According to the website Eliminate Wasps, timing is extremely important when it comes to spraying wasps. Wasps are social insects that attack in groups and can be very dangerous if approached at the wrong time.
Best Time of Day
Wasps are active during the day and dormant at night, which makes night time the best time to spray. Do not spray at dusk or dawn, even if the sunlight is minimal, as wasps are more likely to attack in any sunlight. Wasps are extremely reactive to light, so when spraying in the dark, do not use flashlights or artificial light of any kind. Long-range wasp sprays, which can be applied at 20 feet, are safest when spraying in the dark.
Best Time of Year
Spraying is most necessary in the months from August to October, when wasps are most aggressive and more likely to sting. Wasps will die out in the winter months, so if a nest is discovered in the late fall spraying may be unnecessary.
The reproductive cycle for wasps runs throughout the summer and early fall. Spraying suspected nests in late spring or at the beginning of summer could eradicate the wasp population before the reproductive cycle begins.
Since wasps cannot fly in temperatures below 50 degrees, spraying is safer and more effective if the temperature is below that mark.
Exceptions to Spraying Times
Paper Wasps are the only type of wasps that should ever be sprayed during the daytime, but avoid standing under the wasp nest while you do it.