynamic state of low metabolic activity." Diapause slows down or stops all bodily functions, including eating. Sometimes diapause is a mandatory resting period, meaning that an insect must go through dormancy to continue the life cycle. To further enhance the survival rate of overwintering, insects will purge most of their gut contents and frass to minimize any unnecessary water in the body. Dehydration will lessen the chance of ice crystals forming inside the body. Also, insects will start to produce glycerol, acting like antifreeze to help winterize the body for cold temperatures. A combination of these body changes provides the greatest likelihood of surviving temperatures below freezing.

Several factors can influence the chance of insects surviving the winter, including how soon freezing temperatures begin in the fall and subsequent warming/cooling events. For example, if a sudden freeze hits the Wasatch Front in the fall, insects will not be properly prepared in purging water and finding secluded sites. Most certainly an early freeze will kill a small portion of insects before they can enter diapause. But even if insects have enough time to prepare for diapause, repeated warming and cooling periods during the fall and spring can bring insects out of their resting state too soon. If insects emerge before plants are growing, they will likely starve or not survive subsequent freezes.

Where insects go during the winter will affect their survival the following spring. Some migrate, others try to find hiding places in human structures, while most simply "grin and bear it" by finding secluded areas and producing antifreeze. All are major obstacles for insects to overcome each winter. I wonder if they ever tried wearing gloves and hats?

 

-Erin Hodgson, Extension Entomologist