Gunther Becker is an international authority on big-deterioration of materials, wood preservation, termites and related subjects. He is Vice President and Professor of the Federal Institute for Materials Testing of the Federal Republic of Germany and Honorary Professor of the Technical University, Berlin.
In the tropics termites are among the principal destroyers of wood. The author covers their biology, ecology and behavior, control measures, wood preservation and procedures used in laboratory studies.
The most important terrestrial wood-destroyers are special fungi and insects that use compounds of wood as nourishment. These organisms help to maintain the balance in nature by transforming chemical compounds of dead wood into substances vital for sustaining the development of new life. At the same time, they are a challenge to man in his effort to protect and lengthen the service time of wood and wood products when used under conditions suitable for the development of wood-destroyers. This is particularly so in the tropics, where these organisms are numerous and pose a serious and costly problem.
In tropical and subtropical countries, but also in some northern parts of the world, termites, belonging to the insect order Isoptera, are the most voracious of wood-destroying insects. Unlike most other insects, termites (also, though incorrectly, called "white ants") live together in well-organized groups. Large colonies may contain millions of individuals. Termite colonies are comparable to those of ants, bees and wasps, all members of the order Hymenoptera.
The insects reach the final stage through a number of moults. Unlike the Hymenoptera and beetles (Coleoptera), young termites are similar to the final stages in shape and behaviour (Figure 1). The normal size of termites is several millimetres. Most of them are light coloured. With the exception of the final instars, they are blind, but photophobic.
The social behaviour of termites is based on the fact that only one pair or a small number of individuals of a group are sexually active. Some of the young develop into winged "imagines," the final fertile stage. They reach it by going through successive nymph stages with growing pre-stages of wings (Figures 1a, d). The young adult imagines leave the nest at a special annual season. They are from 5 to 20 mm long. After swarming they shed their wings and may pair (as "queen," and "king") to start a new colony (Figure 2). When the first of their larvae are able to take up food by themselves, they begin feeding the queen, which results in her increased growth and egg production. Among many tropical species of termites with large colonies, old queens become as big as a man's finger and may lay several eggs per minute continuously. This fertility is an important factor in the potentials of termites.
Not all juveniles become fertile imagines: some, and among those species which form large colonies, the majority, of the juveniles develop into a sterile type, the so-called workers (Figure 1b). Others, also sterile, undergo drastic changes of body shape, which renders them capable of defending the colony against enemies, mainly ants. They therefore are called "soldiers" (Figure 1). Their head-capsule either becomes very much enlarged and the mandibles strong "mechanical" weapons, or it is filled by a gland producing a glue-like secretion which, when ejected through a tubular frontal nose, acts as a kind of chemical defence weapon.
The formation of castes, which results in a division of activity, is typical of all termites. The system is even more complicated by the fact that in the event of the loss of the original queens and kings, young stages, nymphs with or without wing-stumps (pre-stages), and sometimes even soldiers may become fertile and take the place of the lost queens and kings. Thus, once established, a colony possesses theoretical immortality. The caste development is governed by hormones, pheromones and nutritional conditions. The influence of these differs with the various termite species.
In tropical countries the presence of termites becomes evident by their nests, the permanent centre of a colony containing the fertile imagines, eggs, young larvae and pre-stages of imagines, and by their runways, which are soil-containing tubes and called "galleries." These connect the nest with sources of food and water and provide shelter and protection for the termites from desiccation, light and enemies (Figure 3).
There are several types of nests. Some termites influence the landscape by their nests. In tropical savannas and forests, one may encounter mounds of impressive proportions, several metres high and containing millions of individuals. Such hills are built of soil cemented by saliva excretions of the termites (Figure 4). Many other species produce only small, less spectacular soil hills.
Another type of nest consists of digested wood more or less combined with soil particles. These carton-like nests of different shapes may be located either on the ground or in trees (Figures 5a, b, c, d).
Numerous other species of termites do not build special nests but live inside the wood they are consuming, and the colony, including the egg-laying females, slowly migrates there. Some of these species have colonies numbering only a few hundred individuals, while others may have many thousands.
There are about 2000 different species of termites, with the largest number found in tropical Africa. The number of species in non-tropical countries is relatively small. However, a few of these species may show high population densities. The composition of termite species in rain forests differs considerably from that found in savanna areas of the same country or region. The distribution of species depends upon various ecological factors.
Some species, mainly those which do not build nests and do not require contact with soil or a water supply, have become dispersed intercontinentally by infesting wood in transport, with serious economic consequences.
Not all termites are wood-destroyers. Grass-, leaf- and humus-eating termites in tropical countries play an important role in soil fertility, like earthworms in cooler zones. Measures taken in a tropical area to destroy all termites indiscriminately, therefore, would result in marked ecological changes and a negative influence on agriculture and forestry in the area. Only a small number of termite species regularly or severely attack crops such as wheat and sugarcane, the heartwood of standing trees in forests (Figure 6), orchards and vineyards, and seedlings or young trees in nurseries.
