Not so good vibrations

Abstract

Pliny the Elder was one of the first to record it 2,000 years ago: termites are noisy eaters. Their loud chewing generates acoustic emissions that have been proposed to be a method of detecting cryptic attack. But a team from the Commonwealth Scientific and Industrial Research Organisation and the University of New South Wales in Canberra, Australia, wondered whether worker termites might be able to detect vibration/acoustic signals generated by their foraging and use these signals to determine the size of the piece of timber they are attacking.TermitTermites have a reputation of being voracious and non-discriminating feeders, consuming all wood that they can find. The 2 million homeowners in the US and many elsewhere who require termite treatment each year know the problem. But Theodore Evans and colleagues, reporting in the Proceedings of the National Academy of Sciences (published online), believe this reputation is not deserved. In fact, termites can be highly selective feeders. Wood species palatability and hardness are important, as are the defensive chemicals made by the plant. But these appear not to be the only criteria by which termites assess a potential food source, as anecdotal accounts abound of termites not consuming a piece of palatable wood after finding it.Termites cause enormous damage, as shown in this house in New Orleans in the southern US, but new evidence suggests that for at least one species the size of potential timber food is of key importance in determining whether or not they decide to feed on it. (Photograph: Oxford Scientific Films.)View Large Image | View Hi-Res Image | Download PowerPoint SlideOne possible issue is the potential quantity of food. Different termite species that live in the same habitat feed on particular sizes of wood, some species targeting smaller fallen twigs and sticks and others targeting larger fallen branches or entire trees. Such variation in choice may have evolved to avoid competition, but how does a termite assess the size of a piece of wood? Termites come into contact with only a tiny piece of any wooden object and decide to eat it based on this minor contact. The decision to eat a piece of wood is made by the termites before the piece of wood is measured physically. They do not assess the linear dimensions, which would expose them to predators, or evaluate it visually as worker termites are blind.The possibility that termites could use vibroacoustic signals to assess wood size has been suggested previously following observations that Cryptotermes termites quickly responded to the volume of food given, without having the possibility of measuring the food physically.The team therefore decided to test whether termites of the Cryptotermes domesticus species might be able to discriminate between different sizes of wood. The team constructed experiments that presented termites with blocks of wood with a standard cross-sectional diameter of 20x20mm but of either 20mm or 160mm length. The insects were placed in a gap between the two faces of the timber blocks so that they were not able physically to assess the length of each block.The team found that the termites, when presented with a choice, preferred the shorter 20mm block. To test whether the termites might be using vibration signal to assess the blocks, the researchers carried out two experiments. They recorded the vibration signals produced by termites placed in a hole in blocks of wood of lengths varying from 20 to 160mm long. They then examined the influence of some of the recorded signals on termites faced with wood blocks of different sizes along with the influence of two artificially synthesized signals.The team found that when they played the vibration signals recorded from the larger block into the smaller block, the termites’ preference for the smaller block disappeared, but when the signal from the smaller block was played the preference was maintained. The termites did not change their behaviour in response to artificial noise signals.The researchers believe the demonstrated preference for smaller pieces of wood may be a mechanism to avoid competition with larger species of termite attracted to larger pieces of wood.But while individual termite species might be fussy, the news is not good for those whose home timbers have been deemed an ideal feast by whatever species finds this source and size of wood attractive.