Development of abamectin resistance in Tetranychus urticae in Australian cotton and the establishment of discriminating doses for T. lambi

Since the 1980s Tetranychus urticae Koch has dominated Australian cotton due to its ability to develop resistance. Here we give screening data for a range of chemicals tested against T. urticae including abamectin, bifenthrin, diafenthiuron, etoxazole and propargite and speculate why abamectin resistance emerged without warning. Abamectin resistance was not detected in T. urticae in Australian cotton before season 2007-2008 when a few resistant individuals were detected in a single strain. Resistance was detected again in season 2010-2011 and continued to be detected in every subsequent season comprising 80% of strains tested in 2018-2019. We speculate the reason may relate to prophylactic abamectin use to prevent mite flare with Creontiades dilutes Stal mirid sprays. With the introduction of transgenic Bt-cotton, spraying significantly reduced and anecdotally Tetranychus lambi became more abundant. Although T. lambi may now be more common than T. urticae its response to chemical controls is completely unknown. Tetranychus lambi conspecific dose responses were established to support resistance monitoring against abamectin, diafenthiuron and propargite that generated discriminating dose (DD) estimates of 0.0007 g/L abamectin, 0.03 g/L diafenthiuron and 0.7 g/L propargite. These DD were used in season 2018-2019 but resistance was not detected against any product including abamectin. The reason why T. lambi may now dominate despite T. urticae being still resistant is speculated and thought related to the progressive reduction in insecticide use in Australian cotton and/or the changing weed complex in the transgenic cotton era.

Automated summary

Tetranychus urticae Koch has become dominant in Australian cotton due to its resistance, with abamectin resistance increasing in recent years possibly due to prophylactic use. The reduction in insecticide spraying with the introduction of transgenic cotton may have led to an increase in Tetranychus lambi, despite T. urticae still being resistant.