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September 23, 2018

Pheromones are not all about sex: Think pest control

Pesticides can polute rivers leading to mass deaths
Pesticides can polute rivers leading to mass deaths

There was a time I used to walk in the bush at night. It was a path through some thickets that connected several homesteads including my grandmother's. I was used to the path because I used it quite often, even during the day. There was this very unique scent coming from the plants close to the path. The scent was absent when there was light, or when it was quiet at daytime. But come darkness or a little disturbance, the scent would fill the air. I did not know then the science behind the scent. But now I know. They are pheromones secreted by a species, be it a plant or an animal, which are used to influence a reaction or behaviour of a different species.

Pheromones are powerful molecules used by both animals and plants as a channel for communication where, and when other senses may not be useful. This could be during darkness for the animals that use visual senses. Pheromones can also remain in the air for long periods without jeopardising the safety of the individual sender. They can also travel through the atmosphere over surprisingly long distances so that the sender and the receiver need not be as close together physically as might be necessary for the exchange, say, of a visual signal.

Pheromones seem to be deployed primarily as a sexual message in the unending search for a suitable mate – talk about nature’s own aftershaves, colognes, and other toiletries. But they can also send other types of messages. Research has found that even inanimate organisms do communicate using scent. Some trees, as an example of inanimate organisms, when attacked by insect predators, will secrete toxic chemical substances in order to ward off the aggressors. Science has discovered that certain trees, when assailed by insect pests in one part of a bush, not only generated its own defence chemicals but appeared to send some form of warning messages to other trees in the vicinity, because other trees immediately emitted their toxins even though they were not themselves under attack at the time. Where there is no root-to-root communication between plants, and yet there is a form of communication going on from observations of reactions within, then the only explanation is that pheromones are being used as communication channels in plants.

There has been a lot of literature on the most modern farming methods that uses less space and the yields are maximised. When the writers talk about the challenges which one would be advised to expect during various farming activities, pests of different kinds are described and their distractive consequences. Remedies are usually chemical-based and they, too, have their side effects on the farmer. Most chemical pesticides will harm other plants that may be useful to the farm. Not much about biological control of pests or parasitic plants, is discussed.

True or not, one can easily see the advantage of knowing more about the pheromonal language of animals, especially those that are a nuisance to farmers and crop growers. If we could discover more about the chemical attractants and alarm system of plant pests, it might be possible to device ways of trapping or otherwise controlling them naturally instead of using insecticides over large tracts of land and end up killing useful insects. We could for example, borrow the pheromonal language of a species of wild potatoes, which is known to secrete alarm chemicals that very effectively chase away harmful aphids. It was a step forward when we heard that the invading plants of hyacinth species in Lake Victoria, were being controlled by introducing a certain larva of a beetle that was feeding on the plants. That was as close as we have gotten in regards to employing biological means to control a menacing plant. Though the project seem to have failed, it was laudable.

It is a sobering thought that in America, a certain species of a moth, whose caterpillar has been eating corn and cotton, is now under study to control it. A scientist, Prof Glenn Prestwich, intends to use the insect’s own chemical secretions to disrupt its mating systems when the mating season is on. The results should be a lesson to our farming researchers since we also grow corn (maize) and cotton as well.

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