The twospotted spider mite, Tetranychus urticae (Acari: Tetranychidae), is a key pest on hops. Hop production is a new industry in the Midwestern USA, and little is known about management of T. urticae in this region. During 2017, we conducted an exclusion trial at four hop yards in Ohio, to determine the services provided by predators already present in hop yards, as well as the ability of the combination of predatory mites, Neoseiulus fallacis and Neoseiulus californicus (Acari: Phytoseiidae), to effectively suppress T. urticae by augmentative releases. To determine the effectiveness of these predatory mites, three treatments were compared: a ratio of two predators per ten adult female T. urticae, a ratio of one predator per ten adult female T. urticae, and a ratio of zero predators per ten adult female T. urticae. Each treatment was established on paired leaves; one leaf was covered with a fine mesh bag and one leaf was left uncovered. After two weeks, the average number of spider mite motiles on the open leaves that received zero predators was significantly less than the initial ten released per leaf, suggesting that naturally occurring predators are capable of suppressing spider mite populations. The average number of spider mite motiles on the closed leaves that received two predators was also significantly less than ten per leaf showing that a ratio of one predator to five spider mites is effective at reducing spider mite populations.
During 2016, we conducted a trial to determine the efficacy of augmentative biological control to suppress T. urticae populations. Treatments compared were Neoseiulus fallacis and Galendromus occidentalis (Acari: Phytoseiidae), each released at a high and a low rate, with eight blocked replicates distributed over four hop yards. T. urticae populations were monitored on the cultivar `Cascade’. When populations reached a threshold of one T. urticae per ten leaves, predatory mites from a commercial insectary were released. Treatment had a significant effect on hop yield, but when we separated it by site, the yields only differed among treatments at two of the three sites. At both of these sites, the yields from the high release rate of both N. fallacis and G. occidentalis were greater than the yield of the control treatment. During 2017, a similar augmentation study was done using earlier and more intense sampling, to ensure early detection of spider mites in a system where mite density varied widely. Treatments compared N. fallacis released at a high rate and a low rate in 17 replicates. Yields did not differ significantly among treatments.
Our studies show that when spider mites are found at low to moderate densities, natural occurring predators are able to suppress their populations in Ohio hop yards. Although spider mite populations never reached economically damaging levels, augmentation using predatory mites did affect the yields at two of our eight sites. Naturally occurring predators were documented to be important in the suppression of spider mite populations and we propose that future studies should focus on biological control by conservation rather than augmentation.