Carpenter bees (Xylocopa spp.) are large, solitary insects known primarily for their wood-boring behavior, which can cause concern for property owners. Their common name, derived from the Greek word Xylocopa meaning “wood worker,” accurately describes the female’s habit of excavating tunnels into wooden structures. The annual activity of these bees is directly tied to the seasons, with their life cycle dictating when they are most visible and when they pose the most risk to exposed wood. This seasonal rhythm maps out their emergence, reproduction, and eventual dormancy.
Spring Emergence and Mating Flights
The annual cycle begins with the emergence of adult carpenter bees from their overwintering sites, typically in early to mid-spring, from March to May. This emergence is triggered by rising ambient temperatures, with sustained activity often occurring when the weather warms into the 70°F to 80°F range. The bees that appear are the same adults that survived the previous winter within the wooden tunnels.
The primary focus during this time is mating and securing nesting sites for the new generation. Male carpenter bees, which are incapable of stinging, become highly visible as they patrol and aggressively guard the territory around potential nest openings. Females, capable of stinging but generally docile, will either begin boring new entrance holes or refurbish existing tunnels to prepare for egg-laying.
Peak Summer Nesting and Tunneling Activity
Following mating, the period from late spring through mid-summer, generally May through August, marks the peak of the female’s tunneling and reproductive activity. The female uses her strong mandibles to chew a perfectly circular entry hole, about half an inch in diameter, into unpainted or weathered wood. After boring a short distance perpendicular to the wood grain, she turns her tunnel 90 degrees and excavates a gallery parallel to the grain.
This tunneling phase is the source of the greatest structural concern, with a single gallery often extending six to twelve inches or more. Within this tunnel, the female provisions a series of cells, each containing a single egg placed on a ball of “bee bread,” which is a mixture of pollen and regurgitated nectar. The visible signs of this work include piles of coarse sawdust, called frass, accumulating beneath the entrance holes.
Late Summer and Fall Feeding of New Generations
The new generation of adult carpenter bees begins to emerge from their sealed brood cells in late summer and early fall, typically around August and September. These newly developed adults chew their way out of the tunnels, signaling the second major period of visible bee activity for the year. This new cohort is distinct from the spring generation.
The main activity for these young adults is intense feeding, often referred to as hyperphagia, to prepare for the coming cold. They forage heavily on late-season nectar and pollen to rapidly build up the necessary fat reserves that will sustain them through the winter dormancy. The young bees then begin searching for a suitable place to settle for the winter.
Overwintering and Dormancy
As late fall approaches and temperatures drop consistently, usually around October or November, the carpenter bees enter a period of inactivity called diapause. They retreat to sheltered locations to conserve energy. This dormant period lasts throughout the winter months, typically until the following March.
The chosen overwintering sites are usually the very tunnels they or their parents created, or other deep crevices in wooden structures. Multiple bees may huddle together within the galleries during this time, remaining protected from the elements. While they are not actively foraging or boring during this phase, their presence confirms that the bee population remains present throughout the year, ready to re-emerge when spring warmth returns.