Answer: I think what you are seeing are carpenter bees. Bare, unpainted, or weathered softwood is especially attractive to carpenter bees. To prevent carpenter bee infestation, paint or varnish all wood surfaces. Apply two coats of a good exterior primer and follow up with at least one coat of finish. However, painting is not practical for the back of most fascia boards and many other wood surfaces. Wood stains provide little repellent action. Carpenter bees do not occupy plywood, but no wood, even painted or pressure-treated lumber, is safe from carpenter bees. The older type of pressure treated lumber has been phased out and no current borate-treated product manufacturer currently makes the claim that it prevents carpenter bee infestation.
Sometimes standard window screening can be used to keep bees from getting into areas where there may be unpainted surfaces or other places where it might be difficult to paint. Cut strips of metal window screening; don?t use the fabric or plastic type (they can bore right through it). Wedge or staple the screening into place where the bees are working.
Paints that include an insecticide may repel bees attempting to nest. However, the insecticide does not remain active for more than one season. Nail holes or exposed saw cuts should be filled in with caulk or dowels and painted. If practical, remove severely damaged wood and replace with chemical pressure treated wood to deter nest construction. Carpenter bees do not damage nonwood surfaces, such as vinyl siding.
To control active pests, spray or dust insecticide directly into the carpenter bees? entrance holes or adjacent wood surface to reduce carpenter bee activity. These control efforts should be attempted in late afternoon or at night when the bees are inside the wood tunnels. Injecting a pesticide into the tunnels can be effective, but a barrier lasts longer than a poison. Therefore, 24 hours after injecting insecticide, tunnel entrances should be sealed with caulking compound to kill the trapped carpenter bees and to prevent recolonization by other nesting bees. This is challenging and labor-intensive. Do not use the foam insulation that comes out of pressurized cans and expands to fill cavities because carpenter bees go through it easily. Sealing holes can be done at any time of year.
Good luck with your project!
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