Biocides

Herbaria with desiccated plants, composed of organic material combined with other organics such as glues, pastes and other cellulose-based and proteinaceous materials are particularly vulnerable to pest infestations. Several chemical compounds have been used either to prevent infestation or to fight an existing pest attack. Most of them are now phased out or restricted in use and production. Biocides pose numerous threats both to artefacts and people working in the heritage sector, such as conservators.

The use of biocides in herbarium collections

Popular and easily accessible biocidal substances are widely used for pest control. Unfortunately, several compounds proved to have negative impacts not only on insects and microorganisms, but also on other forms of life, including humans, and on the artefacts themselves. As the history of the use of biocides in institutions and conservation studios is poorly documented, this article focuses on the issues related to their use. Herbaria with desiccated plant material can be regarded as paper-based artefacts, but they also contain biological (plant) material, often mounted with natural adhesives. This combination of components makes herbaria particularly vulnerable to damage caused by insects and rodents. Unstable storage conditions, climatic fluctuations or random events involving higher moisture levels (such as dampness, flooding) also create conditions for mould growth. Herbaria in which the plants are kept loose between pages are also not free from potential damage by insects and microorganisms. Stored with other collections containing materials “attractive” to pests (i.e. natural adhesives), herbaria also suffered damage by pest attacks. Some plant species are more susceptible to insect attacks than others. The most endangered groups of specimens are petaloids, belonging to the monocotyledons, many dicotyledon species, particularly Asteraceae, and fungi. On the contrary, bryophytes and lichens are more resistant. Problems concerning insect damage were well known already at the beginning of the creation of herbaria. The first publication on herbarium preparation, Isagoge in rem herbariam libri duo by Adriaan van den Spieghel (1606), already mentions biocidal and repellent substances. Spieghel describes cloves powder and aloe as additions to the glue he used to mount specimens. In another archival source by Joseph Pitton de Tournefort, there aloe extract (aloe hepatica) is mentioned, as well as a decoction of common wormwood (Artemisia absinthium L., “absinth”) and santonica (Artemisia cina O. Berg.), which, however, altered the colour of the botanical specimens.

Natural biocidal products

Other plants and plant compounds that were generally used as natural pesticides and repellents were camphor, extracted from camphor tree oil (Cinnamomum camphora (L.) J. Presl.), cassia oil from Chinese cinnamon (Cinnamomum cassia (L.) J. Presl.), thymol and carvacrol – present in thyme oil (Thymus vulgaris), wild thyme (Thymus serpyllum L.) and carom (ajwain, Trachyspermum ammi L. or Carum copticum L.), neem (Azadirachta indica A. Juss.), pyrethrin, extracted from Dalmatian pellitory (Tanacetum cinerariifolium (Trevir.) Sch. Bip., previously called Chrysanthemum cinerariifolium (Trevir.) Sch. Bip.), linalool, from lavender oil (Lavandula angustifolia Mill., previously called Lavandula officinalis Chaix), turpentine, distilled from pine resin (Pinus sylvestris L.), leafy sprouts of wild rosemary (Rhododendron tormentosum Harmaja, syn. Ledum palustre L.), sweet flag (Acorus calamus L.), cassumunar ginger (Zingiber cassumunar Roxb.), cultivated tobacco (Nicotiana tabacum L.), strychnine from the strychnine tree (Strychnos nux-vomica L.), Javanese long pepper (Piper retrofractum Vahl), maidenhair tree seeds (Ginkgo biloba L.), citronella oil (Cymbopogon nardus (L.) Rendle or Cymbopogon winterianus Jowitt), rotenone, present in the roots of many plants of the Fabaceae family, menthol, cedar oil and bitter almond oil. Several of these compounds were at some point synthesised to expedite their production and to standardise their composition and effects. Several are still used as biocides, such as pyrethrins, both in natural and synthetic form.