Dead wood is the main and preferred food for most termites, and most species of wood are susceptible to termites. A number of tropical tree species and a few non-tropical do have a natural chemical resistance of their heartwood to termites, other insects and fungi. In addition, the degree of hardness of wood is in itself a defence against termites. The lighter and softer parts of the annual rings are first destroyed (Figure 7), and then the whole interior of a piece of wood is excavated. Because of the large number of termites in a colony, timber, once under attack, may be quite rapidly destroyed.
The digestive process of termites is biologically complicated. In the breakdown of cellulose, the termite depends upon symbiotic organisms which live in its gut, special flagellates, belonging to the unicellular protozoa, and bacteria in both the intestine of the termites and in the flagellates. The nitrogen content of wood is very small and the symbiotic organisms play an additional important role in the termites' nutrition by increasing their nitrogen sources.
Hemicelluloses, starch and sugars the other carbohydrates besides cellulose in wood, are also digested by termites. About 90 % of the carbohydrates can disappear during digestion, which, by comparison, is much more than in the case of the Coleoptera larvae. Lignin, which accounts quantitatively for about a third of the wood is less dissimilated in the digestive process, probably by intestinal bacteria. Termite faeces, therefore, consist mainly of lignin. This is the building material of one type of nest and is contained in galleries (Figure 4).
Termites prefer wood decayed by fungi. They are attracted by various substances contained in decayed wood and in fungal mycelium. Even the smell of fungi may stimulate food consumption and gallery building (Figure 8).
The nutrition of termites is definitely improved by their feeding on suitable fungi and slightly decayed wood, probably as a result of an increase of nitrogen from the fungi.
While this tendency of termites to feed on fungus-infested wood is based on environmental conditions and instinctive preferences, some highly developed termite species actually cultivate fungi in their nests. It has long been observed that holes in the interior of the large mounds of Macrotermitidae in Africa and Asia are filled with so-called fungus gardens. On a substratum of semi-digested wood and other plant material, an association of various species of fungi grows. These "gardens" have a characteristic shape (Figure 9). Young larvae live in the cultures and feed on them. At the same time, the fungus cultures contribute to conditioning the climate within the nests.
Termites also attack material other than wood, such as paper (Figure 10) and textiles, the cellulose of which is digestible, and plastics (Figure 11), rubber and other soft materials such as coatings, which are indigestible. The susceptibility of these materials to termite attack depends on their hardness and chemical composition. Frequently they are objects of mere gnawing activity. When a large number of termites concentrate their attack on the same spot they may damage many materials in their way. The loss of indigestible materials is normally not very substantial, but the consequences may be expensive or even disastrous. For instance, large quantities of goods may spoil because of ruined packaging and there have been short circuits in power cables and consequent blocking of the work of factories a result of termites' gnawing (Figure 12).
1a. Group of termites with dealated imagines, eggs, larvae and soldiers.
1b. Forms of soldiers.
1c. Forms of workers.
1d. Pre-stages of elated imagines and imago.
1e. Workers of a Nasutitermes species enlarging their nest; soldiers protect them.
2. Incipient colony of termites (Nasutitermes sp. from Mexico) with first workers and soldiers.
3a. "Galleries" of termites on the walls of a culturing container.
3b. "Galleries" of termites on infested wood.
3c. "Galleries" of termites enlarged.
4. Termite mound (India).
5a. Nests made of carton-like material (mainly lignin). Nest of a Nasutitermes sp. on a tree with gallery to the ground (Guatemala).
5b. Nest on a branch (Colombia).
5c. Nest built in the laboratory.
5d. Cross-section through a Microcerotermes sp. nest located on top of soil (India).
6. Nest material of Coptotermes niger in the excavated heart of Sainotrees (Colombia).
7a. Destruction of wood by termites. Attacked transmission pole (Guatemala).
7b. Two patterns of destruction by various species.
7c. Attack of dry-wood termites (Nigeria) after removal of the undestroyed surface layer.
8. Influence of the smell of fungus mycelium on gallery-building activity of termites. Above: controls without fungus mycelium. Below: stimulation by fungus mycelium (removed for photography).
9. Files destroyed by termites.
10. Fungus garden of an Odontotermes species (India) in dried condition.
11a. Plastic material and coating destroyed by termites in a laboratory test.
11b. Lid of a polystyrene container with a termite culture destroyed by the insects.
12. High-power transmission cable destroyed by termites. The termites gnawed a hole through the rubber (above) and attacked the polyvinylchloride cover in the interior (middle). Another part of the cable burnt by a short circuit (below).
