Effect of fungicide timing on twospotted spider mite and hop aphid management in hops

Twospotted spider mite, Tetranychus urticae Koch and hop aphid, Phorodon humuli (Schrank) are major pests of hop. Pesticides may disrupt conservation biological control of spider mites and hop aphid. In previous research, sulfur and stylet oil applied for management of hop powdery mildew (Podosphaera macularis) were shown to be toxic to predatory phytoseiid mites and exacerbate spider mite outbreaks in field studies in Oregon and Washington. Studies were conducted during 2007-2009 in Oregon and Washington hop yards to evaluate the effect of the timing of sulfur fungicide applications on T. urticae and P. humuli and their predators. In Oregon, late-season applications of sulfur tended to exacerbate spider mite outbreaks, particularly in the upper canopy. Treatment differences in the Washington trials were not as clear, although there was a trend for greater numbers of spider mites in plots treated with sulfur later in the season. Aphid abundance was not consistently affected by fungicide treatments. Relatively few phytoseiid mites were observed in all years in both states; although reduced numbers were observed in the late-season sulfur treatment. Consistent differences in abundance of predatory insects between treatments were not detected. The mechanisms associated with spider mite outbreaks in relation to sulfur applications remain unclear, although it is clear that toxicity to predatory mites is not the only factor involved. Spider mite outbreaks appeared to be induced by sulfur in the near absence of predatory mites. This work provides evidence that biological control could be enhanced by avoiding the use of late season sulfur.