Absence of mowing prevents resurgence of Tetranychus urticae and Panonychus ulmi (Acari: Tetranychidae) after broad-spectrum insecticide use in apple orchards

We investigated whether mowing the undergrowth in apple orchards affects the occurrence of spider mites and their predators, phytoseiid mites. We compared their occurrence between one unmown and one mown plot in an apple orchard in Akita Prefecture, northern Japan. Neither plot was mown in 2014 or 2015, but one plot was mown monthly from late May to late August in 2016 and 2017. Both plots were sprayed with narrow-spectrum insecticides in 2014 and 2015 and broad-spectrum insecticides in 2016 and 2017. In 2014 and 2015, some Tetranychus urticae were present on leaves in both plots, but their numbers did not increase greatly. In 2016, numbers of Panonychus ulmi and T. urticae surged in the mown plot, but grew slightly in the early growing season and then decreased in the unmown plot in association with rapid population increases of the predacious mites Typhlodromus vulgaris and Neoseiulus womersleyi . In 2017 also, numbers of T. urticae surged in the mown plot but remained low in the unmown plot. In addition, the numbers of T. vulgaris until July were significantly greater in the unmown plot in both years. Moreover, the population density of T. urticae when N. womersleyi began to appear was significantly lower in the unmown plot in both years. These results suggest that the resurgence of spider mites can be avoided by not mowing in apple orchards.