Monitoring of spirodiclofen susceptibility in field populations of European red mites, Panonychus ulmi (Koch) (Acari: Tetranychidae), and the cross-resistance pattern of a laboratory-selected strain.

BACKGROUND: Phytophagous mites such as the European red mite, Panonychus ulmi (Koch), are serious pests in European fruit tree orchards, and a number of acaricides are frequently used to control them. Spirodiclofen (Envidor(®)) has been a commonly used acaricide for several years. In the present study, European field populations collected in 2009 and 2010 were checked for their susceptibility to spirodiclofen by using discriminating dose and full dose response bioassays.
RESULTS: In 2009 and 2010, a total of 63 field populations (including winter eggs) of European red mites were collected in different European countries, and in several populations from south-western Germany a shifting in susceptibility against spirodiclofen was observed. Full dose response bioassays on different developmental stages of field-collected strains suggested an age-dependent expression of resistance because eggs remain fully susceptible to spirodiclofen. Artificial selection with spirodiclofen of one of the field strains resulted in resistance ratios of > 7000. Synergism studies suggest a possible role of cytochrome-P450-dependent monooxygenases in spirodiclofen detoxification. Most of the other acaricides from different chemical classes displayed no or low cross-resistance in a spirodiclofen-selected strain.
CONCLUSION: In order to preserve spirodiclofen as an important tool in spider mite resistance management, the efficacy situation should be continuously monitored, and it is suggested that spirodiclofen be alternated with acaricides coming from different mode-of-action classes. An observed age-specific expression of resistance revealed full susceptibility of eggs, so targeting spirodiclofen particularly against eggs is likely to reduce the selection pressures imposed on other life stages.