Powderpost and other wood-destroying beetles in Minnesota structures
Wood destroying beetles, including powderpost, deathwatch, and other beetles, are united by the type of damage that they cause to wood in service. In a technical sense, wood destroying beetles in Minnesota are represented by the families Lyctidae (true powderpost beetles), Anobiidae (deathwatch beetles), Bostrichidae (false powderpost beetles), and Curculionidae (broadnosed bark weevils). These beetles get their common name because they produce fine, powdered dust as they feed. This material, called frass, is a combination of boring dust and fecal material, and is helpful in determining which groups of beetles are causing the damage.
Hosts, habits and diagnosis
Lyctidae: Lyctids infest hardwoods with large pores, such as oak, walnut, ash, and hickory. Hardwoods with smaller pores, such as birch or maple, are rarely infested. Softwoods like pine, spruce, and fir are not susceptible to powderpost beetles. Powderpost beetles can also infest some woods of tropical origin (e.g., mahogany and obeche) and can attack bamboo although it is not wood. Powderpost beetles usually infest relatively new lumber, which has a higher starch content than older wood. Therefore, recently seasoned lumber and semi-manufactured wood products such as hardwood flooring, floor joists, and paneling are most vulnerable to attack. As wood ages, starch content decreases, making reinfestation less likely with time. Nonetheless, infestations have been observed in wood 40 years of age and older. Once starch levels drops below 3 percent, the beetles will no longer infest the wood. These beetles are often "built-in" to structures because lumber is infested when it is installed (as flooring), but they can also be brought inside the structure within wooden items, such as furniture or knick-knacks. The frass of true powderpost beetles is diagnostic; it has the consistency of talcum powder and has no grittiness at all.
Anobiidae: Anobiids infest the sapwood of both hardwoods and softwoods, although at times they will colonize heartwood as well. They are frequently found in older wood (10 or more years of age) and in high moisture areas. Some anobiids prefer to feed on wood infected with rot fungi. The frass produced by anobiids is often found in clumps or pellets and has a distinctively gritty or coarse feel. These beetles are infrequently reported in Minnesota, but cause extensive damage in the western and southeastern United States. In these areas, anobiids normally enter homes from outside, rather than being built in or brought in. In any region of the United States, anobiids may be brought into structures with imported antique furniture. One species of note in Minnesota is Ptilinus ruficornis, which attacks logs of aspen (Populus spp.) in rustic cabins.
Bostrichidae: Bostrichids are often found in both hardwoods and softwoods, though hardwoods are preferred. Bostrichids frequently attack woods of tropical origin such as lauan and mahogany. The frass of these beetles is a combination of fine powder, pellets, and larger wood chips, and is frequently cemented into the feeding galleries made by the larvae. In Minnesota, most reports of bostrichids result from furniture imported from Asian countries.
Curculionidae: Hexarthrum ulkei is a weevil that prefers moisture-damaged softwoods (like pine). It is generally found in older structures near bathrooms and other areas that are consistently exposed to water. The frass produced by H. ulkei consists of small oval pellets and is slightly gritty. Infestations of these beetles often occur in hidden places, and consequently, damage can be extensive.
Description and life history of insects
Lyctidae: Adult lyctids are small (1/32"-1/4"), slender, and uniform reddish-brown to black in color, with a prominent head easily visible from above. They are similar in appearance to some beetles that infest stored food (e.g., flour and grain beetles). Adult beetles lay tiny, cylindrical eggs in the pores of wood. Once the eggs hatch, larvae bore into the wood though larval entry holes are generally not visible to the naked eye. The larval galleries run with the grain of the wood and are generally short. The creamy-white, c-shaped larvae feed for 3-12 months (several years in some cases) and are responsible for all the damage. Damage is generally first noticed 6-12 months after initial infestation when the adult beetles emerge from the wood. When conditions permit, powderpost beetles will reinfest the same wood from which they emerge. In most cases, other wood products are unsuitable for attack because they are too dry, too moist, or are covered with a sealant (e.g. paint, varnish, or wax) that prevents egg laying.
