The monitoring technique employed in this study (discriminating dose screening of larvae reared from field collected eggs) proved extremely successful in documenting the impact of the strategy on both pyrethroid and endosulfan resistance, without the problems of alternative techniques. Because of the sensitivity of this technique, strategy users have been able to verify the anticipated impact of the strategy, identify problems, adjust their management practices accordingly and assess the effectiveness of these procedures. This has resulted in the maintenance of the strategy's excellent compliance rate.
Pyrethroids imposed selection for resistance in both moths and larvae, resulting in increases in resistance within the Stage II window and the early Stage III period, respectively. These two peaks effectively merged into one large peak while the period of pyrethroid use remained at 42 days. However, the initiative to reduce the pyrethroid window to 35 days separated the two peaks and proved to be a successful delaying tactic. The two main factors influencing pyrethroid resistance appear to be dilution by susceptibles immigrating from the refugia, followed by pyrethroid selection pressure. However, as the refugia became increasingly contaminated, their effectiveness as a source of susceptibles for dilution declined, resulting in gradually increasing pyrethroid resistance levels in all areas over time. Adult selection was more important in the mixed crop Emerald study area because of premating selection. This, along with the higher Helicoverpa armigera pressure at Emerald, probably offset any potential benefit of the longer crop season at this site. Inadequate cultivation of post-harvest fields harbouring overwintering pupae, presumably due to low price forecasts in the economically sensitive cotton industry, resulted in the survival of large numbers of resistant pupae. As a result, cultural control of overwintering pupae has become a major component of the integrated Australian resistance management strategy. The strategy has not overcome the pyrethroid resistance problem but has proved to be a successful delaying tactic in ‘buying time’ and extending the useful life of the pyrethroids.
However, the strategy has been much more successful in managing endosulfan resistance and some possible reasons for this are discussed: effectively lower selection pressure, fitness deficit, fewer life stages selected, or lower genetic dominance. However, it was not possible from this study to determine the relative importance of these factors or their interactions.