My incoming email box this last month was full of questions about the powderpost beetle and its holes in kiln dried wood, and bamboo too. So let’s put everything we know about this pest in one place.
Name of insect
There is only one insect, other than the termite, that is able to survive in kiln-dried, low moisture content (7 percent MC) hardwood: The lyctid powderpost beetle. There are other PPBs, but the lyctid is the only one in dry hardwoods, so always include this “lyctid” name in your discussions.
This insect, after living on the wood for a year or so, matures into a flying insect at the end of its life cycle. It burrows a hole about 1/32 to 1/16 inch in diameter, moves to the outside and then looks for a mate to breed. The dust (technical name is frass) pushed out of this exit hole is extremely fine. So, if we see these small holes and frass appears that wasn’t there yesterday, we can be quite sure that we have living lyctid PPB.
It is rare to find the insect as it does not like light, but if you find one, one characteristic is that it has two “antenna” that look like there is miniature club on the end. The insect itself is quite small (1/16 to 3/16 inches long) and is not a good flier (compared to mosquitos).
Once the male and female mate, outside of the wood, probably in wood dust or other debris on the floor, the female looks for small holes or devices in the wood (this means red oak and ash are ideal with their open pores) to lay her 10 to 50 eggs. She has a special, long tube that extends from the rear of her body into the holes or nooks and crannies that she finds, so the eggs are well protected just under the surface. Really smooth wood with fine pores (e.g., soft maple) or wood with a finish on it does not have the nooks she needs.
In about 10 days (times vary with the temperature mainly; they are active between 60 F to 105 F, with 90 F being best), the eggs hatch and a little ¼ inch long, cream-colored worm, called a larva, appears. The larva then eats small tunnels in the wood for food; preferred moisture is 7 to 15 percent MC. After maybe 10 to 36 months (12 months seems common) of making these small tunnels, the larva pupates (turns into a pupa, which looks more like an insect than a worm) and burrows to the surface, probably not eating the wood anymore, where it emerges as a full grown insect. At this point, breeding is the insect’s goal, followed by death.
Once you see the exit holes, it is likely too late to control the initial infestation. Further, although the holes indicate the insect has left the wood, there are likely other larva and pupa in the wood that are not yet burrowing to the surface.
Hence, using an insecticide spray or powder will not stop those still deep inside the wood. An insecticide might control future infestations, but then only if it gets into the holes where the eggs are laid.
Fumigating the wood with a very powerful gas is possible, but is expensive and cannot be done safely without fancy equipment.
So, the only practical control is to kill the insects, eggs, larvae and pupae with heat. Research from years ago showed that 133 F throughout a piece of wood was adequate. However, it seems that the USDA is now requiring 160 F throughout the wood imported into the U.S.
The air temperature must be 20 F hotter than 130 F or 160 F, whichever standard you are targeting, in order to achieve the target temperature in a reasonable length of time. The time at the required temperature varies, with some rules indicating 10 minutes and some indicating 75 minutes. The National Hardwood Lumber Assn. is a good source for the latest information.
It is key to understand that almost every dry kiln will use 160 F as the hottest temperature for several days, so when the wood leaves the kiln, the wood is “sterilized.” However, once the wood is heated to 133 F, or 160 F, it is not protected from future infestations. So, how do we prevent an infection after the lumber leaves the kiln? The key to having zero risk of lyctid PPB is to prevent the insect from getting anywhere close to this sterilized lumber.
Because of the time delay between the eggs being laid and the insect making the exit hole, it is possible that any porous, hardwood lumber adjacent to the infected lumber with exit holes is likely infected: we won’t know for a year! So, prevention is the real cure for avoiding lyctid PPB infections.
The main source of the lyctid PPB that will infect freshly dried lumber is other lumber or wood. This includes any hardwood or bamboo dried by someone else, with tropical wood or bamboo being a common, but not the only source. The source also includes dust and debris as well as lumber. I have seen a PPB infection spread from wood stored or left as junk outside a building (closer than 30 to 50 feet perhaps) that has KD lumber inside. Think clean!
Once the wood is heated to 133 F, or 160 F, it is not protected from future infestations. Therefore, any stickers or bolsters (4x4s, etc.) must also have been in the kiln immediately before every use. Even hardwood dunnage used for shipping needs to be heated just before use. The storage area for lumber leaving the kiln must be free of debris…use a vacuum.
Prevention by avoiding contact of “sterilized” KD lumber with infected lumber is the key. When infections do occur, heat is the most practical way to stop additional damage and spread. ✚
Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 35 years. He is extension specialist emeritus at the University of Wisconsin-Madison.
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