A woman purchased some furniture made from air-dried western juniper lumber. She loved the furniture, but had a heck of a time getting rid of the occasional bug, which would insist on leaving wood dust on the floor of her new $500,000 log home, and small holes in the furniture. These critters were noisy - you could actually hear them chewing through wood. She obtained professional advice, none of which worked, and eventually figured-out her own solution.
Cerambycidae is the Latin word for a particular family of insects. You might want to know something about them because this insect family contains beetles who can wreak havoc on carefully-finished, value-added products made from conifers, such as air-dried western juniper. There are several varieties of these beetles, but they are usually clumped together and called either round-headed borers or long-horned beetles. The generic term "wood borer", is used in this article.
Wood-boring insects develop from eggs deposited in the bark of dead or dying trees, as well as logs. Upon hatching, larvae bore into the inner bark, forage there for several weeks, and then enter the wood. They slowly tunnel their way along and derive nourishment by ingesting soluble carbohydrates in the wood particles and/or available fungal tissue. The larval stage lasts from several months to several years and is the only phase in the life cycle of these insects when wood is used as food.
Following the larval state, the insect pupates and emerges as an adult to mate and fly away in search of susceptible trees or logs in which to deposit eggs. It is hypothesized that the oleoresins released when juniper is cut makes them especially attractive to the adult wood borers.
Wood borers seldom attack healthy trees, and in reality, do not cause serious structural damage. Nor do wood borers of the type so far observed in western juniper infest other dry lumber in storage or in wood structures.
Wood borers are a manageable problem and endemic to all resinous conifers, not just western juniper. Historically, the most effective method of eliminating wood borer activity has been through application of heat. Published dry kiln schedules for western juniper should be effective, and no wood borer evidence has been reported by end-users of kiln-dried lumber produced during the last four years of the Western Juniper Commercialization Project. Water sprays and immersion may also reduce wood borer activity, and any wood borers which have taken-up residence in a log are normally removed in slabs and edgings.
If solar kilns are used to dry juniper, additional heat may be needed to achieve the minimum kiln temperatures and length of time necessary to eliminate wood borer activity. For example, at least 130° F for eight to twelve hours for 4/4 lumber.
Many small producers do not have access to a kiln. What can they do?
First and foremost, process logs as soon as possible, especially if cut in the spring or summer, and eliminate wane (bark) during primary breakdown. Then as a security measure, especially if sawing during the spring or summer, apply a borax-based product, such as Bora-care® or Tim-bor® , both of which can be sprayed or used in a dip solution. A license is not required in the State of Oregon to purchase or apply these borax-based products. Oregon distributors include Van Waters & Rogers, Inc. (Portland, 1-800-452-4912) and Kemi-K Products, Inc. (Portland, 1-800-323-5364).
According to Stan Galyon, Vice President for Research and Development for Nisus Corporation, which produces Bora-care®, spray or dip lumber when green (moisture content greater than 20%). For example, it might take one week for a wetting spray to penetrate 4/4 green lumber, but months to penetrate dry lumber. Estimated cost of Bora-care® treatment is about $60/1000 board feet of 4/4 lumber, less for dipped lumber.
There may be other ways to reduce the risk of wood borer problems when air-dried lumber is intended for value-added products. According to several manufacturers, wood borer activity is significantly reduced, if not eliminated, when cutting is restricted to late fall and wintr, and logs are processed before the weather begins to warm-up in March.
The question of whether or not to treat air-dried lumber depends in large part on target product line and intended end-use. Rough-cut "farm lumber" may not require the same handling as lumber intended for rustic interior furniture.
Commercial market acceptance of juniper will be enhanced if producers treat or otherwise handle air-dried lumber to minimize infestation potential, and if end-users are advised of this potential.
So what did the woman do who had wood borer residue appear on the floor and carpets of her now log home? She said: "Every time I'd see evidence of the bugs, I'd got my can of WD40® , put that little plastic straw which comes with it on the nozzle, and give 'em a squirt. Worked like a charm."
For more information, contact Larry Swan, USDA Forest Service, Winema National Forest (541) 883-6714 or Scott Leavengood, OSU Extension Service, at (541) 883-7131. See also "Insects in Western Wood" (1990), available from the Western Wood Products Association.
Another issue related to the use of air-dried juniper lumber has been related to shrinkage. Wood shrinks when it loses moisture and western juniper is no different than other wood species in this regard. In fact, tests have shown that juniper shrinks less than many other commonly-used Northwest wood species. The problem is; 1) even minor shrinkage can be devastating to joinery in furniture, millwork, cabinets, etc. and 2) at this stage in the development of the juniper industry, many juniper producers do not have access to dry kilns.
The moisture content of wood varies according to the surrounding temperature and humidity. Wood will reach an equilibrium moisture content (EMC) appropriate for the ambient temperature and humidity. For example, at 70° F and 25 percent humidity, wood will equilibrate to approximately 5.4 percent moisture content and at 80° F and 70 percent humidity, wood will equilibrate to approximately 13.4 percent moisture content.
One result of EMC is that the minimum moisture content attainable in air-drying depends on the climate. In most of the U.S., including the dry climate of eastern Oregon, air-dried wood rarely gets below 12 percent moisture content. The typical EMC for wood indoors is 6 to 8 percent. Therefore, when products made from air-dried wood are placed indoors they lose moisture and shrink. As an example, a 4/4 x 6 flatsawn juniper board will shrink a little less than 1/16 inch in width from 12 percent to 6 percent moisture content (for purposes of comparison, red oak would shrink almost twice as much, about 1/8 of an inch, under the same conditions).
The only real solution is to ensure that the lumber's moisture content is suited to the end-use. For interior purposes, the moisture content should be approximately 6 to 8 percent. Ask your suppliers if their lumber is kiln-dried and if not, if they have access to dry kilns. From a customer satisfaction standpoint, it is worth the extra investment to kiln-dry the lumber.
This article is not meant to say that kiln-dried lumber is "better" than air-dried lumber. I know that many woodworkers feel that kiln-dried wood is inferior to air-dried wood because they have heard that kiln-drying degrades the wood structure. Research literature neither supports nor debates this view. What is well-known, however, is that kiln-drying schedules must be suited to the wood species and the intended end-use. Very aggressive kiln schedules (high temperature and short time) can degrade wood. For higher value end-uses, the appropriate kiln schedule is typically more "gentle" (lower initial temperatures and higher initial humidities).