Springtime is pollen season. Most insects have come from hibernation and are looking to establish nests and to begin pollination. Some insects are solitary while others comprise large colonies or nests, these include many of the stinging insects.
Carpenter bees are most active during the pollen season. Wasps, yellow jackets and hornets are more problematic during the summer and fall after the nests have become extremely large.
Wasps can also pose a problem during the winter months. Large numbers of female paper wasps will hibernate in and around chimneys and will fly around during warm sunny days and then retreat back to their hibernacula. Some will wind up inside the house or attic during this time.
Some homeowners stumble upon nests by accident. Simply cutting grass or cleaning around the yard can disturb a nest.