INSECT CONTROL WITH THE THERMO LIGNUM TREATMENTConservation News Number 59 March 1996
David Pinniger, Consultant Entomologist, 83 Westwood Green, Cookham, Berks SL6 9DE, UK
The use of heat to kill insects and control infestations in food and possessions goes back a long way in the history of pest control [eg Howard and Marlett 1902, Back 1923, Jensen 1933]. The practice of placing insect or plant specimens in an oven overnight at 60°C to kill beetles was widespread in many museum departments and the technique was also used for the control of moths [Rawle 1951] and beetles in textiles and wood objects. The supporting data [summarised in Strang, 1992 and Fields, 1992] shows that death can be caused at high temperatures because lipids in the nerve membranes and waxy layers of the insect cuticle degrade.
There is also evidence that for some species certain enzymes important to the metabolism of insects are inactivated at temperatures above 55 to 60°C which leads to the death of the insect. Although high temperatures were used for many years for certain types of collections, the effects of heating particularly on objects which were treated repeatedIy was shown to be damaging. In many cases drawers which contained specimens such as insects were subject to shrinkage, cracks and distortion. These side effects of the treatment due to moisture loss and shrinkage of the wood made the treatment unacceptable for sensitive material and most museums have now stopped using heat treatment because of damage to storage furniture and specimens.
The search for alternatives to persistent insecticides and fumigant gases has intensified in recent years because of concerns over the the effects of chemicals on objects, staff and the environment. The particular concerns over the environmental effects of methyl bromide which has been classified as an ozone-depleting chemical have resulted in restrictions on its use and could result in the withdrawal of this fumigant in the next few years. The options for alternative treatments to control insects in infested objects include the use of atmospheric gases such as carbon dioxide and nitrogen and the use of low temperatures.
These alternatives can be very effective for the control of pests although in some circumstances they may not always be practical or appropriate. At temperatures of 20°C or below long exposure to gases may be needed to kill wood boring beetles and it is not possible to expose certain sensitive objects to -30°C without risk of damage. The development of the Thermo Lignum heat treatment has now provided another alternative for the control of insects in objects.
The Thermo Lignum technique, which was developed in Germany, uses a purpose-built chamber equipped with very sensitive and responsive temperature and humidification control. The principle is to ensure stability of moisture in objects during the cycle of elevation of the temperature to 52°C and the return to ambient temperature. Figure 1 shows a typical temperature and humidity record from one of the test runs described in this paper and details of the method and the theory behind the process are given in Ertelt . The questions and concerns of conservators are addressed by Child  and a considerable amount of data has been generated which confirm the lack of observable or measurable effects on treated objects of varying types and composition.
|Whatever the effect of a treatment on materials, it must achieve complete control of insects to be of practical value as a replacement for methyl bromide for disinfesting objects. As an entomologist I was particularly interested to see data generated by exposing test insects to the Thermo Lignum treatment. A range of pest species including clothes moth Tineola bisselliella, furniture beetle Anobium punstatum, powder post beetle Lyctus brunneus, death watch beetle Xestobium rufovillosum and house long horn hylotrupes bajulus were tested by Ertelt 1994 and the results are summarised in Table 1.|
|Exposure needed to achieve 100% kill of test insects [data summarised from Ertelt 1994]|
|Species||Temperature °C producing 100% kill of test insects|
|House Longhorn beetle||55|
|Death Watch beetle||47|
|Powder Post beetle||50|
|It is clear that exposure to 52°C in the chamber was sufficient to
kill all stages of the species tested with the exception of the house
longhorn Hylotrupes bajulus which required 55°C. The actual period of
exposure to the peak temperature needed to kill all stages of the insects
is probably much less than an hour but in order to achieve the stability
of temperature and humidity in the object during the heating and cooling
period the actual treatment in the chamber from start to finish will
normally take between 15 and 24 hours.
Carpet beetles Anthrenus sp are one of the most serious pests of textiles and natural history collections. As these pests were not included in the programme of tests carried out in Germany, it was decided to place test specimens of larvae of the varied carpet beetle in the Thermo Lignum chamber during two commercial treatments of objects to check the effect of the heat exposure on this species. For each treatment 12 batches of ten larvae from CSL laboratory cultures were placed in tubes containing wheat bran as a heat buffer and 10 of these were plased in the chamber and two batches were left in the outer room as untreated controls. Treatment 1 was an exposure to 52°C at 36% relative humidity with a total treatment time of 15 hours. Treatment 2 was also to 52°C but the relative humidity was 50% and the total exposure period was 19 hours. With both treatments, 100% of the test insects were dead when examined in the laboratory, whereas 92% of the untreated insects were still alive. These tests showed that the treatments were completely effective in killing the larvae of this species.
From past and present work the evidence shows that the temperature conditions of 52°C achieved by the Thermo Lignum treatment are effective in controlling the eggs, larvae, pupae and adults of the major museum pest insects. In some countries and areas there may be problems with the house longhorn beetle and if this species is present then a temperature of 55°C will be needed to ensure complete kill. There appear to be some advantages to the Thermo Lignum treatment in terms of speed of treatment and lack of requirement for bagging of objects compared to low temperature treatments or use of atmospheric gases. In addition, Thermo Lignum guarantee that an object is free from living infestation when it leaves the chamber. Strang  discusses the important relationships between moisture in an object and the surrounding environment and the rates of change expected during the lowering or elevation of temperature and conoludes that "Thermal control methods can be used selectively on a large proportion of general museum collections without undue fear of damage". Given that these data on the effects of treatments of objects satisfies the needs and concerns of conservators, this means that museums have another valuable weapon in the war against the pests of their collections. It is understood that further developments are in progress which will enable the chamber to be used at lower elevated temperatures in combination with nitrogen or carbon dioxide to substantially shorten the time of exposure to gases which is normally needed. It must be emphasised that although the ability to kill insects in infested objects is very important, it is only a part of the pest control strategy which should include a planned programme of pest monitoring, cleaning and other targeted control methods [Pinniger 1994].
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Fields, P G [199y The control of stored produot insects and mites with extreme temperatures. J Stored Product Research 28 
Howard, L O and Marlatt, C L  Species injurious to woolen goods, clothing, carpets, upholstery, etc. The principle household insects of the United States. USDA Bulletin No 4, Washington DC
Jensen. K  Wärme als Bekämpfung gegen Hausböcke. Mitt. Ges. Vorratsschutz, Heft 9
Pinniger, D B  Insect pests in museums. 3rd Ed, Archetype Press London
Rawle, S G  The effects of high temperature on the common clothes moth Tineola bisselliella Bull Ent Res 42 [ 1 ]
Strang, T J K , A review of published temperatures for the control of pest insects in museums. Collection Forum 8 
Strang, T J K [19951, The effect of thermal methods of pest control on museum collections. Proceedings 3rd Internatio Thailand 1995.