Temperatures have been warmer than average this winter, but in most regions, insects don’t have the insulating benefit of snow either. Will these conditions make a difference to the winterkill of important canola insect pests? In general, overwintering mortality is not a major factor in reducing insect populations, but we know of a few important exceptions:
Bertha armyworm. Studies have shown that cold soil temperatures, which can result from a lack of an insulating layer of snow, can reduce survival of the overwintering pupae of bertha armyworm. Soil temperatures need to get below -7°C for a sustained period of time to cause significant mortality of bertha armyworm. As the soil temperature gets further below -7°C the length of time needed to cause significant pupal mortality decreases.
Cabbage seedpod weevil. Soil temperatures below -7°C have also been shown to kill the cabbage seedpod weevil.
Grasshoppers. Cold soil temperatures have also been shown to kill grasshopper eggs, but the soil temperatures generally need to be quite cold (about -15°C) in the regions where the eggs are deposited to cause substantial mortality.