Brief description of treatment
Heat treatment for infested collection items is an effective method for assuring 100% efficacy in killing insect pests. In addition, it is a method that can be carried out in situ and has been applied at scales ranging from small individual objects to entire multi-story buildings. It is particularly useful for treating large immovable items with minimal impact on surrounding operations. Heat treatment can also be a good option where resources are limited.
A short exposure of 55oC (130oF) is sufficient for eradication of all life stages of insects (Strang, 1992). The limiting factor for treatment is the time needed to heat through the affected object. Thus, it will take longer for the effective temperature to reach the center of thick or dense items that are effective thermal insulators, than for thin and more thermally conductive items. Temperature probes may be necessary to insure that the required temperature has been reached. A useful thermal treatment guide (for both low temperature treatment and heat treatment) including estimated treatment times can be found in Strang and Kigawa, 2009:
Note that microwaving as a heat treatment is not recommended for any collection material due to the risk of excess heating around metallic inclusions.
Thermo-lignum® treatment is a proprietary method of eliminating insects with heat using a climate controlled heating chamber which controls moisture content during heating and cooling. See Ackery et al. 2005 for more information.
What collections materials cannot be treated this way?
There are some contra-indications for heat treatment:
Changes in temperature can drive changes in relative humidity and moisture content. Some heat treatment procedures allow for control of object moisture content by containment (bagged in moisture barrier film) or active control (humidity adjusted during treatment to maintain moisture content). Other heat treatment procedures do not allow for this humidity control. Therefore other treatment options might be preferable for items that are extremely sensitive to changes in relative humidity treatments if controlling the object moisture content cannot be achieved.
Do not use heat treatment for low melting point waxes, certain adhesives which may slump (if in doubt, ask a conservator), flammables, explosives and plastic materials that melt or deform from heat in the treatment range (usually less than 60°C).
DNA extraction and PCR amplification have been shown to be relatively insensitive to heat treatments for pests (again, in the 52°C to 55°C range). Rarity, high scientific value, and other concerns should be considered before applying any treatment to collections, but there is strong evidence that heat is far less harmful to DNA and protein structure than many of the fumigants applied in the past (Ackery et al. 2004, Kigawa et al 2003, Kigawa 2012, Kigawa et al. 2011.)
Items with low activation energies, e.g. magnetic media and urethane binders, may be particularly susceptible to damage from accelerated aging caused by heat treatment as heat increases the rate of hydrolysis. However, this factor should be weighed against the relatively small cost given the short duration of heat treatments. Many widely-accepted routine conservation treatments for paintings, paper, textiles and magnetic media (for example relieving “sticky shed” syndrome) involve temperatures as high, or even higher, than those required for heat treatment for pest eradication. If in doubt, consult a conservator, and refer to published estimates of the severity of this factor (Strang 1995, 2001, 2012; Michalski).
Heat treatments can be carried out in ovens, environmental chambers with properly engineered room heaters and circulating fans, or in enclosures or buildings with heat supplied by outside heat generators through temporary ducts.
Heat treatment is now a common strategy for killing bed bugs, so exterminators offering this service use industrial machines that can heat whole rooms to the desired temperature, with monitoring equipment to assure that the proper levels are achieved. Note that if entire building structures are being treated attention should be paid to possible loss of heat via heat sinks such as ground floor building pads and openings. Risks to building components should also be considered and mitigated.
Solar bagging is a variation of the heat treatment. With this method, objects are bagged in black polyethylene sheeting before heat treatment to stabilize the moisture content of the objects and thus minimize any damage which would otherwise be induced by excessive shrinkage. A solar bagging method, developed by Tom Strang at the Canadian Conservation Institute, uses sunlight to produce the temperatures lethal to insects. uArtifacts are wrapped in cotton to buffer moisture fluctuation and sealed in black plastic bags which prevent moisture loss and damage from ultraviolet and visible light. This bag is placed within a clear plastic enclosure outdoors that maximizes interior temperatures, delivers heat to the shade side, and screens out rain and contaminants. The wrapped object packets are then exposed to the sun, and the maximum heat rise is controlled by the angle of incidence. Inside the packets, temperatures may rise as much as 40°F to 70° F above the outdoor environment. This causes adult insects, eggs, and the stages in between to be killed in a matter of hours, due to increased rate of dehydration and enzymatic dysfunction.
