Oh...but they are there. They are in hibernation (or larvae stage) on the ground or trees. To be effective you want to kill the larvae on the ground before they mature and fly away.
In general, insects are able to survive cold temperatures easiest when the temperatures are stable, not fluctuating through alternate thaws and freezes. Many insects can gain shelter and nourishment through the winter in a variety of micro-habitats. Among these niches are under the soil, inside the wood of logs and trees, and even in plant galls. One kind of fly is known by fishermen to be present in certain galls in winter, and the fly larvae are consequently used as fish-bait. Blankets of snow benefit insects by insulating the ground and keeping the temperature surprisingly constant. Honeybees have been studied during the winter and are found to remain semi-active in hollow trees through the generation of body heat. The consumption of up to 30 pounds of stored honey during the winter months makes this possible. Heat energy is produced by the oxidation of the honey, and circulated throughout the hive by the wing-fanning of worker bees. Insects that are inactive during the winter months undergo a state in which their growth, development, and activities are suspended temporarily, with a metabolic rate that is high enough to keep them alive. This dormant condition is termed diapause. In comparison, vertebrates undergo hibernation, during which they have minor activity and add tissues to their bodies.
José