The present invention relates to a method of preventing or reducing insecticidal resistance of adult insect pests of the family Nitidulidae, the family Curculionidae and Psylliodes spp. which method comprises contacting the plant or the soil or water in which the plant is growing, or the pest or its food supply, habitat, breeding grounds or locus, with pesticidally effective amounts of piperonyl butoxide and at least one pyrethroid, wherein piperonyl butoxide and the pyrethroid are applied in a weight ratio of from 0.0001 to 10000.