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Research Article
Received: 17 November 2010 Revised: 1 March 2011 Accepted: 18 March 2011 Published online in Wiley Online Library: 5 May 2011

(wileyonlinelibrary.com) DOI 10.1002/ps.2191

Acaricide resistance and resistance mechanisms in Tetranychus urticae populations from rose greenhouses in the Netherlands
Jahangir Khajehali, Pieter Van Nieuwenhuyse, Peter Demaeght, Luc Tirry andThomas Van Leeuwen∗
Abstract
BACKGROUND: Spider mites are important crop pests that rapidly develop resistance to acaricides. To investigate whether acaricide resistance is a threat to greenhouse rose culture in the Netherlands, the susceptibility of 15 strains of Tetranychus urticae was tested to several currently used acaricides, and resistance mechanisms were investigated. RESULTS: Although theobserved levels of susceptibility differed between strains and acaricides, resistance was detected in most strains. The activity of detoxifying enzymes was significantly increased in most field-collected strains, and a number of amino acid substitutions known to be involved in resistance were detected. CONCLUSIONS: Resistance levels to traditional acaricides such as bifenthrin and abamectin wereprominent, and might result in control failure under field conditions. Resistance to more recently registered compounds was detected in several populations. Resistance levels were generally unstable in the laboratory without selection pressure. The toxicological, biochemical and genetic data in this study will be essential in devising an efficient resistant management for Dutch rose culture. c 2011Society of Chemical Industry Keywords: resistance monitoring; point mutations; cyflumetofen; glutamate-gated chloride channel; cytochrome b; P450 monooxygenases; esterases; mechanisms

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INTRODUCTION

Control of two-spotted spider mite, Tetranychus urticae Koch, populations mostly relies on the use of acaricides. However, this species rapidly develops resistance to acaricides owing to itshigh reproductive potential, extremely short life cycle and arrhenotokous reproduction.1 Genetically fixed resistance mechanisms in spider mites are thought to be similar to those documented in insects and involve enhanced detoxification through the enzymatic activity of esterases, glutathione-S-transferases and P450 monooxygenases and/or modification of the acaricide target site (reviewed in VanLeeuwen et al.2 ). The risk of developing resistance is particularly high in greenhouses because of the relative isolation of mite populations, the extended growing season and the intensive use of pesticides.3 The most threatening situation probably occurs in ornamental horticultural crops such as potted plants and cut flowers, where biological control is not well established owing to a zero pesttolerance. Rose is the top cut-flower species sold at the Dutch auctions, with more than ¤0.8 billion turnover in 2008 (http://www. flowercouncil.org). Next to the fact that consumers insist on cut flowers being pest and blemish free, they are also not edible and thus are excluded from regulations on pesticide residues, resulting in intensive pesticide use. In rose greenhouses, pesticide use poses athreat to occupational safety, which is as important as the enhanced risk of resistance development.4 Tetranychus urticae is one of the economically most important pests on rose.

Its quick resistance build-up to acaricides is threatening proper rose production in greenhouses. In the present study, the efficacy of eight commercially important acaricides applied at different rates to 15 T. urticaestrains collected at different locations in the Dutch rose cultivation areas was investigated. The selected acaricides included some older compounds with a long history of widespread use (such as bifenthrin, abamectin and milbemectin), some currently registered and frequently used compounds (bifenazate, spiromesifen, etoxazole and acequinoclyl) and a potential candidate for European registration...
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