Adult Helicoverpa armigera were shown to be capable of expressing pyrethroid resistance to both direct and indirect exposure. Male and female moths were equally resistant. Pyrethroid field sprays selected for resistance in moths which were repelled onto neighbouring unsprayed cotton and possibly also, lower into the unsprayed portion of the canopy. Selection of moths prior to egg laying is suggested as the explanation for the immediate increase in pyrethroid resistance which occurs within the Stage II window.

The phenotypic expression of pyrethroid resistance in adult H. armigera declined significantly with age, beginning sometime between four and six days and continuing for the effective life span of the moth. This was the main reason for the poor correlation of adult resistance (determined from pheromone trapped males) and resistance in field collected eggs found in a detailed study comparing both techniques on two properties. On a young emerging moth population, pyrethroids remained selective for four to five days. On an ageing population, pheromone trapped males significantly underestimated resistance. Endosulfan and profenofos sprays were neutral in their impact on pyrethroid resistance in both eggs and adults. The resistance peaks in eggs lagged four to seven days behind the pyrethroid applications. The advantages and disadvantages of monitoring resistance using pheromone trapped males are discussed.