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

Abstract

Twospotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae), and hop aphid, Phorodon humuli (Schrank) (Hemiptera: Aphididae), 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 1.4-3.3-fold greater spider mite populations during summer. Near the cessation of the sulfur applications, or after a lag of 20-30 d, 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 varied, leading to exacerbation of spider mites in Oregon and Washington in 2005, suppression of mites in Oregon in 2006, and no significant effect compared with water in Washington in 2006. Significant relative treatment effects for cone damage due to spider mite feeding were detected in Oregon in 2005 in plots treated with sulfur and paraffinic oil compared with water and 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.

Organic farming and host density affect parasitism rates of tortricid moths in vineyards

“…These products are known to affect arthropods in general and natural enemies in particular so that could explain lower parasitism rates in organic vineyards (Nash et al, 2010). For instance, several studies have shown the negative effect of sulphur on a wide range of natural enemies including parasitoids, predatory mites and spiders (Gent et al, 2009;Martinson et al, 2001;Nash et al, 2010). Our findings suggest the need for a more detailed description of farming systems to provide a more mechanistic understanding of the way farming systems shape ecological processes such as biological control of pests (Puech et al, 2014;Rusch et al, 2011).…”

Evaluation of airborne methyl salicylate for improved conservation biological control of two-spotted spider mite and hop aphid in Oregon hop yards

“…Commercial deployment of MeSA dispensers could potentially replace one or more miticide applications, therefore we tested a lower rate (123/ha) that is near the typical cost of a single miticide application (approximately $120US/ha). Deployment in both years was timed to coincide with the natural occurrence of pest species, and corresponded with known activity of natural enemies in Oregon (Gent et al 2009). Cooperating growers were asked to avoid using broad-spectrum insecticides to conserve natural enemies, although given the practicalities of commercial crop production this request was not followed in all cases.…”

Suppression of Hop Looper (Lepidoptera: Noctuidae) by the Fungicide Pyraclostrobin

“…Management required to minimize crop damage from powdery mildew involves the application of multiple fungicide chemistries, each with varying levels of effects on nontarget organisms (James and Coyle 2001;Mahaffee et al 2003;Gent et al 2008Gent et al , 2009Woods et al 2012).…”