Response to fenbutatin oxide in populations of Tetranychus urticae Koch (Acari: Tetranychidae) in UK protected crops

Abstract

Control of spider mites, Tetranychus urticae Koch (Acari: Tetranychidae), in protected edible crops in the UK has depended on the combined use of the predatory mite, Phytoseiulus persimilis Athias-Henriot (Acari: Phytoseiidae), and the selective acaricide, fenbutatin oxide. However, in recent seasons there has been concern that the chemical has not performed well against all T. urticae populations. This paper reports the results of laboratory-based tests to determine the response to fenbutatin oxide of five populations of T. urticae collected from various sources with different histories of exposure to the acaricide. There were significant differences in the response of the mite populations; one appeared to be resistant, one partially resistant and three susceptible to fenbutatin oxide. Three of the mite populations became significantly more resistant following a single exposure to fenbutatin oxide but there was evidence that they reverted to being more susceptible when the selection pressure was relaxed. The results explain why many growers have experienced intermittent difficulties controlling T. urticae with fenbutatin oxide for several years without the widespread development of complete resistance.