Pheromones are the basis of chemical communication in insects. Although not as well studied as some insects, pheromones are nonetheless important in termites.
Articles in this magazine often discuss termites. For many years, termites have been the subject of dedicated research into their taxonomy, biology, behaviour, distribution patterns and their preferences for materials. Then there is the research into termite control, eradication and prevention. Plus the significant cost associated with product registration, CodeMark certification and product distribution.
Chemical communication in insects is fundamental to their success. While there has been considerable research on the use of pheromones in eusocial insects, much of this research has focused on ants and bees. However, although there have been fewer studies investigating the role of pheromones in termites, this research has nonetheless produced many interesting insights. Of particular interest, from an evolutionary perspective, is the fact that it appears the same compound is often secreted by different glands in different species, and often for a different purpose.
These various substances are considered to be particularly important for communication in termite societies since termites are blind, with these compounds therefore responsible for maintaining the social structure of the colony. The social structure of a termite colony depends very much on the ratios of worker termites to soldiers to reproductives, and their effective communications.
Consequently, we see pheromones used for foraging, defence, commencement of feeding on food sources, colony segregation, mate location, and caste regulation.
Trail pheromones are secreted by the sternal glands, which are present in all species of termites, and are also known to produce sex pheromones. Trail pheromones are released when the termite presses its abdomen down onto the surface it is tracking over, to release them from the sternal gland. Termites lay these pheromone trails while searching for food sources so they can later recruit their nestmates to the location.
The process commences with scouts and foragers (usually the older workers) leaving the colony. They leave a dotted trail, a process akin to trail hunting, except in the case of termites, the distance between scent points tends to be a consistent distance apart.
After traveling several centimetres, scouts stop and then quickly head back along the trail they have just laid. Other scout workers will then later extend the trail, either in the same direction, or moving off into a new direction. When a suitable food source is located, the scouts maintain the trail while carefully examining the food source with their palpi and antennae. Having confirmed the viability of a food source, which is often done through the use of sounds, scouts will return to the nest, laying now a recruitment trail to attract forager termites to follow. So, the recruitment trail is laid in reverse, heading back to the colony.
These trail pheromones are considered to be a multi-component blend of chemicals containing a single primary active component. The latter is perhaps common across species but is supported by a range of secondary compounds that are more species specific. Those chemicals that have been identified include dodecatrien-1-ol in Rhinotermitinae including Coptotermes spp.; neocembrene in Nasutitermes spp.; and dodecenol in Macrotermes spp. and Odontotermes spp.
Coptotermes spp. are known to secrete an aggregation pheromone that also acts as a feeding stimulant. This is released from the termites’ salivary glands when they feed on timber. It is believed this helps with colony security since it is easier for the soldier termites to monitor workers when they are grouped together.
When it comes to the protection of the colony, we’re all aware of the vibratory cues produced by soldiers tapping their heads on the timber, but chemical compounds created in the endocrine glands are also present. The projection of these chemicals is also commonly observed by pest management professionals when dealing with Nasutitermes spp. and C. acinaciformis. These volatile, projectile chemicals include monoterpenes, diterpenes, and sesquiterpenes. The scents from these will also draw more termites to the defence of the colony.
In Nasutitermes spp., worker and soldier termites react differently to the chemicals produced in order to optimise the defence of the colony. Soldiers become excited and patrol the area surrounding where the compounds were emitted. Mature workers are also attracted to these defence pheromones, but their activity is much more focused around the actual point at which the chemicals were emitted, than is the case with the soldiers. The workers’ reaction is also governed by the presence of soldiers and they are less active when larger numbers of soldiers are present. This seems to indicate they play a support role to the soldiers, rather like Australia’s Army Reserves. Earlier instar workers show little reaction to the secretions.
The soldiers therefore provide the first line of defence, immobilising and incapacitating mobile enemies with their sticky secretion. The older, larger workers, then assist in eliminating the enemy.
These alarm reactions are elicited by (+)-α-pinene, the major monoterpene in the secretion. Many of the compounds present are not pheromones, and they provide a vast arsenal of defensive solutions. These chemicals are usually projected through a fontanelle on the head of the termites, or from the end of the nasute projection in Nasutitermes. Some of the compounds present are toxic to ants, the main predators on termite colonies, whilst others mimic ant alarm pheromones.
Sex pheromones are most commonly emitted by female alates. Dodecatrien-1-ol, mentioned earlier as a trail pheromone, is the main sex pheromone identified with Coptotermes spp. Again, the available evidence indicates that termite sex pheromones consist of a blend of chemical substances which provide a synergistic response. Sex pheromones are produced from the sternal glands, as well as the tergal glands, with the latter mostly involved during post-flight mating behaviour.
Whilst these sex pheromones are often either identical, or closely similar, to the trail pheromones, they are produced in much greater amounts, especially by female termites. Both long distance and short distance pheromone cues have been observed. The latter are involved in the final stages of mate attraction, after the flight and loss of the wings; they are known to lead to the commencement of the tandem running behaviour observed in freshly ‘joined’ alates, as they seek to establish a nesting site.
In this behavioural pattern, the males place their prothoracic legs on the abdominal pleural membranes of females, effectively linking themselves to the females, who then lead the search in tandem with their attached male. The males follow, using only their mesothoracic and metathoracic legs for movement.
These are also known as recognition pheromones and are mainly hydrocarbon compounds formed by the termite’s exoskeleton. They provide for nestmate recognition as they are considered to be unique to each colony within a species. With increased knowledge of termite pheromones and communication systems we can hopefully gain a greater understanding of termite societies, which may lead to the development of alternative methods for termite control.
Steve Broadbent, Regional Director, Ensystex Australasia