13. Frass of termites on a dead tree under the cover of gallery material.
14. Plastic container (diameter and height 9 cm each) with a termite group.
The biology and activity of termites cannot fully be understood, nor can prevention against attack or control measures be successfully undertaken, without knowledge of the ecological conditions upon which termites depend. They need, first of all, high air humidity. Most species depend on saturated water content of the air most of the time or on access to water. Only a small group of species can develop as well under conditions of about 90 % relative air humidity. These so-called dry-wood termites can live in construction timber of buildings or in furniture without contact with the soil in regions where the average air humidity is high, such as in coastal or other normally moist areas. The other species, which depend on contact with the ground and its humidity, are called subterranean termites.
Optimum temperature for most termites is 28 to 30°C. A constant temperature of more than 32°C can be lethal for many species, but termites can tolerate temporary increases in temperature and are adept at avoiding places which are too hot. There are, however, exceptions: some species can tolerate rather high temperatures. The lower temperature limits differ very much. While species living in areas with cold winters may survive temperatures near the freezing point, tropical species die within several weeks when permanently kept at 18°C.
Termite nests and gallery systems provide a favourable environment for maintaining the temperature and humidity needed by their builder-inhabitants. Geographical and local distribution of termites depends much on the reactions of the various species to climatic conditions, just as, within the same tropical country, the fauna of the savanna differs markedly from that of the rain forests.
Young termite instars, workers and most soldiers are eyeless. Their negative phototaxis also serves as protection against desiccation of the body, which would soon lead to death. Most species collect their food in darkness, underground, in the interior of wood, or under protection of their gallery runways (Figure 13). Exceptions to this are a few species which seek food without protection in daylight and, in general, elated imagines when they are swarming at dusk.
In the absence of sight, the orientation and communication of termite individuals are based on smell and contact reactions. Termites produce a pheromone substance with which they mark their paths and this trail is recognized by all individual members of the same species. In addition to these pheromones, which are produced by a gland, other substances have been identified, of completely different composition, which are of trail-producing efficiency, such as special glycols contained in the ink of ball-point pens.
The main natural enemies of termites are ants. If a nest or gallery is opened, ants will attack the termites and carry them off into their own nests. Also a number of mammals feed on termites and some will open nests in order to collect them. Birds catch flying imagines.
Termites may suffer from bacterial diseases and are seriously affected by mould fungi, many species of which produce toxic substances such as the aflatoxins contained in Aspergillus flavus, a fungus which is also dangerous to poultry, livestock and human beings. Certain fungus species are also parasitic in termites. Toxic mould fungi can cause great damage, especially to small laboratory cultures of termites.
The avoidance of loss of timber due to destruction by termites as well as other organisms benefits the economy of any country concerned. Large quantities of wood for repairs after attacks could otherwise be used for export or for meeting other demands.
In the neighborhood of buildings, colonies of wood-destroying termites should be eradicated. Termite species living in large mounds can be killed by the application of chemicals to the opened mounds. In such an operation it is essential to eradicate the queens since they are the sources of reproduction. Poisoning the soil under wooden buildings, another anti-termite measure undertaken frequently in some countries, can be recommended only if the poisons do not affect ground water.
In construction work, metal shields placed between concrete piles and the wooden frame carrying the construction inhibit the access of termites to the timber.
Buildings should be regularly inspected on the ground level to see whether termite galleries are coming up from the soil.
To combat dry-wood termites which may enter a building as flying imagines, powdered chemicals arc spread in attics or other rooms where the application of poison is without risk to human life. There are also nontoxic dusts which kill termites by desiccation.
Wood without natural durability which may come into contact with subterranean termites should be treated chemically. The most efficient preservatives are oils with contact insecticides, which kill either when the termites eat the wood or simply on contact, some of these chemicals also act as repellents. In selecting such oils, care should be taken to choose those which will not leave strong odours in the structure and which are proven not to increase flammability of the treated timber. It should be noted that some of these insecticides, such as DDT and dieldrin, are banned in quite a few countries because of their toxicity.
For open-air use on products such as poles, posts and railway sleepers, tar-oil creosote is commonly used with success. It has the advantage of reducing cracking and thus the risk of termites getting at untreated heart-wood is decreased. This is particularly important in dry, hot climates such as in savanna lands.
Water-soluble salts constitute another group of wood preservatives. With the exception of fluorides, they act entirely as stomach poisons. Arsenic compounds are the most efficient substances in this group against termites. Their addition to creosote has been recommended as a means of increasing the depth of penetration by diffusion.
Anti-termite wood preservatives must have deep penetration into the wood in order to perform well. When only an outer layer is treated, the protection may fail since cracks will expose the untreated interior or, if repellency or contact-insecticidal efficacy is insufficient, termites will gnaw through the surface. The best treatment process is the application of preservatives in closed cylinders under low and high pressure. The effectiveness of the more simple methods of applying chemicals depends upon the treatability of the wood species involved and the properties of the preservatives. Other factors influencing performance of anti-termite chemicals are evaporation of oils and leaching of salts.
