We undertook studies in three commercial vineyards to answer questions about Willamette mite (Eotetranychus willametti) on grapes, including how cultural practices such as sulfur and water management affect mite population density, as well as the economic injury level (EIL) of the mite. At the field site which we used for the irrigation studies (Steinbeck-Paso Robles), Willamette mite density was too low to make any evaluations. The sulfur and economic injury level studies took place at the Shandon site (San Juan Ranch, Fillipponi and Thompson).
For the EIL study, we worked in a block of Chardonnay and attempted to regulate mite density in four categories, control, 10 mites/leaf, 30 mites/leaf and >40 mites/leaf, representing low, mid-low, mid-high and high densities through selective use of a miticide (Nexter® [pyridaben]). These levels ultimately corresponded to <1000 mite days (MD)/leaf, ca. 2000 MD/leaf, ca. 4000 MD/leaf and ca. 8000 MD/leaf (a mite-day is equivalent to one mite/leaf for one day). Treatment vines were allowed to reach the threshold and were subsequently sprayed with Nexter®.
For the sulfur study, we worked in a block of Chardonnay which was treated in early spring twice with wettable sulfur, and subsequently with DMI fungicides (Rally®), but not with sulfur dust. Our treatments were sulfur dust and attapulgite clay applied weekly, compared to an untreated control.
In the EIL study, end of the season mite densities were 725 MD, 1607 MD, 3478 MD and 7760 MD for categories 1-4, respectively. Mite density peaked at 65 mites/leaf, 75 mites/leaf, 105 mites/leaf and 192 mites/leaf in categories 1-4, respectively. Overall average mites/leaf were 4.8, 10.2, 26.9 and 91.8 for categories 1-4, respectively. The only measured vine parameters that were different among treatments was berry weight.