Croft, Brian A.

Strong, W. B.

Undetermined

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97Biological and Cultural ControlSection VBiological Control of Two-Spotted Spider Mite on HopsW. B. Strong and B. A. CroftDepartment of Entomology, 2046 Cordley Hall,Oregon State University, Corvallis, Oregon, 97331.INTRODUCTIONBiological control of insects on hops is a desirable means of control due to loss of effectivepesticides, development of insecticide resistance in pests, and public concern about pesticideresidues. This research project was initiated in 1991 to study the potential of and developbiocontrol against Two-Spotted Spider Mites (TSSM), Tetranychus urticae, using phytoseiidpredatory mites in the Willamette Valley of Oregon.SURVEYS OF COMMERCIAL AND ESCAPED HOPSThese surveys were conducted in early, mid, and late-season in both 1991 and 1992.Leaves from commercial and escaped (unsprayed) hops were collected and examined for phytoseiidsand TSSM. Overall, the phytoseiid incidence in commercial fields was variable; the majority wereAmblyseius fallacis (Table 1). Metaseiulus occidentalis, a warm-weather species, was found only in1992. In escaped sites phytoseiid numbers were more uniform between samples and years, themost common being Typhlodromus pyri, followed by a mixture of several other species. TSSM werefound in commercial sites from low to high levels (up to 38 mites/leaf) despite miticide applications,whereas TSSM levels in unsprayed escaped hops rarely exceeded 1/leaf and never exceeded 3/Ieaf.It seems that biocontrol of TSSM using phytoseiids is possible and indications are that it will workeven in commercial fields, if certain cultural and spray modifications are made.REPLICATED BIOCONTROL STUDIESA biocontrol experiment was conducted which tested the effect of releases of predators onspider mite numbers in commercial hops. Four species of predator were released: A. fallacis, M.occidentalis, T. pyri, and Amblyseius andersoni, alone or in combination. The trial was repeated onfive farms. On 3 of the farms, biocontrol was deemed successful. Although mites were notcontrolled to low levels, they were maintained at levels well below the controls (no phytoseiids) (seeFigure 1). On the other two farms, we were sprayed out before success could be determined.Overall, A. fallacis had the most potential as a biocontrol agent, followed by M. occidentalis.Table 1. Phytoseiids and TSSM levels found in escaped and commercial hops in the WillametteValley, Oregon.Commer :ial fields Escaped sites1991 1992 1991 1992# leaves 5100 4600 700 1200Mites/leaf 1.37 + -1.43 1.63 + -1.70 .24+ -.07 .96 + -.21Phytoseiids/sample 1.02+ -.67 9.82 + -5.48 4.70 + -1.28 5.44 + -2.14Total A. fallacis 78 791 6 4M. occidentalis 0 26 2 25T. pyri 4 9 58 120A. andersoni 0 0 2 38Other 0 0 6 798Amixture of the species may prove beneficial, since A. fallacis is a cool, wet-adapted species andM. occidentalis is a warm, dry-adapted species. There are strong indications that if released in adifferent manner (more frequent, smaller releases on the tops of every plant) they can providecontrol acceptable to a commercial grower. Whether this is feasible and economical remains to beseen.Figure 1: Spider mite levels from Farm 2A. fallacis18'13'Plant height 9'M. occidentalisControl18'13'Plant height 9'

Biological Control of Twospotted Spider Mite on Hops

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Biological Control of Twospotted Spider Mite on Hops

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