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United States Department of Agriculture

Agricultural Research Service

Title: Effects of Powdery Mildew Fungicide Programs on Twospotted Spider Mite (Acari: Tetranychidae), Hop Aphid (Homoptera: Aphididae), and Their Natural Enemies in Hop Yards

Authors
item Gent, David
item James, D - WSU
item Wright, L - WSU
item Brooks, D - WSU
item Barbour, J - U OF I
item Dreves, A - OSU
item Fisher, G - OSU
item Walton, V - OSU

Submitted to: Journal of Economic Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 21, 2008
Publication Date: January 1, 2009
Citation: Gent, D.H., James, D.G., Wright, L.C., Brooks, D.J., Barbour, J.D., Dreves, A.J., Fisher, G.C., Walton, V.M. 2009. Effects of Powdery Mildew Fungicide Programs on Twospotted Spider Mite (Acari: Tetranychidae), Hop Aphid (Homoptera: Aphididae), and Their Natural Enemies in Hop Yards. Journal of Economic Entomology. 102(1):274-286.

Interpretive Summary: Spider mites, hop aphid, and powdery mildew are important pests affecting hops in the Pacific Northwest. A potential barrier for greater adoption of conservation biological control strategies for spider mites and hop aphid is the extensive use of fungicides for management of powdery mildew. Field studies conducted in experimental plots in Oregon and Washington in 2005 and 2006 quantified the effects of powdery mildew fungicide programs (i.e., sulfur, paraffinic oil, and synthetic fungicides) on arthropod pests and natural enemies on hop. Fungicide treatments significantly affected spider mite populations in all four studies, with a tendency for spider mite populations to increase faster on sulfur and paraffinic oil treated plants compared with water treated plants. Populations of predacious insect were not consistently suppressed or stimulated by treatments in all trials. Based on the results of these studies, powdery mildew fungicide programs that minimize or eliminate applications of sulfur and paraffinic oil would tend to conserve predatory mites and minimize the severity of spider mite outbreaks.

Technical Abstract: Twospotted spider mite, Tetranychus urticae Koch (Acari:Tetranychidae), and hop aphid, Phorodon humuli (Schrank) (Homoptera:Aphidiae), are the most important arthropod pests of hop (Humulus lupulus L.) in the Northern Hemisphere. A potential barrier for greater adoption of conservation biological control strategies for spider mites and hop aphid is the extensive use of fungicides for management of hop powdery mildew, Podosphaera macularis (Wallr.:Fr.) U. Braun & S. Takamatsu. Field studies conducted in experimental plots in Oregon and Washington in 2005 and 2006 quantified the effects of powdery mildew fungicide programs (i.e., sulfur, paraffinic oil, and synthetic fungicides) on arthropod pests and natural enemies on hop. Fungicide treatment significantly affected spider mite populations in all four studies. Multiple applications of sulfur fungicides applied before burr development resulted in 1.4 to 3.3 fold greater spider mite populations during summer. Near the end of the sulfur applications, or after a lag of 20 to 30 days, spider mite populations increased significantly faster on sulfur treated plants compared with water treated plants in three of four experiments. The effect of paraffinic oil on spider mites was mixed, leading to exacerbation of spider mites in Oregon and Washington in 2005, suppression of mites in Oregon in 2006, or no significant effect compared with water in Washington in 2006. Significant relative treatment effects for cone damage from spider mite feeding were detected in Oregon in 2005 for plots treated with sulfur and paraffinic oil compared to water or synthetic fungicides. Mean populations of hop aphids were similar among treatments in Oregon, although sulfur treatment suppressed hop aphid populations in Washington in 2005 and 2006. Populations of individual predacious insect species and cumulative abundance of macropredators were not consistently suppressed or stimulated by treatments in all trials. However, predatory mite abundance in Washington was affected by fungicide treatments, with plots treated with sulfur consistently having 10-fold fewer phytoseiids per leaf compared with the other treatments. Based on the results of these studies, powdery mildew fungicide programs that minimize or eliminate applications of sulfur and paraffinic oil would tend to conserve predatory mites and minimize the severity of spider mite outbreaks. However, mechanisms other than direct or indirect toxicity to phytoseiid mites likely are associated with exacerbation of spider mite outbreaks on hop.

Last Modified: 7/28/2014
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