The development of resistance to pesticides is linked with the use of broad spectrum pesticides which often kill the pests and their natural enemies.

As many phytophagous insects are at least partially controlled by natural enemies, the use of selective insecticides in the ecosystem has been suggested as a way to prevent the development of resistant strains because the destruction of the survivors of the pesticide treatment interferes with the mechanism of the Darwinian selection which leads to resistance.

There are a number of explored cases of the use of selective insecticides in which the reduction of the pests by the pesticide has led to such concentration of the natural enemies’ activity on the survivors that the survivors were wiped out. There are no cases on record where a selective insecticide when applied in the correct concentration and against a past which had natural enemies in the ecosystem, has developed resistance.

Selective insecticides may be either physiologically or ecologically selective.

It has now been possible to test the above mentioned hypothesis in the field: In a replicated field experiment using the cabbage aphis (Breviroryne brassicae (L.)) as the pest and the parasite Aphidius rapae (McIntosh) as parasite, repeated treatment of parathion and schradan respectively were applied to control the aphis. The LC₅₀ was then determined after each 6 generations of aphids, and it was found that after 12 generations the LC₅₀ was increased seven-fold. While a resistance against parathion had thus been demonstrated, none had developed against schradan as there was no increase in the LC₅₀. This experimental proof of the prevention of the resistance by the use of a selective pesticide is in keeping with the current theory of the mechanism of the development of resistance.

If therefore the spread of resistant strains of phytophagous of other inserts under partial biological control assumes the size of a major problem, then it would make sense to readjust the research objectives of organizations responsible for the development of new pesticides to the discovery of selective insecticides.