Augmentation and conservation biological control of Tetranychus urticae on hops in Ohio

Abstract

The twospotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae), is a key pest on hops grown in the Midwestern USA, where hop production is a new industry, and little research has been done on the management of T. urticae. In 2016 and 2017, we conducted an experiment to determine the efficacy of augmentative biological control of T. urticae populations on the cultivar ‘Cascade’ at four hop yards. In both years, treatments compared Neoseiulus fallacis Garman (Acari: Phytoseiidae), released at a high rate and a low rate, and an untreated control, with eight replicates in 2016 and 17 replicates in 2017. Additional treatments in 2016 evaluated Galendromus occidentalis Nesbitt (Acari: Phytoseiidae) released at a high and a low rate. The target low rate in both years was one predator per ten T. urticae. The target high rate was one predator per five T. urticae in 2016, and one predator per two T. urticae in 2017. When weekly monitoring showed that the population reached an action threshold of one T. urticae per ten leaves, predatory mites were released. If the T. urticae population continued to increase, a second release was made. By the time of harvest, the cumulative number of mite-days for T. urticae did not differ significantly among treatments in either year. Hop yields showed a significant treatment effect in 2016, with higher yield where the high rate of G. occidentalis was released than in other treatments, but yields did not show any significant treatment effect in 2017. In 2017, we also conducted an exclusion experiment 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, N. fallacis and Neoseiulus californicus McGregor (Acari: Phytoseiidae), to suppress T. urticae by augmentative releases at three different predator to prey ratios: zero to ten, one to ten, and two to ten. Samples were paired; one leaf was covered with a fine mesh bag and one leaf was left uncovered, in each of 50 replicates. After two weeks, the average number of T. urticae motiles on the open leaves that received zero phytoseiids was significantly less than the starting number of ten, suggesting that ambient predation is capable of suppressing T. urticae populations. The average number of T. urticae motiles on the enclosed leaves that received two phytoseiids was also significantly less than the starting number of ten, while the average number of T. urticae motiles on the enclosed leaf that received one phytoseiid was not, showing that a ratio of one phytoseiid to five T. urticae is effective at reducing T. urticae populations. Our experiments showed that when T. urticae is found at low to moderate densities, naturally occurring predators are able to suppress their populations in Ohio hop yards. Augmentation using phytoseiid mites did not have a consistent beneficial effect on yields. Given that naturally occurring predators are important in the suppression of T. urticae populations, future studies thus might concentrate on conservation biological control.