Termites are an ancient order of insects whose origins date back more than 100 million years to the Cretaceous period. Although they are commonly called ‘white ants’, the resemblance to ants is superficial and they are more closely related to cockroaches and in fact have been recently included into the cockroach order Blattodea.
Termites can be grouped into three basic categories: damp wood, drywood and subterranean. Dampwood termites generally live in damp rotting logs or rot pockets in dead or living trees. Drywood termites obtain water from the wood in which they live and have no contact with the soil, or with any other source of moisture. Subterranean termites are generally ground-dwelling or require contact with the soil or some constant source of moisture and are the main threat posed to timber in the built environment (timber-in-service).
Termites play a prominent part in the recycling of plant nutrients through the disintegration and decomposition of dead wood and plant debris. Their excavations alter the structure of trees and provide spaces which have become a necessary part of the habitat of many vertebrate species including bats, birds, reptiles and arboreal mammals. Many species of termite feed on materials such as grass. Only a handful are of economic importance to timber-in-service.
1. The termite colony
Termites are social insects, working and living together in groups (colonies). Each colony contains several types (castes) which differ in body shape and behaviour, and each caste is specialised to perform different tasks. Three principal castes are recognised: workers, soldiers and reproductives (the primary king and queen and sometimes supplementary reproductives).
Figure 1 The subterranean termite Schedorhinotermes sp. Soldier on right with worker on left
The worker caste dominates the colony numerically and is wingless, sterile and blind. The workers are aptly named because they build the nest and galleries, tend to the eggs and young, gather food, and feed other castes incapable of feeding themselves. Older workers may predominate in activities outside the nest. The primitive termite family (Mastotermitidae) lacks a worker caste. Instead, the tasks of workers are performed by a ‘worker-like caste’ (pseudergates), which may develop into other castes.
The soldier caste is the most distinctive and easiest from which to identify the species. The soldiers are distinguished from other castes by their heads which are heavily armoured and coloured. Like the workers they are wingless, sterile and blind. Because their mandibles are so modified or specialised, soldiers must be fed by the workers. The primary function of the soldiers is to defend the colony against predators such as ants.
The alate caste, the potential kings and queens of new colonies, possesses eyes, functional reproductive systems and wings. They usually swarm (leave the colony) in spring to early summer or late summer to early autumn, often through specially constructed exits. They normally swarm at dusk and may be attracted to lights at night. Alates do not fly strongly and, unless assisted by winds, their dispersal is limited.
Following swarming, alates shed their wings and each mated pair seeks out a suitable place to establish a new colony. The king alters little in shape, but the queen’s abdomen may become enormously distended with eggs until she is little more than a large, immobile egg laying machine. Some subterranean termite queens are capable of producing 2000 eggs per day. Mature termite colonies may number up to two million individuals and exist for as long as 50 years.
Termites build various types of nest. Some termites have a completely underground existence, apparently without a central nest. Examples include some species of Amitermes. Others build a central nest in the soil, or in dead or living trees. Many economically-important termites build nests of this type, notably Mastotermes darwiniensis and species of Coptotermes and Schedorhinotermes. Still, other species, for example in the genera Microcerotermes (Figure 2) and Nasutitermes (Figure 3), attach their nest to a tree but maintain a soil connection via galleries running down the surface of the trunk. A termite mound is the most familiar form of termite nest.
Mounds are often of very distinctive form, and their size and shape vary from hardened, flat areas to the tall, columnar structures of the spinifex termite Nasutitermes triodiae in northern Australia, which may be more than 7m high (Figure 4). Typically, each species builds a characteristic mound, although there may be geographical variation in the size and shape of the mound within species. In the mounds of Coptotermes (Figure 5) the outer wall is hard and built of soil and the inner region is generally composed of woody faecal material (carton) and soil.
3. Feeding habits and behaviour
Cellulose, found in plants, is the basic food requirement of all termites and, in turn, all types of plant material can be damaged by termites. Most termite species eat grass and other surface vegetation and have an important role in maintaining soil fertility. They recycle nutrients, in particular nitrogen which is essential for healthy plant growth. When termite mounds erode, the soil particles rich in nutrients such as calcium, magnesium and potassium are washed into the soil from the mound to become available for plant growth. Termite galleries improve soil structure, and assist water entry and storage in soil; surface rainwater runoff and subsequent soil erosion are thereby reduced by the galleries.
Other termite species infest timber and particularly timber which is in an early state of decay by wood rotting fungi.
- Figure 2 Arboreal nest of Microcerotermes
- Figure 3 Arboreal nest of Nasutitermes
- Figure 4 Tall, columnar structures of the spinifex termite Nasutitermes triodiae in northern Australia
- Figure 5 Coptotermes acinaciformis constructs a mound north of the Tropic of Capricorn
Some species of timber are resistant to termites, but none is entirely ‘termite proof’. Termites will often damage materials they cannot digest, for example, plastics, rubber, metal or mortar. Primarily, this damage occurs when the indigestible items are encountered during the termites search for food.
Some termites forage for food by means of subterranean galleries or covered runways, which extend from the central nest to food sources above or below ground. The gallery system of a single colony may be used to exploit food sources over as much as one hectare, with individual galleries extending up to 50m in length for most species. In the case of the giant northern termite M. darwiniensis, individual galleries may extend as far as 100–200m. Apart from grass-eating species, which forage in the open; all termites remain within a closed system of galleries, devoid of light. The only exceptions are during a swarming flight, or when repair or new construction is occurring. The advantages to the termites of this closed system are twofold. They are protected from natural enemies such as ants, and they gain a measure of protection from temperature and humidity extremes. Termites have a thin external covering and have relatively little resistance to drying out.
4. Natural enemies
The most important natural enemies of termites are predators of various kinds, especially ants. Winged reproductives emerging on their colonising flight are eaten in large numbers by lizards, snakes, frogs, insectivorous and omnivorous birds, ants and other predatory insects, especially dragonflies. Workers and soldiers of a wide range of species form an important part of the diet of the echidna, which has strong, long-clawed feet with which it damages mounds and subterranean galleries.
Although the coastal belt and northern parts of the country are generally regarded as high hazard areas for subterranean termite infestation, species which damage timber-in-service occur throughout mainland Australia. In practice, any structure containing wood is exposed to possible subterranean termite infestation whether in the business heart of a city, in the suburbs or out in the country, unless protective measures are taken.
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