THE CONTROL OF INSECT PESTS IN MUSEUM COLLECTIONS:THE EFFECTS OF LOW TEMPERATURE ON STEGOBIUM PANICEUM (LINNEAUS), THE DRUGSTORE BEETLE
MARK GILBERG, & AGNES BROKERHOF
ABSTRACT—The effect of time on mortality of various developmental stages of Stegobium paniceum (Linneaus) at −20° was studied. Eggs, larvae, pupae and adult insects were collected and exposed to −20°C for 0–4 hours. Complete mortality was observed for all developmental stages after 1-hour exposure. The use of low temperatures as an alternative to conventional fumigation techniques for the treatment of insect-infested museum objects is discussed in terms of the results of these experimental trials.
CONCERN FOR the safety of museum personnel when using chemical fumigants for the treatment of insect-infested museum objects has in recent years led to a greater emphasis on the development of nonchemical methods of pest control. One alternative that has been considered is freezing. Indeed, freezing has been employed for a number of years in libraries, herbaria, and natural history collections for the treatment of insect-infested materials (Florian 1986). More recently, some consideration has also been given to its possible application as an alternative to chemical fumigants for the treatment of insect-infested museum objects in anthropological collections.
Though freezing is generally considered to be a highly effective means of insect control, in reality, very little information exists with regard to the lethal effect of low temperatures upon many of the more common museum pests. Much of the literature is devoted to stored-product pests; moreover, these studies were conducted over a wide range of temperatures and exposure periods (Adler 1960; Back 1924; Cantwell and Smith 1971; Childs et al. 1968; Childs et al. 1970; Crumb and Chamberlain 1934; Gerber 1981; Howe 1957; Knipling and Sullivan 1957; Mullen and Arbogast 1979; Myburgh and Bass 1969; Nagel 1934; Nowosielski-Slepowron and Strevens 1973; Swingle 1938; Tenhet et al. 1957; Waggoner 1985). Only a few systematic studies have been conducted on museum pests (Arevad 1975; Arevad 1980; Ketcham-Troszak 1984).
However, a tentative treatment schedule for the disinfestation of museum objects has been proposed by Florian (1986), who recommended repeated exposures to −20°C for two days. This treatment schedule was derived primarily from a review of the scientific literature on the effect of low temperature on insects as well as from existing freezing procedures used in various museums that have been found to be successful in eradicating insect pests (Crisafulli 1980; Florian 1986; Nesheim 1984). In most cases a commercial freezer was used, which generally operates at this temperature. To avoid freeze-resistance, Florian (1986) recommended a high cooling rate and multiple exposures to sub-zero temperatures.
In the absence of a systematic study of the time-mortality relationship for the major museum pests, it is difficult to establish appropriate treatment procedures for the disinfestation of museum objects. For this reason a study of the lethal effect of prolonged exposure to low temperatures was undertaken for a number of museum pests. The results of the experimental trials for Stegobium paniceum. (Linneaus), commonly refered to as the drugstore beetle, are presented.
Stegobium paniceum is a cosmopolitan pest that may infest almost any dry animal or plant product (Ebeling 1975). Only the larval stage of this species feeds. Within museums it has been known to attack a wide variety of materials including botanical and entomological specimens as well as anthropological artifacts (Kingsolver 1988; Story 1985). The life cycle is dependent upon temperature and humidity, but one or two generations per year is common at room temperatures (Lefkovitch 1967).
2 METHODS AND MATERIALS
A WALK-IN freezer unit was used for all low-temperature tests. The temperature was maintained at a constant −20°C. The relative humidity was monitored using a Lambrecht hygrometer and varied between 75% and 85%.
2.2 INSECT CULTURES
Stock cultures of Stegobium paniceum were obtained from the Stored Grain Research Laboratory, CSIRO Division of Entomology, Canberra, ACT. Laboratory stocks reared on a culture medium consisting of 90% cracked wheat and 10% brewers' yeast at 25°C and 70% RH were used for all experiments.
One-to-three-day-old eggs were obtained by placing young adult insects on 10g of culture medium, which had been sieved through a 250-micron mesh. After three days the adults were removed, and the medium resieved. The eggs were then collected and counted under a binocular microscope.
2.4 LARVAE AND PUPAE
Larvae and pupae were obtained by allowing 100 young adults to oviposit on 200 g culture medium for 24–48 hours. The adults were then removed, and the cultures were incubated as described above for 32 and 42 days, respectively. The larvae and pupae were then separated from the medium by hand and counted.
