Glenn C. Fisher

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 signifi-cantly 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.

Spring emergence and seasonal flight of Delia radicum L. (Diptera: Anthomyiidae) in Western Oregon

Abstract Field research was conducted to describe and characterize spring emergence and seasonal flight activity of the cabbage maggot, Delia radicum L. (Diptera: Anthomyiidae), in relationship to degree-day accumulations. Turnip and rutabaga fields were monitored in the northern Willamette Valley in western Oregon from 2001 through 2004. Spring emergence from overwintering puparia was monitored using emergence cages. A bimodal spring emergence pattern was observed, with ≈70% of the spring population emerging in an early peak in late March, 2 mo before a later peak near the end of May. The mean degree-day accumulations at 10, 50, and 95% of spring emergence using a lower and upper developmental threshold of 4.3 and 30°C beginning 1 January had corresponding degree-day values of 200 ± 50.2 (8 March), 330 ± 22.2 (4 April), and 762 ± 60.1 (28 May), respectively. Seasonal flight activity was monitored using yellow water traps. Spring flight patterns mirrored the bimodal emergence patterns but with a delay of 3 d to 5 wk between emergence and detection of flies in the water traps. The mean degree-day accumulations recorded from the beginning to the end of spring flight had corresponding degree-day values of 303 ± 61.5 (31 March) to 839 ± 51.9 (4 June). Fly activity was lower over the summer from the beginning of June until the end of August (2,138 ± 82.3 DD). A fall flush of activity was observed each year beginning in late August to early September and extending through October (2,860 ± 170.6 DD).

Population Density and Phenology of Tetranychus urticae (Acari: Tetranychidae) in Hop is Linked to the Timing of Sulfur Applications

ABSTRACT The twospotted spider mite, Tetranychus urticae Koch, is a worldwide pest of numerous agronomic and horticultural plants. Sulfur fungicides are known to induce outbreaks of this pest on several crops, although mechanisms associated with sulfur-induced mite outbreaks are largely unknown. Studies were conducted during 2007–2009 in Oregon and Washington hop yards to evaluate the effect of timing of sulfur applications on T. urticae and key predators. In both regions, applications of sulfur made relatively late in the growing season (mid-June to mid-July) were associated with the greatest exacerbation of spider mite outbreaks, particularly in the upper canopy of the crop. The severity of mite outbreaks was closely associated with sulfur applications made during a relatively narrow time period coincident with the early exponential phase of spider mite increase and rapid host growth. A nonlinear model relating mean cumulative mite days during the time of sulfur sprays to the percent increase in total cumulative mite days (standardized to a nontreated plot) explained 58% of the variability observed in increased spider mite severity related to sulfur spray timing. Spatial patterns of spider mites in the Oregon plots indicated similar dispersal of motile stages of spider mites among leaves treated with sulfur versus nontreated leaves; however, in two of three years, eggs were less aggregated on leaves of sulfur-treated plants, pointing to enhanced dispersal. Apart from one experiment in Washington, relatively few predatory mites were observed during the course of these studies, and sulfur-induced mite outbreaks generally occurred irrespective of predatory mite abundance. Collectively, these studies indicate sulfur induces mite outbreaks through direct or indirect effects on T. urticae, mostly independent of predatory mite abundance or toxicity to these predators. Avoidance of exacerbation of spider mite outbreaks by sulfur sprays was achieved by carefully timing applications to periods of low spider mite abundance and slower host development, which is generally early to mid-spring for hop.

Spatial modelling of the influence of corn planting and wind blocking features on catch of Helicoverpa zea (Boddie) in pheromone traps and subsequent pest damage

Abstract A spatial model of capture of Helicoverpa zea (Boddie) males in pheromone traps was developed as a UNIX script file running geographical information system commands (GRASS, v. 4.0). This model was developed to explain how spatial patterns of corn planting and wind-blocking features act to modify pheromone-trap-based earworm damage predictions on sweet corn. In the model, we sought to create a modified cumulative moth catch value that correlated more strongly with subsequent pest damage than unmodified cumulative trap catch values. The model looks at daily changes in wind direction and pheromone plume movement, calculates contribution to catch levels for all locations around the trap, and modifies the catch so that it only includes moths within the planting area of interest. We describe this model in detail and show field data results from 2 years.