AThe woodpeckers are probably going after the bee larvae because of the noise they make as they grow within the wood, according to the Cornell Lab of Ornithology.
To thwart them, it helps to know a little about the life cycle of the bees, which resemble bumblebees but have shiny, rather than fuzzy, abdomens. Beginning in April or May, the females flit about, then chew into wood to create half-inch-wide tunnels where they lay their eggs. The tunnels usually go lengthwise within a single board — a piece of siding, a railing or, as you’ve seen, a part of a trellis. The bees typically don’t reach the framing of a house and thus don’t cause structural damage.
At the far end of the tunnel, the female bee deposits an egg and a little food packet, mostly pollen. Then she seals off that chamber with a thin wall, puts down another egg and food packet, and builds another wall. Over the summer, the eggs hatch and the larvae grow fat on the food left for them. They make enough noise to tip off woodpeckers, which then use their beaks to unearth the tasty treats. This, of course, expands the damage. Instead of having a single half-inch entry hole, you get a series of holes where the woodpeckers got into each little chamber.
If the woodpeckers don’t get to them first, the larvae chew their way out and emerge as adult bees after about three months, in reverse order of when the eggs were laid. The adults overwinter, often in the then-empty tunnel. In April or May in the Washington area, the adults emerge and the cycle repeats.
Knowing all this, you can target your actions to the most effective times of the year. One solution is to spray into the bee entry holes with an aerosol labeled for use against wasps, hornets and bees. Or puff a little insecticide dust labeled for use against carpenter bees into each hole. Do this in early spring when you first spot the bees flying about, or in late summer, when the adult bees emerge. Wait a week after you apply the insecticide so there’s time for the bees to come in contact with it, then plug the holes with wood putty or stainless steel wool.
Another solution is to remove and replace all boards that the bees have infested. Do this during the summer, when the larvae are within the boards. In the spring, there’s too much risk that the adults will already be out and about. Carpenter bees are considered “solitary bees” because they don’t create the elaborate social structure of a hive. But generations do go back to where they were born. It’s common for offspring to extend branch tunnels from ones that their mother created. So the trick is to wipe out an entire generation fixated on your trellis.