Miraculous chemicals and health risks

Over time, users and custodians began to notice that the use of biocides is detrimental not only to insects and microorganisms, but also to human health. Our civilization is still struggling with the use of “miraculous” biocidal chemicals that facilitate agricultural production in many areas, reducing its costs, but also cause irreversible changes in natural environments. The most frequently cited example is DDT, which is now conditionally phased out. The production of DDT provided its developer, Paul Hermann Müller, with the 1948 Nobel Prize in Physiology or Medicine because of its insecticidal properties and effectiveness in fighting malaria and yellow fever. Today, DDT is still used to treat malaria. At the time of its intensive use in agriculture, this chemical compound almost led to the extermination of entire bird populations in the USA; being one of the most persistent polluting pesticides, it is still detected in populations of animals inhabiting the Arctic. Such examples remind us, as conservators, that an awareness of the threats connected with pesticides is crucial to responsibly and effectively care not only for the safety of historical items, but also to protect both the natural environment and human health.

Classification of biocides

Biocides can be grouped according to their mode of action: stomach poisons that must be consumed by pests, contact pesticides that must come into direct contact with an insect, residual products, desiccant insecticides causing dehydration and, eventually, death, insect growth regulators and fumigants. They are available in different formulations: in mixtures with solvents or inert substances, as oil concentrates, as emulsifiable concentrates, wettable powders, dusts, baited insecticides, microcapsules diffusing poison and plastic strips impregnated with insecticide. The best known and most commonly used pesticide in herbaria is mercury chloride. The first reference to its use is the description of Tournefort (1694), who used mercury compounds in both monovalent (mercure doux) and divalent form (sublimate, French sublimé corrosif) as additives to the glue used to mount the specimens. However, the list of toxic substances used as preventive measures is considerably longer. Synthesised biocides, also synthetic versions of natural compounds, are listed in Table 1. This table contains references to publications that provide information on individual substances used in herbarium collections. The literature on biocides themselves is far more extensive, in addition to general studies, in which the use of pesticides is discussed in relation to all museum objects, also covering ethnographic, taxidermy, fluid entomological and economic botany collections. These publications mention all biocides listed in the table below, as well as other substances such as borax, methoxychlor or glyphosate, which were, however, most likely not used in herbaria collections. It cannot be excluded that in the past, these biocides might have been used for the preservation of herbarium specimens, and the development and availability of sensitive analytical techniques, mainly chromatographic, still provides new discoveries. Pesticide residues and degradation products are less evident in herbaria than in zoological items. The most extensive literature concerns the use of the above-mentioned mercury chloride, which is one of the earliest known biocidal substances and one of those pesticides that permanently contaminate and damage specimens and are dangerous to human health, even many years after the original application. In addition, the CAS (Chemical Abstract Service registry) identification number is listed under the name of each pesticide. When an entire group of compounds (e.g. pyrethrins or carbamates) is listed, the CAS number for the representative substances is given. However, this does not mean that there are no other substances with different registration numbers in the same group. The third column of the table indicates whether the compound has a negative effect on the genetic material of objects and artefacts. Due to the limited number of studies in this area, information on the impact on genetic material has been generalised and concerns a wider group of natural history collections, not only herbaria. In some cases, the results of research in this area are not clear or consistent. In such situations, the table provides a less “optimistic” result,assuming that if in one study, a given compound showed a destructive effect, it is a potential threat to other historic objects. The last column contains information on the genotoxicity (mutagenicity or carcinogenicity) of the listed materials, based on the EPA (United States Environmental Protection Agency) and IARC (International Agency for Research on Cancer) databases. All listed pesticides are toxins with a variety of risks to human health. Some of them have a confirmed muta- and carcinogenic status, and for others, genotoxicity is defined as “possible” or “probable”, according to the classification implemented in the above-mentioned sources. In some cases, although a compound is known to be a neurotoxin and interferes with identification of genetic material in museum objects (as in the case of mercury chloride), there are insufficient data to classify it as a carcinogen or mutagen. There is also often a lack of research on chemicals currently phased out (at least in some regions of the world and most previous uses), such as DDT. Descriptions of individual pesticides indicate whether they are listed in the Stockholm Convention, an agreement governing the use of persistent organic pollutants (POPs). The Stockholm Convention divides toxic substances into three groups: group A – intentionally produced toxic agents that should be eliminated, with specific exemption for certain uses; group B – toxic agents, of which international production should be restricted to manufacture and use to fulfil specific purposes (e.g. DDT for the treatment of malaria); group C – unintentional toxic agents that are products of the decomposition of other substances (e.g. production process side effects) and which should be kept to a minimum.