If a building is infested by subterranean termites, one of the actions to take is to inhibit further access of termites by destroying the galleries going to the ground. Contact and stomach poisons, blown into holes and galleries of infested wood, will kill subterranean and dry-wood termites. But the most efficient way to treat an entire building is with a gas, such as methyl bromide, applied under cover of a large plastic tent.
Experiments have been carried out with attractive substances existing in fungus-decayed wood and with a combination of baits and poison for reducing the termite population in special areas. Another possibility of control is upsetting the balance of the termite caste structure through the use of hormones.
If there is insufficient knowledge about the natural durability of wood species or the efficacy of new materials against termites, tests have to be made. For screening purposes and fundamental investigations, laboratory tests are advisable before final evaluations take place under practical conditions. In a number of countries where termites do not naturally occur but materials for export and chemical compounds for their preservation are produced, suitable methods for laboratory tests with termites have been developed.
A number of field tests have also been devised. One common field-test method is the exposure of wooden stakes in soil; details of this method have been unified on an international basis.
In laboratory as well as field tests it should be observed that different species of termites show different degrees of aggressiveness as well as tolerance to toxic substances. Therefore tests must be conducted with several species.
The basis for efficient laboratory work with termites is a thorough knowledge of the biology, ecology and physiology, behaviour and reaction of the various species used. A number of economically important species have now been well studied.
Advantages of laboratory tests are that they can easily be repeated and results can be obtained within a relatively short period. The climatic and other ecological conditions of the area where the wood will be exposed will influence its performance and still more the permanence of the chemical preservative used. Nevertheless, differences between laboratory and natural conditions should always be taken into account when evaluating results.
The principles of laboratory testing procedures are briefly the following. Equal groups of hundreds or thousands of termite workers - with or without other castes - are kept in small glass or plastic containers (Figure 14) under controlled conditions. As minimum requirements, soil material, humidity, temperature and nutrition should be constant. In special tests the insects are kept under starvation conditions. The degree to which termites attack the materials being tested and the number and condition of the termites are recorded over set periods of time. At short intervals the activity of the termites, their gallery building and attack on the material are checked. Untreated or susceptible material is used for comparison under the same conditions.
Investigations of termites and in most cases laboratory tests as well are conducted notably in Australia, Brazil, the - Federal Republic of Germany, France, the German Democratic Republic, Ghana, India, Italy, Japan, Nigeria, Pakistan, the Philippines, South Africa, Switzerland, the United Kingdom and the United States.
The most extensive laboratory culture of termite species in the world is probably the one kept at the Federal Institute for Materials Testing located in Berlin-Dahlem, Federal Republic of Germany. It contains about 40 species from all continents, and some of the colonies consist of millions of individuals. For some termite species, cultures are kept from different areas of their occurrence. The collection is housed in three basement rooms at constant temperatures of 26, 28 and 30°C with relative air humidity of 90 and 97 to 98% respectively. Water barriers prevent the termites from escaping (Figure 3a) or from making contact between species. Details of culture depend on special behaviour or needs of the individual species. The reason for the relatively large number of species and races is a programme to study the importance of differences among species and of racial variations for laboratory tests and other studies of termites. Comparative tests of preservatives and materials and investigations on biology, ecology and physiology should contribute to better knowledge of termites and their control or prevention and thus to the economy of tropical and subtropical countries.
(The following are mainly textbooks and general information)
BECKER, G. 1968 Protection of timber. An introduction into some problems. In Biodeterioration of materials, microbiological and allied aspects, p. 205-222. Amsterdam, London, New York, Elsevier.
BECKER, G. 1969 Rearing of termites and testing methods used in the laboratory. In Biology of termites. Vol. 1, p. 351-385. New York, Academic Press Inc.
GRASSÉ, P.P. 1949 Ordre des Isoptères ou termites. In Traité de zoologie. Vol. IX, p. 408-544. Paris, Masson.
HARRIS, W.V. 1971 Termites, their recognition and control. 2nd ed. London, Longmans, Green. 187 p.
HICKIN, N.E. 1971 Termites: a world problem. London, Hutchinson. 232 p.
KRISHNA, K. & WEESNER, F.M., 1969-70 eds. Biology of termites. 2 vols. New York, London, Academic Press.
LEE, K.E. & WOOD, T.G. 1971 Termites and soil. London, New York, Academic Press. 251 p.
WEIDNER, H. I 1970 Isoptera (Termiten). In Handbuch der Zoologie. IV/2/2 (2. Aufl.) Berlin, Walter de Gruyter. 147 p.