Section 10. Pyrethroid resistance: resistance breaking pyrethroids
Authors: Forrester Neil W. , Cahill Matthew , Bird Lisa J. , Layland Jacquelyn K. 
Published: October 2013

The structural requirements for designing a resistance breaking pyrethroid to overcome oxidative metabolic pyrethroid resistance in Helicoverpa armigera were studied. A range of pyrethroid structures were tested on a well defined pure breeding pyrethroid resistant strain of H. armigera (homozygous for a metabolic detoxification mechanism fully suppressible by piperonyl butoxide, presumably via a microsomal monooxygenase system). Highest resistance factors were to the ester bonded phenoxybenzyl alcohol pyrethroids, particularly to those with an aromatic acid moiety. Changes to the alcohol moiety alone could overcome most, if not all, resistance. Simple benzyl alcohols were the most effective followed by cyclopentenolones and a methylated biphenyl alcohol. However, the benzylfurylmethyl alcohol (bioresmethrin) was not effective. The incorporation of a synergophore grouping into the alcohol moiety was fully effective for Scott's Py III (methylenedioxyphenyl) and prallethrin (propynyl) but only partially effective for tetramethrin (N-alkyl). Changes to the acid moiety had little effect except for the incorporation of a synergophore methylenedioxyphenyl grouping (Cheminova I) which was just as effective as for the same insertion in the alcohol moiety. The change to a central ether bond from the conventional ester bond lowered resistance. Reversion to an unsubstituted alpha carbon analogue from the conventional alpha cyano group also lowered resistance.

Piperonyl butoxide (Pbo) had little effect on pyrethroid toxicity in the susceptible strain except for the single isomers deltamethrin and esfenvalerate. However, it was more than fully effective in overcoming resistance and actually reduced resistance factors to significantly below one in the resistant strain. This indicated the possibility that Pbo could be acting both as a classical monooxygenase inhibitor and a preferential penetration synergist in resistant larvae.

Partial or full resolution of racemic mixtures had minimal impact on increasing toxicity in the susceptible strain. However, partially or fully resolved isomers were clearly much more toxic on resistant strains, indicating a possible blocking effect of the inactive isomers during the toxication process with the higher pyrethroid doses applied to resistant larvae. Cis isomers had only slightly higher resistance factors than trans isomers.

Seven fully effective resistance breaking pyrethroids were identified in this study and one of these (the simple benzyl alcohol, Series Two) was shown to be equally effective on both adults and larvae of H. armigera. It was also shown to work equally well on laboratory or field material and gave results similar to a pyrethroid/Pbo combination. However, none of the resistance breakers identified so far are able to satisfy all of the requirements necessary for an ideal resistance breaking pyrethroid (i.e. good resistance breaking activity at low rates, photostability, residual activity similar to current pyrethroids and safety to mammals). Factors acting against the possible commercialization of successful resistance breaking compounds are discussed.