This question comes up every year. Many of our insect pests have had it easy for the last few winters because of the mild temperatures. But now that we have experienced a much more normal (?) Canadian winter, does this mean that the insects won't be a problem this spring? Well, not entirely.
How Insects Cope With Winter
First, it depends on how they cope with winter. Some insects migrate south for the winter. Wouldn't it be nice if we could all do that? These include black cutworm, corn earworm and leafhoppers for example. Others bury themselves as adults under leaf litter, in ditches and in sheltered areas like woodlots where the temperatures are warmer than if they were exposed on the soil surface in the field. These insects undergo a state of diapause where all growth and development is suspended until temperatures and daylength increase again in the spring. They may also cluster together to help increase and stabilize their surrounding temperatures. Examples of these insects include bean leaf beetle, corn flea beetles and ladybird beetles.
If the insects overwinter as larvae they too may bury themselves under leaf litter or could migrate down under the frost line in the soil, avoiding the freezing temperatures. Some insects can even tolerate freezing temperatures all together. These insects have the ability to replace the water in their bodies with glycerol, a type of antifreeze to prevent them from freezing. No matter what method, no insect can survive below -40 F.
So, what about this winter? Considering temperatures alone, yes, insect mortality will be high. But we have also received a lot of snow this winter, which acts as an insulator, increasing as well as stabilizing temperatures around the insect. It is much more difficult for an insect to survive when temperatures fluctuate. Especially if the temperatures warm up enough in the day to cause the insect to break diapause, thinking it is spring. Out of diapause, they are much more vulnerable and if the night temperatures were to drop back down below freezing it would be difficult for them to survive.
A study conducted in Iowa found that even when the average air temperature was below 10oC, the average temperature under leaf litter was only 2oC. Results from this study also found that the bean leaf beetle had a 100% mortality rate when overwintering in sites exposed to the air temperatures (that is no snow cover), whereas those under snow covered leaf litter had only a 65% mortality rate.
By understanding both the abiotic and biotic factors involved that enable an insect to survive winter, we can monitor these factors to better predict this survivability. Several models currently exist in the US that do just that. Research conducted at Iowa State University found that the accumulative temperatures in the months of December, January and February determine the survivability of corn flea beetles. Results demonstrated that mild winters, that is, the sum of the average monthly temperatures for December, January and February greater than 32oC, greatly improves the survivability of both the flea beetles as well as the bacteria they carry that cause Stewarts' Wilt.
We have been monitoring soil temperatures at several sites in Essex and Chatham-Kent Counties this winter to determine if this model can be used to predict the survivability of Ontario's flea beetle populations. Those results will be published in the first issue of this years CropPest newsletter.
Finally, the number of insects that go into winter plays a role too. For example, if there were only 100 flea beetles in a field in the fall that went into winter, a 90% mortality would mean that only 10 beetles would survive to come out in the spring. Not much to get concerned about. But if there were 100,000 flea beetles in that field that went into the winter, a 90% mortality rate would mean that there could still be 10,000 that survived to come out in the spring. Now that would be something to be concerned about. Especially if they were carrying the Stewart's Wilt bacteria.
What Can You Do?
What can you do to help reduce the chance of overwintering survival rate of insect pests? Reduce the amount of crop debris left on the soil surface in the fall. Reduce the number of volunteer weeds along ditchbanks and fencerows where insects may be overwintering. Also, in the spring following a mild winter, think about your planting schedule. Plant earlier or later, depending on the insect, to avoid or reduce crop damage potential for spring emerging pests. And become more aware of what insect problems you have from year to year. Good management begins with good monitoring.