Chemical control

Synthetic compounds

The first nicotine-based insecticides, derived from the natural compound, date from the 1920s. The development of the chemical industry saw the creation of numerous insecticide products. The supply of these agents rapidly increased, with the marketing of organochlorines in the 1960s, organophosphates, carbamates and then synthetic pyrethrinoids in the early 1980s and the neonicotinoids in the late 1990s. The latter are currently by far the most commonly used against aphids, by direct spraying on the plants for pyrethrinoids and by both spraying and seed treatment for neonicotinoids. 

Over the course of this period, the recommended dosage has steadily decreased, as have the toxicity of the active ingredients (AI): between 1970 and 2000, the recommended doses of AA per hectare were cut by 10 on average. At the same time, products have become more selective, more respectful of the auxiliary fauna of the enemies of aphids, but still to a highly unsatisfactory extent. They have also become rapidly degradables after use, thereby reducing the quantities of residues detectable in the food chain. Chemical control has in fact favoured selection of clones of a range of aphid species mostly resistant to the great majority of insecticide familles.

Current regulations limit the number of chemical substances and their use

They are substances powerfully toxic for both the central and peripheral nervous systems. Their neurotoxicity explains at once their efficacy on insects and their toxic effects in humans. Recent regulatory changes have brought about the withdrawal of many of the active agents, notably organophosphates and carbamates. Currently, the pyrethrinoids are the insecticides most often called on for manufacturing products for agricultural use because they have a better effectiveness/toxicity ratio. New substances such as fipronil and imidaclopride, although potentially advantageous from the operator-safety point of view, have been the subject of considerable application restrictions owing to their supposed deleterious effects on bee populations.

Mode of action

Insecticides can work by contact, or after absorption of the sap (systemic products). Other, agents are available that work through an intermediate process, by translaminar activity (penetrating plant tissue).
Oils extracted from petroleum act by asphyxia and are used in orchards and nurseries as winter treatments to destroy the eggs of aphids. Restricting outbreaks of aphids can be done with the aid of systemic agents, which show various advantages.
· The aphids feeding on sap will be poisoned
· The insecticide can be employed for treating the seed: it diffuses through the plant during germination..
· Natural enemies will not be poisoned but they could suffer from a scarcity of prey.
Seed applications are permitted for control of aphid vectors of viruses of annual crops (wheat, barley, beet). However, they have the great drawback that treatments inevitably have as systematic options, seeing that there is no way of predicting the forthcoming risks of attack at the moment of sowing the treated seeds

At the current stage of techniques available, chemical control must be used with discretion to limit its undesirable effects.

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