Powderpost beetles

Дата канвертавання22.04.2016
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Small beetles lumped into the classification of powderpost beetles are found in three separate families, Lyctidae, Anobiidae and Bostrichidae. Generally speaking, Lyctidae larvae are found in bamboo and hardwoods. The anobiids are usually associated with furniture and structural timbers. Bostrichidae are infrequently encountered.

The larvae of powderpost beetles feed only on the sapwood and not in the heartwood. The sapwood contains nutrients which will support the life cycle of the beetle larvae. The heartwood is the center or dead portion of the tree and provides no nutritive value for powderpost beetles. Therefore, the amount of sapwood in a particular timber will dictate whether or not the feeding of powderpost beetles is structurally damaging. Most commonly in historic buildings, where large, oversized timbers are used, particularly in post and beam construction, powderpost beetles cause minimal structural damage.

Historically, old-timers knew the sapwood of timbers was particularly vulnerable to beetle attack, and often chopped away most of it prior to putting a timber in a building. Smaller dimensional lumber, such as floorboards, often had powderpost beetle on the outside edges where the sapwood was located, with the center of the floorboard never having been attacked.

In historic house settings, powderpost beetles are generally introduced into the building when it is built. As the lumber is stacked outside of the construction site, depending on the time of the year, beetles fly onto this seasoned wood and lay eggs on it. The larvae are then built into the building during construction. Slowly over the years, the larvae eat all of the available sapwood and the infestation dies out naturally. However, in the interim, some beetle infestations may move into furniture or other susceptible wooden elements and artifacts found in the structure. This is particularly true in unheated historic house settings.

Another way powderpost beetles are inadvertently introduced into historic houses is via firewood. Often firewood is stacked outside where the ends of it are subject to attack by powderpost beetles. When the firewood is brought inside, the infestation is introduced into the structure. Beetles emerging from the firewood may mate and fly off to other parts of the building to infest furniture, unfinished wood, and artifacts. Often firewood is stored for a period of time in basements of historic houses, again providing a very moist site for infestations to act as a reservoir for movement into the rest of the building.

Peeled-log homes in particular are susceptible to powderpost beetle attack. Modern-day, as well as historic round-log homes, are vulnerable. As the log home ages, the beetle larvae consume most of the available sapwood and the populations eventually die out. On the exterior, the exit or flight holes of the adult beetles may leave the logs exposed and vulnerable to the infiltration of rainwater. Once the powder-packed galleries of the larvae are wetted, rot begins to develop in the logs and may eventually cause extensive deterioration. Additionally, small wasps often excavate the powder from existing powderpost beetle galleries and use these cavities for nesting purposes. Large amounts of powder (frass) can be seen cascading out of old powderpost beetle holes onto the face of the logs.

The reason powderpost beetle infestations seem to be most significant in crawl spaces with earthen floors, unventilated attics of old buildings, and unheated historic homes, is these situations maintain high moisture levels with the wood providing necessary moisture for the development of the larvae of these beetles. Information enclosed with this section details some of the particulars of the life history, habits and importance of wood moisture in maintaining populations in wooden elements.

The Anobiidae female lays her eggs in cracks and checks in the wood. The tiny larvae hatch out, enter the sapwood and generally feed with the grain, packing the gallery with powdered frass. As the life cycle is completed, the larvae moves to just beneath the surface of the wood, chews a neat round hole in the wood, packs it with powder, and then backs off to pupate into an adult beetle. When the adult beetle emerges from the hole, it pushes this plug of powder ahead of it upon exiting. Fresh exit holes are quite bright and the color of the wood as if it were freshly sawn. Old inactive infestations have virtually no powder associated with them, no plugged holes and the exit or flight holes are quite dark.

The Lyctidae female has a very long egg-laying tube (ovipositor) for laying eggs deep in the pores of hardwoods. From here the Lyctidae larvae begin to tunnel with the grain creating very fine, talcum powder-like frass which continues to fall out of the natural pores of the wood. Lyctid infestations therefore have lots of fine powder associated with them. There will be minute holes with drift, which are the pore sites of original egg deposition. There will also be powder drifting from the larger exit or flight holes out of which the adults have emerged. Lyctidae infestations are much less commonly found for they are restricted to hardwood species high in sugar content or bamboo. Lyctids are therefore found in furniture, basketry, hardwood flooring, and bamboo. They are also commonly found in oak beams used for post and beam construction. As previously stated, the larvae are restricted to the sapwood portions.

Often control of powderpost beetles can be obtained by simply turning on the heating system in a home and drying it out in the winter time. Powderpost beetles require fairly high moisture content in the wood in order to survive. By heating a home in the winter time, moisture levels in the wood will be reduced below that necessary for successful survival of the larvae of powderpost beetles. However, well insulated attics or crawl spaces may continue to have infestation, even in a heated structure.

Practices such as providing proper cross ventilation in crawl spaces, the utilization of vapor barriers on the floor of the crawl space, the combination of vapor barriers and concrete slabs on the floor of crawl spaces and basements, the elimination of vapor barriers on the undersides of floors directly over basements or crawl spaces, and the elimination of vapor barriers in attics of unheated historic homes will go a long way towards reducing the overall moisture content of the wooden elements of the structure.

Care should be taken not to inadvertently introduce powderpost beetle infestations into historic properties via basketry, both modern and antique; firewood; infested artifacts; and infested building materials. Because the temperatures used for kiln drying lumber are usually not hot enough to kill larvae of powderpost beetles, care must be taken in the selection of modern-day building materials and flooring.

Insecticides labeled for powderpost beetle control fall into two groups, the borates and the phosphates. Generally speaking, the borate material, Bora-Care, is a water soluble protectant which can be used in situations which are not exposed to weathering. In wood with high moisture content, this water soluble borate material migrates deep into the wood to provide long-term control against powderpost beetles. In most situations however, the borates will not migrate into a dimensional timber much further than one-quarter inch. Topical spraying of borates to unfinished wood, such as in attics and crawl spaces is cost effective and generally provides long-term residual control. It is not very effective in hardwood flooring because of the finish.

Dursban TC and Equity (chlorpyrifos), organophosphate insecticides, are no longer labeled for powderpost beetle control. These insecticides formerly were available in formulations designed to be mixed with water and sprayed on wood. The basic ingredient is not water soluble and therefore would not migrate as deeply into the wood as would the water soluble borates. They penetrated the wooden surface to a degree and would then recrystallize, leaving long-term protection.

In exterior areas where weathering may be a problem, the borates alone are not suitable for long-term residual control. They are water soluble and therefore will leach their protection from the wood. If they are used on the exterior, after drying, the wooden elements should be treated with a moisture sealant.

The approach of a topical application is to provide a chemical barrier through which the powderpost beetle larvae and adults cannot penetrate. Upon penetration or emergence, the larvae or beetles will be killed. Successive egg deposition will not result in continuing infestations and eventually will be totally controlled.

2003 Thomas A. Parker, PhD

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