Penetration and Metabolism of Synthetic Pyrethroids in New Zealand Pasture Pests

Abstract

The penetration, excretion and metabolism of topically applied [14C]permethrin and [14C]cypermethrin have been examined in porina, grass grub, white-fringed weevil, wax moth and puriri moth larvae. Rates of penetration of the cis- and trans-isomers of pyrethroids depended on the insecticide and insect species. Similarly the rate of penetration was influenced by the dose size depending on the pyrethroid isomer and insect species. Cypermethrin was more stable to metabolism than permethrin in porina and grass grub larvae but both pyrethroids were metabolised to the same extent in white fringed weevil larvae. Pyrethroids were extensively metabolised by porina and puriri moth larvae but less extensively in grass grub larvae. Pyrethroids were poorly metabolised in white-fringed weevil larvae. Metabolism of pyrethroids was by hydrolysis and to a lesser extent oxidation. Primary metabolites were quickly conjugated to water-soluble products in porina, grass grub and puriri moth larvae. Hydrolysis was the only significant metabolic pathway in white-fringed weevil larvae. Little excretion occurred throughout the incubation periods and body levels of unchanged pyrethroids were dependent on the interaction of penetration and metabolism. Optimal assay conditions for the gut pyrethroid-hydrolysing esterase of porina larvae were studied. Larval gut pyrethroid-hydrolysing esterases of porina larvae were studied. Larval gut pyrethroid-hydrolysing esterases hydrolysed trans-permethrin 2.4-fold more than cis-permethrin which was consistent with the in vivo findings. The gut esterase hydrolysing cis- and trans-permethrin was very sensitive to inhibition by pirimiphos-methyl and carbaryl but less sensitive to l-dodecylimidazole and oxidase inhibition. In whole insects esterase inhibitors significantly prolonged the lifetime of permethrin indicating that metabolism of permethrin was mainly by hydrolysis. Smaller effects were shown by oxidase inhibitors. Porina larvae pretreated with oxidase or esterase inhibitors did not have altered permethrin penetration patterns suggesting that metabolism did not affect rates of penetration. Penetration, metabolism and excretion of lindane and pyrethroids were compared in white-fringed weevil larvae. Lindane was absorbed much more rapidly than the pyrethroids indicating that penetration patterns varied among insecticides within a species. Less than 20% of lindane was metabolised to water-soluble metabolites. Considerable quantities of unchanged pyrethroids accumulated in the bodies of porina, white-fringed weevil and puriri moth larvae which however resisted poisoning by permethrin and cypermethrin. Under these circumstances storages or nervous system insensitivity appeared to be responsible for pyrethroid resistance.