Cross Resistance Patterns and Biochemical Mechanisms in a Chinese Helicoverpa armigera Strain Selected with an Organophosphate/Pyrethroid Mixture

Selection of a field strain of Helicoverpa armigera(HN) was undertaken by treatment with a mixture of an organophosphate(phoxim) + a pyrethroid(cyhalothrin)(25:1,a.i.) for 33 generations.The resistance to phoxim of the selected strain(HN-R) increased to 17.4-fold and the resistance to cyhalothrin increased to 144.7-fold.High level cross-resistance to two pyrethroids was detected(86.2-fold to deltamethrin and 23.4-fold to fenvalerate) in the HN-R strain.In addition,the HN-R strain also possessed a moderate level of cross-resistance to monocrotophos(5.2-fold),a low level cross-resistance to methyl parathion(1.6-fold),profenofos(1.8-fold),and methomyl(2.4-fold).The esterase inhibitor S,S,S-tributylphosphorotrithioate(DEF) was very effective in reducing phoxim resistance of the HN-R strain(with a synergism ratio of 15.3-fold) but the mixed function oxidase(MFO) inhibitory synergist piperonyl butoxide(PBO) had no synergism to it at all.In contrast,PBO dramatically removed most of resistance to three pyrethroids with synergism ratios of 21.9-fold to cyhalothrin,19.1-fold to deltamethrin,and 21.4-fold to fenvalerate,but DEF had no synergism to them.Both DEF and PBO can remove about half of the resistance to monocrotophos(with synergism ratio of 2.2-and 2.5-fold,respectively).Compared to the reference HN strain,the MFO activity of the resistant HN-R strain to p-nitroanisole(pNA) was 6.5-fold and total esterase activity to 1-naphthyl acetate(a-NA) was 1.6-fold.At least two metabolic resistance mechanisms are involved in this resistant HN-R strain: esteratic detoxification(to phoxim and monocrotophos) and oxidative metabolism by mixed function oxidases(to pyrethroids and monocrotophos).The results suggest the use of pyrethroid/organophosphate mixtures in the field is likely to lead the simultaneous selection of multiple metabolic mechanisms of resistance,and thus a broad cross-resistance spectrum.