Pesticide-mediated displacement of a phytoseiid predator, Neoseiulus womersleyi, by another phytoseiid predator, N. californicus (Acari: Phytoseiidae)

Neoseiulus womersleyi and N. californicus are two predators that are frequently used to control spider mites in fruit-tree orchards. Neoseiulus womersleyi used to be the dominant predator species in Japan, but since the 1990s in central and southwestern Japan, N. californicus populations have been increasing and have displaced populations of N. womersleyi. We previously observed the same phenomenon under laboratory conditions when these species were released at a 1:1 ratio, and attributed the displacement to asymmetrical intraguild predation. However, the ratio in fruit-tree orchards could be different from 1:1. Therefore, we hypothesized that differential susceptibilities to pesticides might accelerate species displacement of N. womersleyi by N. californicus, even if the ratio between these two species was extremely skewed in favor of N. womersleyi and no species displacement occurred otherwise. We examined the effects of 21 pesticides on egg-to-adult and adult survivorship in N. womersleyi and N. californicus. Among these pesticides, two neonicotinoids (acetamiprid and imidacloprid) had much severer effects on N. womersleyi than on N. californicus and thus could possibly account for the species displacement. When the two species were released onto leaf arenas at an N. californicus: N. womersleyi ratio of 1:9 in the absence of insecticide, no displacement was observed. However, just after acetamiprid or imidacloprid application, the proportion of N. californicus increased, causing N. californicus to displace N. womersleyi. Our results indicate that displacement in predator complexes of fruit-tree orchards could be due to different degrees of pesticide susceptibility.