One-to-five-day-old adults were obtained by placing 100 young adults on 200 g culture medium for 24–48 hours. The adults were then removed, and the culture was incubated as in the above until the emergences of the next generation of adult insects. Following the first emergence the adults were collected five days later and counted.
3 EXPOSURE PROCEDURE
FOR TEST purposes the various developmental stages were counted into batches of 50 insects and placed on the bottom of a small petri dish lined with black filter paper. To prevent the possible escape of any surviving insects, the petri dishes were subsequently transferred to wide-mouth glass jars (fitted with filter paper lids), which had been cooled to −20°C inside a walk-in freezer unit. All developmental stages were then exposed to −20°C for periods ranging from 1 minute to 4 hours. After freezing, 10 g of finely sieved culture medium was added to each jar which was then incubated at 25°C and 70% RH. After 24 hours the jars containing the larvae and adults were resieved, and the number of survivors counted. The jars containing the eggs and pupae were resieved after two weeks. The number of eggs that had hatched and the number of pupae that had emerged as adults were then determined.
Parallel tests with an equivalent number of untreated insects acting as controls were conducted for each developmental stage. These were incubated at 25°C and 70% RH. The tests on eggs, larvae, pupae, and adults were conducted without replication because of the difficulty of obtaining large numbers of these stages at any one time. Treatments were repeated three times for each developmental stage to confirm the results. The percent mortality for each developmental stage was calculated using the formula (x−y)/x (100), where x is the percentage survival in the control and y that in the frozen sample.
THE RESULTS of the experimental trials for Stegobium paniceum and its various developmental stages are given in table 1. In general, mortality increased with exposure time. Eggs and pupae appeared to be the most resistant developmental stage, though complete mortality was observed for all developmental stages after 2-hour exposure to −20°C.
TABLE 1 Average Percent Mortality(s) of the Various Development Stages of Stegobium paniceum Following Exposure to −20°C for Varying Periods of Time
No attempt was made to determine mortality at temperatures other than −20°C, given that this temperature can be readily achieved using domestic freezers and therefore has immediate application. Indeed, it is precisely for this reason that this temperature is commonly used in museum practice.
Though only a brief exposure to −20°C is necessary to achieve high mortality rates for unprotected Stegobium paniceum, under practical working conditions insects may well be insulated against the cold, and thus much longer exposure periods may be required. Under these circumstances various forms of freeze-resistance such as cold acclimation may occur, in which case it would be difficult to establish the appropriate exposure period without first determining the effects of low-temperature acclimation on the developmental stages of Stegobium paniceum. In the absence of such information the rate of cooling should be maximized, as recommended by Florian (1986).
THOUGH FURTHER studies are required to determine the time-mortality relationship for other common museum pests at −20°C, the observed results for Stegobium paniceum support the provisional treatment schedule proposed by Florian (1986) for treating insect-infested materials in museum collections. Additional studies are also necessary to establish the rate of cooling of various museum materials.
THE AUTHORS wish to thank Dr. H. J. Banks, CSIRO Division of Entomology, Canberra, ACT, for his assistance in providing insect cultures for study and for numerous discussions regarding the experimental methodology.
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MARK GILBERG has a B.S. and an M.S. from Stanford University, where he investigated the redox properties of transition metal centers. He served as research assistant in the Department of Psychiatry, Stanford University Medical Center. He received his Ph.D. from the University of London Institute of Archaeology, with a dissertation on applications of liquid ammonia in conservation. Subsequently he joined the Conservation Processes Research Division of the Canadian Conservation Institute. He is presently scientific officer in the Materials Conservation Division of the Australian Museum. Address: Australian Museum, Materials Conservation, 6–8 College Street, P.O. Box A285, Sydney South, N.S.W. 2000.
AGNESW. BROKERHOF studied chemistry at the University of Leiden, the Netherlands, majoring in analytical chemistry and receiving an M.Sc. in 1987. After receiving her B.A. in art history from the University of Leiden in 1988, she combined both disciplines in conservation research and worked at the Central Research Laboratory for Objects of Art and Science in Amsterdam. In 1989 she went to Australia and worked at the Australian Museum in Sydney. Currently she is undertaking research on the control of clothes moths at CSIRO/University of Canberra, with a scholarship from the Australian European Awards Program. Address: CSIRO Division of Entomology, Stored Grain Research Division Laboratory, Canberra, ACT, Australia.