A novel fumigation system has been employed by the Center for Wood Anatomy Research at the United States Department of Agriculture, Forest Service, Forest Products Laboratory (FPL). The FPL is located on the campus of the University of Wisconsin in Madison (43°04 Nand 89°22 W) and is an important American research center devoted to the study of wood and cellulose products. One of its major resources is the world's largest research wood collection (over 100,000 specimens) and a small accompanying herbarium. The wood specimens are stored in wooden cabinets on the fourth floor and the herbarium is stored in metal cabinets in an annex above the wood collection. In temperate climates such as Madison, there is rarely an infestation of insects in the wood collection and insect control practices are therefore seldom required. When evidence of insects, particularly powderpost beetles, is found, infected drawers of wood samples are removed and either frozen (-20°C) for several weeks or placed in an oven (90°C) for a few days. Freezing techniques are preferred because the wood samples retain the same moisture content. In the herbarium, however, insects are often a problem and insect control is necessary. Before 1975, para-dichlorobenzene was routinely used to fumigate the herbarium cabinets. After 1975, several alternative methods were explored, including 'pest strips', biodegradable compounds carried in an aerosol silica gel spray, and other, less toxic chemicals. All these methods killed insects to a greater or lesser extent, but the toxicity to humans was always of concern. About this time, several authors showed that freezing herbarium sheets for several days at -4°C killed insects and their eggs [1-3]. Herbarium sheets are removed from the cabinet, placed in plastic bags, and stored in a sub-zero freezer. After several days, the bags are removed, allowed to return to room temperature to prevent condensation, and then replaced in the cabinet. Modifications of this general technique are now employed at many herbaria throughout the country. Since the FPL herbarium is situated in a room atop the main building, with its four walls and ceiling fully exposed to the elements, it was decided to freeze the entire herbarium in situ. In Madison, the winter temperatures sometimes fall below -30°C (often in January) and remain below -18°C (O°F) for several days. In 1985, in order to test natural freezing conditions on insects, we installed a valve to shut off the steam heat, drained the radiators, shut off the water to the sink, added alcohol to the drain, and waited for a frigid northwest storm. To test the conditions in the herbarium, seven thermocouples were installed and readings were taken at the following places: outside, downstairs, two sites in herbarium room, in closed herbarium cabinet, in closed herbarium cabinet deep within bundle of herbarium sheets, and in open herbarium cabinet deep within bundle of herbarium sheets. During a frigid storm in January, we opened the windows and some herbarium cabinet doors and began taking readings. Since it was windy, the temperatures plummeted at all sites. The coldest readings occurred in the room where the wind could continually remove the latent heat. Little difference was noted between open and closed herbarium cabinets, or between the dead space surrounding the bundles of herbarium sheets and deep within the bundles. After the wind abated, the temperatures in the herbarium began to rise although the outside temperature remained approximately the same. After a few days the windows were closed and the heat was turned on. No condensation was evident because the relative humidity of the air was very low. January 1985 was the only time we froze the herbarium. For three years no insects were found and the mild winters prevented further testing. In 1988 and again in 1991, insects were found in one cabinet. In these cases we followed standard practices and froze the herbarium material in a freezer. Further testing has been prevented by the persistence of mild winters and failure to be prepared for frigid storms. Freezing an entire herbarium may not be practical for many institutions, but it is a unique option if the conditions are right. There are no health risks, the cost is minimal, the treatment is effective, and it is a good alternative to the use of chemicals. Authors' addresses: (Miller) Forest Products Laboratory, 1 Gifford Drive, Madison, WI53701, USA; (Rajer) P.O. Box 567, Madison, WI53701, USA.
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Feb 20, 1998
JüRgen Weissenberger 
Open Access