Mite abundance and phenology on commercial and escaped hops in Washington State, USA

Abstract

The abundance and phenology of two-spotted spider mite (Tetranychus urticae) and its phytoseiid and nonphytoseiid predators, were monitored monthly on commercial (pesticide-treated) and escaped (pesticide-free) hops in the Yakima Valley, Washington over two seasons. Mean abundance of T. urticae for both seasons was generally low (2.8-27.7 mites/leaf) and did not differ significantly between commercial and escaped hop sites, despite application of up to four miticides at commercial sites. Miticide control of T. urticae failed at three hop yards in 1999 producing substantial populations (300/leaf) in August. Six phytoseiid species (Galendromus occidentalis, Neoseiulus fallacis, Typhlodromus pyri, Amblyseius andersoni, Typhlodromus caudiglans, Proprioseiopsis lindquisti), were identified with G. occidentalis and N. fallacis accounting for > 95% of phytoseiids sampled. Mean abundance of phytoseiids for both seasons did not differ significantly between commercial and escaped hops. Six nonphytoseiid predators of T. urticae were recorded with the mite-eating ladybird, Stethorus picipes, most abundant. The incidence of nonphytoseiid predators was more than three times greater in escaped than in commercial hops. The possible importance and utilization of an assemblage of natural enemies containing phytoseiid and nonphytoseiid predators for biological control of T. urticae populations in Washington hops, is discussed.