Options for managing pesticide resistance in the redlegged earth mite (Halotydeus destructor Tucker): an experimental test involving altered selection pressures and alternative chemicals

Chemical strategies to manage pest populations that have evolved pesticide resistance can involve killing resistant individuals by using higher rates of the same pesticide or applying alternative pesticides for which resistance does not exist. However, the dilemma is that alternative pesticides available for many pest species are limited, while higher pesticide rates may lead to the evolution of higher resistance levels. Here, both strategies are tested in a resistant population of the redlegged earth mite, Halotydeus destructor, which contained resistance against two groups of chemicals: synthetic pyrethroids and organophosphates. Resistant mites were introduced into exclusion field plots, which were then treated with pyrethroid, organophosphate or alternative chemicals for 2 years at a low pressure (one spray at the registered field rate per year) or a high pressure (two sprays at the registered field rate per year). A single pyrethroid application suppressed mite numbers, but pyrethroid-resistant allele (kdr) frequencies quickly rose from ∼50% to nearly 100%. Thereafter, pyrethroid chemicals lost effectiveness. However, kdr frequencies declined across mite generations when pyrethroids were not used, regardless of other treatments. Organophosphates continued to suppress mite populations under both high and low pressures, irrespective of kdr allele frequencies, and laboratory bioassays showed no increase in organophosphate resistance levels. One of the alternative chemicals, diafenthiuron, applied once per year over two years, successfully suppressed mite numbers, whereas other treatments did not control mites. These findings demonstrate that different strategies are required to manage H. destructor with pyrethroid resistance and organophosphate resistance.