We undertook studies in three commercial vineyards to answer questions about the
Willamette mite on grapes, including how cultural practices such as sulfur and water
management affect mite population density, as well as the economic injury level of the
mite. At two of the field sites (Santa Maria and Paso Robles), which we used for the
sulfur and irrigation studies, Willamette mite density was too low to make any
evaluations. However, at the third site (Shandon) the mite population reached a high
enough density for us to evaluate economic injury.
At the Shandon site we worked in a block of Chardonnay and attempted to regulate mite
density at pre-set static levels through selective use of a miticide (Nexter® [pyridaben]).
These levels were: 0 mites/leaf (control), 10 mites/leaf, 20 mites/leaf and >40 mites/leaf
(unregulated) did not reach a treatable threshold until early August. The control was
sprayed once a week with Nexter® for the next four weeks, and the 10 and 20 mites/leaf
treatments were sprayed at low rates for the next two weeks.
Mite density peaked at over 120 mites/leaf in the unregulated treatment, but reached 9.5
mites/leaf in the control. Overall average mites/leaf were fairly close to the pre-set
categories, at 3.1, 13.5, 23.3 and 93.13 mites/leaf for the 0, 10, 20 and >40 mites/leaf
treatments, respectively. However, in terms of cumulative mite-days (a mite-day is
equivalent to one mite/leaf for one day), there was no significant difference among the 0,
10 and 20 mite/leaf treatments. The >40/leaf treatment had 3900 cumulative mite-days,
compared to just over 500 for the average of the 0, 10 and 20 mite/leaf treatments. None
of the measured vine parameters (berry weight, °Brix, pH, yield and pruning weight)
yielded significant differences among the treatments. The only wine parameter that was
close to significant was malate, which was about 15%higher in the >40 treatment than
Previous studies have shown a negative effect on either berry sugar accumulation or yield
with Zinfandel that had higher peak mite density or mite-days than in our study. This
could be due to differences in varietal susceptibility or timing of infestation. Anecdotal
evidence has it that Zinfandel is particularly susceptible to Willamette mite. Perhaps
Chardonnay can tolerate much higher pressure. Or, the increase in mite density may have
occurred too late in our study to affect vine performance. Another possibility is that there
were differences in vine canopies between our study and previous studies that were not
accounted for. Whereas we found little difference in wine among the treatments, we will
continue the analysis in the form of an organoleptic evaluation.