More information on solar bagging is available: Baskin 2001 provides a case study, Strang and Kigawa 2009, and Brokerhof 2001 provide various solar treatment designs, and papers in Strang 2012 discuss heat risks.
For small items of lesser value: place a pan of water on the bottom rack of a kitchen oven in order to maintain high humidity in the oven. Place the object onto an upper rack. Turn the oven to its “warm” setting, which is usually 140 degrees F/60 degrees C. (170 degrees F or 77 degrees C is too high.). Leave the object for three hours, and then turn the oven off, leaving the object inside. Remove the object when the oven is cool.
Pros and Cons of this treatment
- Inexpensive and low-tech solutions available
- Easy to conduct
- Effective over a short period of time
- Low equipment maintenance
- Not all materials may be treated this way.
Ackery, P. R., J. M. Testa, P. D. Ready, A. M. Doyle, and D. B. Pinniger. 2004. “Effects of High Temperature Pest Eradication on DNA in Entomological Collections.” Studies in Conservation 49 (1): 35–40.
Ackery, P. R., David Pinniger, Adrian Doyle, and Karen Roux. 2005. “Heat Treatment of Entomological Drawers Using the Thermo Lignum Heat Process.” Collections Forum 19 (1-2): 15–22.
Baskin, Bonnie. 2001. “Solar Bagging: Putting Sunlight to Work to Eliminate Insect Infestations in Mere Hours.” Newsletter – Western Association for Art Conservation 23 (2) (May): 20–21.
Brokerhof, Agnes W. 2004. “The Solar Tent: Cheap and Effective Pest Control in Museums.” AICCM Bulletin 28: 93–97.
Chalfoun, David J., and Noreen Tuross. 1999. “Botanical Remains: Utility in Protein and DNA Research.” Ancient Biomolecules 3: 67–79.
Kigawa, Rika, Tom Strang, Noriko Hayakawa, Naoto Yoshida, Hiroshi Kimura, Gregory Young. 2011. “Investigation of Effects of Fumigants on Proteinaceous Components of Museum Objects (Muscle, Animal Glue and Silk) in Comparison with Other Non-Chemical Pest Eradicating Measures.” Studies in Conservation 56 (3): 191–215.
Michalski, Stefan. 2000. Technical Bulletin 23: Guidelines for Humidity and Temperature in Canadian Archives. Ottawa: Canadian Conservation Institute, Canadian Heritage.http://www.cci-icc.gc.ca/publications/downloads/technicalbulletins/eng/TB23-GuidelinesforHumidityandTemperatureforCanadianArc.pdf
Pinniger, David. 1996. “Insect Control with the Thermo Lignum Treatment.” Conservation News (United Kingdom Institute for Conservation of Historic and Artistic Works) 59: 27–28. http://www.bcin.ca/Interface/openbcin.cgi?submit=submit&Chinkey=166154.
Strang, T .J. K. 1994. “Reducing the Risk to Collections from Pests.” CCI Newsletter 14. Canadian Conservation Institute Newsletter: 8–10.
Strang, T .J. K. 1995. “A Brief Guide to Thermal and Controlled Atmosphere Treatments for Insect Eradication.” ICOM-CC Preventive Conservation Working Group Newsletter: 4–5.
Strang, T. J. K. 1995. “The Effect of Thermal Methods of Pest Control on Museum Collections.” In Proceedings of the 3rd International Conference on Biodeterioration of Cultural Property, 334–353. Thammasat University, Bangkok, Thailand: Thammasat University Press.
Michalski, Stefan. 2000. Technical Bulletin 23: Guidelines for Humidity and Temperature in Canadian Archives. Ottawa: Canadian Conservation Institute, Canadian Heritage.
Nicholson, Mark and von Rotberg, Werner. 1996. Controlled environment heat treatment as a safe and efficient method of pest control. The 2nd International Conference on Insect Pests in the Urban Environment. Edinburgh, Scotland.
Strang, Tom, and Rika Kigawa, 2009. Technical Bulletin 29: Combatting Pests of Cultural Property. Ottawa: Canadian Conservation Institute, Canadian Heritage.
Strang, T. J. K. 2012. “Studies in Pest Control for Cultural Property.” Doctoral thesis, Gothenburg, Sweden: University of Gothenburg. Faculty of Science.
Strang, T.J. K. 2001. “Principles of Heat Disinfestation.” In Integrated Pest Management for Collections: Proceedings of 2001: A Pest Odyssey, 114–129.
Integrated Pest Management Working Group
Treatment Subgroup March 2014