Sixty-five synergists and seven solvents were tested on larvae of a pure breeding pyrethroid resistant strain of Helicoverpa armigera (homozygous for a metabolic detoxification mechanism fully suppressible by piperonyl butoxide (Pbo), presumably via a microsomal monooxygenase system). The methylenedioxyphenyl and acetylenic compounds were the most effective synergists with moderate activity from some organophosphate compounds, particularly the N-alkyl phthalimidomethyl phosphorodithioate, phosmet. All the other compounds tested were either ineffective or only marginally effective including the majority of organophosphates (12 out of 18 tested), pyrethroid analogues, N-alkyls, esterase and glutathione transferase inhibitors, various nitrogen heterocycles, juvenile hormone and analogues, formamidines, organochlorines, anti-oxidants and kojic acid. The most promising synergists indicated for further evaluation were synthetic analogues of Pbo, phosmet, propargite and possibly also fenthion, phosalone, azinphos-ethyl, pyrazophos and kinoprene. All solvents (except dodecane) had no impact on improving kill, indicating that the mode of action of Pbo and the other synergists in this study is principally true biochemical inhibition and not quasi-synergism (improved penetration). Dodecane marginally improved kill in the resistant but not the susceptible strain, indicating some preferential penetration synergism in the resistant strain.

A number of trials with Pbo were designed to facilitate the development of this synergist for commercial field use within the Australian IRM strategy. From these, it was found that: 1. A set rate of Pbo should be used, irrespective of the activity of the accompanying pyrethroid. 2. Residual activity of Pbo is poor (less than 50% of initial activity by two days post spray) but that this could be partially overcome by increasing the rate. 3. Straight Pbo applied onto a weathered pyrethroid deposit could restore control but only temporarily and would probably be of little practical field use. 4. No difference in residual activity could be found between the four formulations of Pbo tested. The synthetic analogue PBX was slightly less intrinsically active than the natural product (Pbo) but when tested in the field against Pbo at the same rate, was equivalent in efficacy to both the standard commercial and research Pbo formulations.

In order to preserve the long-term effectiveness of Pbo as a pyrethroid synergist within the Australian IRM strategy, an optimal use strategy (based on synergist rotation within the present insecticide rotation scheme) is proposed.