Who survives under the snow?
by Dr. Hector Carcamo
How cold is cold enough to reduce insect pest numbers like the evil weevils that attack our crops? Well the answer, as is often the case, is ‘it depends.’
The first one will freeze and die when exposed to around -7 degrees C, but the other two can take it up to around -14C. These temperature thresholds are called supercooling points.
Insects can produce antifreeze molecules that allow them to resist freezing to certain cold temperatures. The supercooling points for pea leaf and alfalfa weevils are well matched with where they overwinter.
The cabbage seedpod weevil prefers tree shelters, ditches, road margins where there is more leaf litter or where more snow may accumulate. This will result in better insulation so that the soil temperature where they overwinter will seldom if ever get cold enough to kill them.
For alfalfa and pea leaf weevils, they often overwinter in semi-perennial forage fields such as alfalfa where temperatures may get colder at the soil level, but still seldom close to their -14C threshold. Another important factor is the length of the winter. Even if the temperatures in the microhabitat are mild, a very long winter, like the one we saw in 2017-2018, may force insects to run out of fat reserves so that they are weaker in the spring. This would make them more vulnerable to diseases and limit their dispersal abilities. So here, here, let us hope for a long and very cold winter to get rid of some bugs.