The answer to 'when to treat for bees' is that the best time to treat for bees on your property is in the spring and early summer, before the bees become more active and establish a hive. This is when bees are most vulnerable and the treatment will be most effective.
Bees are most active during the warmer months when they are foraging for nectar and pollen to feed their colony. Treating for bees in the spring and early summer, before the peak of their activity, will help prevent a full-blown infestation and the establishment of a hive on your property. This is the ideal time to apply insecticides or other bee control methods to eliminate the threat.