Anobiidae: Adult anobiids are also small, and vary in length from 1/16"-5/16". They are reddish-brown to black and are covered with fine hairs. Unlike lyctids, the head is not visible when viewed from above. They lay their round eggs in cracks and furrows in the wood, and bore both with and across the grain of the wood. The whitish, grub-like larvae feed for a long period of time and complete their life cycle in 1 to 5 years. Like lyctids, reinfestation occurs when conditions permit.
Bostrichidae: Bostrichids are large, normally between 1/8"-1/4" in length, though some can be much larger. They are reddish-brown to black in color. Like anobiids, the head of the bostrichid is not visible from above (with the exception of the black polycaon, Polycaon stoutii). Their eggs are slender and are deposited in pores exposed in "egg tunnels" that are constructed in the wood by the adult female. Beetle larvae typically feed for less than one year, though this time is lengthened as the wood dries out.
Curculionidae: Hexarthrum ulkei is a small weevil (snout beetle), between 1/8"-1/4" in length. The weevils are reddish-brown to dark brown in color with long rows of deep pits on their wing covers. Little is known about the basic biology of these weevils. They can easily reinfest moisture prone areas.
Powderpost, deathwatch, false-powderpost, and H. ulkei beetles are important because they feed on wood, damaging it and detracting from its appearance. Adults emerging from the wood leave small, circular exit holes (2-3 mm diameter), while larvae tunnel beneath the wood surface, converting solid wood into powder. In many cases, the holes can become so numerous that the damage looks like it was created by a shotgun blast. Most powderpost and related beetles do not normally cause structural damage in a short period of time, but they can quickly destroy smaller items like picture frames and tool handles.
The first step in managing powderpost beetles is to determine whether the infestation is active. Wipe and vacuum all dust from the wood, and examine the area a week later. If the infestation is active, new holes and fresh sawdust should be visible. Be sure that what is seen is not old dust that has dislodged due to vibrations. Check more than once (even many months later) if it is unclear whether old or new dust is found. If no new dust is found, the infestation is not active, and no control is necessary. Another technique is to circle all the exit holes you see with a pencil. If weeks or months later no new holes have appeared, the infestation is not active.
The best control method for an active infestation depends upon the type and size of the wood product involved. Beetles can be killed by freezing the material at or below 0° F for four to seven days. Because powderpost beetles can acclimate to gradually falling temperatures, the decrease must be sudden for this technique to be effective. Wood left outdoors for extended periods of time should be placed in a plastic bag or wrapped in plastic. This prevents absorption of moisture which can damage wood products. Heating smaller wood products at 120° F for 2 hours should kill all stages of the beetle, but be sure that the item is heated all the way through. Use caution, however, since some wood products and their finishes may be damaged by prolonged freezing or heating.
Another control method is to protect the wood surface with varnish, paint, or other similar sealants. While this does not kill insects in the wood, it does prevent reinfestation by eliminating all sites appropriate for egg laying.
An application of insecticide can kill emerging adult beetles and prevent reinfestation, but it will not kill most insects already present in the wood. All insecticides effective for control of powderpost beetles, such as borates (e.g. Tim-Bor, Bora-Care), or cyfluthrin (e.g. Tempo), can be purchased and applied only by licensed applicators. All of these insecticide treatments must be done over raw wood. Fumigation with methyl bromide or sulfuryl fluoride is another option that is suitable in situations where an outside source of reinfesting beetles is not present. Fumigation can be done to individual items (in small "tents") or to an entire structure, and is the only method that will kill beetles (in any stage) deep inside the wood that is part of the structure.
If the wood has been so badly infested that structural damage is evident, replace it with kiln-dried lumber. Because beetles may be present in the area of infestation, all new, unfinished, or otherwise susceptible wood should be protected with paint, varnish, another sealant, or an insecticide.
Published in Yard & Garden Brief October 2002