ABSTRACT
The predatory mite Neoseiulus californicus McGregor (Acari: Phytoseiidae) is an important natural enemy of phytophagous mites, and naturally established populations are often found in apple orchards. However, insecticide applications to control pests cause side effects to non-target organisms such as N. californicus. Pirimicarb, a widely used carbamate insecticide in apple orchards, is generally considered a selective aphidicide, however, toxicity to beneficial insects and predatory mites has been reported. Furthermore, the molecular basis for this selectivity, if present in N. californicus, is still largely unknown. In this study, 8 field-collected N. californicus populations were investigated and showed up to 27-fold resistance compared to a susceptible laboratory population. Selection in the laboratory for 5 consecutive generations resulted in a 69-fold pirimicarb resistance. Although there were no significant difference in terms of the acetlycholinesterase (AChE) activity between susceptible and field-collected populations, the selected population exhibited a significantly higher AChE activity. In addition, gene copy number variation of acetylcholinesterase (ace) gene among populations was detected and ranged from 1.6 to 2.1-fold relative to the susceptible population. All field-collected populations, but not the selected population, had a significantly higher ace copy number compared to the susceptible population (t-test, p < 0.05). Molecular analysis of the target-site (AChE) revealed, for the first time, a phenylalanine to tryptophan substition at position 331 in AChE (Torpedo californica numbering), both in field-collected and the selected population, but not in the susceptible population. Last, the selected F5 population consumed significantly more Tetranychusurticae adults than the parental population. Together, the results of this study shed light on the molecular determinants of acaricide selectivity in predatory mites, and will contribute to a better design of an integrated mite management program, including the use of pesticide resistant N. californicus in apple orchards.
