The frequency of resistance to synthetic pyrethroids in Heliothis armigera has been monitored in the Namoi Valley since the introduction of the insecticide resistance management strategy in 1983. During this time the frequency has varied cyclically. Although the magnitude of the changes varies from year to year, the same pattern is evident each cropping season: the frequency increases in stage 2 (the six week period during which pyrethroids are used), continues to increase in stage 3 (autumn) and then falls by stage 1 (spring and early summer) of the following cropping season (Forrester and Cahill 1987).