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

Control of spider mites, Tetranychus urticae Koch (Acari: Tetranychidae), i n 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 season s there has been concern that the chemical has not performed well against a ll T. urticae populations. This paper reports the results of laboratory-bas ed tests to determine the response to fenbutatin oxide of five populations of T. urticae collected from various sources with different histories of ex posure to the acaricide. There were significant differences in the response of the mite populations; one appeared to be resistant, one partially resis tant and three susceptible to fenbutatin oxide. Three of the mite populatio ns became significantly more resistant following a single exposure to fenbu tatin oxide but there was evidence that they reverted to being more suscept ible when the selection pressure was relaxed. The results explain why many growers have experienced intermittent difficulties controlling T. urticae w ith fenbutatin oxide for several years without the widespread development o f complete resistance. (C) 1999 Elsevier Science Ltd. All rights reserved.