Woodpeckers drum on house siding for four reasons: to obtain food (insects) within the siding, to build a nest, to establish a male's territory, or just for the heck of it. The most common woodpecker damage to houses occurs on houses sided with rough-sawn plywood siding. The woodpeckers damage the plywood siding by drilling a series of evenly-spaced holes through the face of the plywood in long, straight rows. The holes are about 1/2 inch apart, 1/2 inch in diameter and extend to the middle layers of the plywood. In this situation, the woodpeckers are feeding on the larvae of leafcutter bees that have nested within the siding. This has been a moderately common problem on plywood sided houses throughout the U.S. for the past 15 years or more. Damage is found in both rural and urban environments.
Woodpecker damage to plywood siding occurs as a result of the following contingent events:
- Rough-sawn cedar plywood siding that has been routed ("grooved") to create a "reverse board and batten" appearance is applied in sheets as the external siding of the house. The routing of the plywood's top surface exposed the "inner core gaps" within the plywood where interior layers of the laminated plywood were not continuous. These gaps may extend all or part way across the plywood sheet.
- Leafcutter bees (any of many species) that normally nest in hollow plant stems and other natural cavities discover the exposed plywood gaps and begin nesting in them. The discovery of nesting sites by these small to medium-sized bees is by a random search process that goes on throughout each spring and summer.
- The female leafcutter bees cut dime-sized, circular disks of foliage from nearby plants and use these to construct cells or chambers within the plywood gaps. Each 1/2 inch long cell is provisioned with a ball of plant nectar and pollen. One egg is laid in each cell.
- The eggs hatch into small bee larvae ("grubs") that consume the stored food as they grow and develop.
- Any of several species of woodpeckers (commonly the downy woodpecker), in their normal, random search for food, find the bee larvae within the siding (probably by the sense of hearing) and begin drilling holes through the outer plywood layers. As each larva is consumed, the woodpecker moves slightly to one side and repeats the drilling to obtain the next larva. The result is the long row of holes across the face of the plywood.
The time for leafcutter be nesting is late spring though summer. We do not know how long after the bees complete the nest building that the woodpeckers begin the destructive search for food, but once the woodpeckers find the first bee, they quickly move down the line causing a significant amount of damage in a short period of time. Damage is confined to the face of the plywood. The bees do not tunnel or otherwise weaken the plywood - they utilize gaps that already existed. Woodpecker damage is usually confined the top and middle layers of the plywood.
To my knowledge, there is no effective chemical control for this problem. The bees are a valuable part of the natural ecosystem and it would be unwise and impractical to control them by spraying plants where they forage. Woodpeckers are protected birds and cannot be legally killed. The best prevention, in my opinion, is to examine plywood siding and plug with caulk any gaps exposed along the routed grooves. Siding that is already damaged is not usually reattacked because the gaps are destroyed and no longer suitable for bee nesting. Damaged siding can be replaced or the surface holes patched and stained.