The honeydew excreted by aphids naturally attracts their predators. Researchers at Gembloux Agro-Bio Tech – University of Liège have looked into its composition in order to isolate the volatile compounds responsible for this attraction effect. In managing to identify and artificially reproduce them they have taken a spectacular step forward in the development of biological control methods against aphids.

In countries with a temperate climate, aphids are the main destroyers of crops. How do these miniscule insects with an inoffensive appearance affect plants? On one hand, they feed on their phloem (the sap tissue feeder) and, on the other hand, through the saliva they inject into the plant, they transmit viruses and bacteria which are the source of serious diseases for the plants. Weakened, the plants colonised by these insects of several millimetres in length experience reduced growth. It is thus not surprising that the aphid has become public enemy N°1 for farmers.

The only effective method in the battle against these pests was for a long time spraying the crops with insecticides. These active substances, which kill the insects, their larvae and/or eggs, are part of the pesticide family.

The pesticide era began at the beginning of the 20^th century and new products have been produced constantly since. Nevertheless for some years pesticides have been closely monitored and are subject to controversy. Certain of them have been withdrawn from the market because of the danger they present to the environment and/or human health. Their defenders estimate that they enable an improvement in product quality. Their detractors for their part denounce, besides the risks to health and the environment, the risk of the pathogens developing a resistance to pesticides, much in the same way as the resistance which bacteria develop against antibiotics which are over used.

In the light of the potential harmful effects of pesticides, the chemical war against the pests is slowly but surely giving way to the biological control. Everywhere throughout the world scientists are looking how to fight against pests through more natural methods which are also more respectful to the environment.

The inside story of the attractive power of honeydew

In order to instigate a biological control against aphids it is necessary to understand their ecology, in other words the interactions – and their consequences – between these insects and their biotic and abiotic environment. That is the task which researchers at the Department of Functional and Evolutionary Entomology, at Gembloux Agro-Bio Tech, directed by Professor Eric Haubruge, have been getting down to for a number of years. They study the interactions between the host plants, the pests and auxiliary insects, in other words potential predators of these pests. In September 2010, in the article entitled ‘Aphids betrayed by their odours’, it was a question of the aphid’s alarm pheromone. The researchers had discovered that the predators of these pests, such as ladybirds, had learnt to recognise their alarm pheromone over the course of evolution, thus allowing them to locate aphid colonies more easily.