A summary of studies on Willamette mite damage to grape vines.
By Devin Carroll - 2005
Main points:
Researchers typically found reductions in sugar content (% sugar, degrees Brix, or specific gravity). But Flaharty found no specific gravity difference in Thompson raisins, and Battig found no reduction in sugar.
Sugar reduction was typically 11-13%, as high as 25%.
Reduction in sugar began to occur somewhere in the 30-50 peak mites per leaf range.
Reduced sugar may be related to delayed maturity.
No researcher found yield reductions in the first year.
Welter studied two years of damage followed by two years of clean vines. He found yield reductions of 18% in the second year of damage and 15% in the first year of recovery. In other words, heavy Willamette one year may cause yield reductions the following year.
It’s possible that vigorous vines such as Thompsons can tolerate more Willamette than weaker vines.
Results:
Frazier & Smith 1946: fewer berries, smaller berries, apparent delayed maturity.
When Willamette populations were high.
Flaharty & Huffaker 1970: Found no effect in berry weight, specific gravity, raisin yield.
Thompson raisins in Shafter. Max 92 per leaf in July.
Note: Carbaryl application harmful to yield.
McNally & Farnham 1985: Found less sugar. No effect on cluster weight or yield.
Zinfandel in Plymouth & Chenin Blanc in Clements and Lodi.
Sugar reductions:
Zinfandel 1983: 25% reduction. (23.6% at 0 mites, 17.6% at max 78 per leaf, Sept 21)
Zinfandel 1984: 11 % reduction.
0 mites per leaf 22.9b % sugar (Sept 4) 27.3b (Sept 20)
14 21.1ab 26.3ab
51 20.1a 24.0a
175 20.3a 24.9ab
Chenin Blanc Lodi (August infestation) 11% reduction.
0 21.6b (Sept 4)
45 19.2a
Chenin Blanc Clements (June-July infestation) 13% reduction.
0 18.7b (Aug 16)
37 16.2a
Welter et al. 1989a: First year, reduced soluble solids
236 mite-days or max 12 per leaf 20.0a Brix (Aug 21) 22.9a (Sep 1)
465 20 20.1a 23.0a
716 29 19.1ab 22.2ab
1667 57 18.8b 21.7b
First year no effect on fruit weight, number of clusters, pruning weight, trunk girth.
Second year, reduced total fruit weight 17.9%, berry size 8%, number fruit clusters.
Second year, no effect on soluble solids, pruning weight, shoot length.
Max 1106 mite days (37 per leaf) or 930 mite days (48 per leaf)
Zinfandel in “dry land foothill” (Amador?)
Welter et al. 1991: (recovery phase after two years damage, see 1989a)
First year of recovery: Total fruit weight reduced 14.9%.
No difference number of clusters, soluble solids, berry weight.
10.5% non-significant reduction in pruning weight.
Second year of recovery: no reductions.
Zinfandel in “dry land foothill” (Amador?)
Welter et al. 1989b: mites decreased photosynthesis and stomatal conductance.
Zinfandel in Amador Co. Effects seen in 1000-6000 mite day range.
Note: Omite did not harm measured parameters.
Battig & Costello 2004: slightly higher malate.
No effect on yield, sugar, pH, wine acidity, wine phenols.
Chardonnay. Mite days up to 3500, max 126 per leaf, late infestation.
San Luis Obispo?
Papers:
Battig, Jonathan and Michael J. Costello, 2004. No Damage Found from Late Season Willamette Mite Infestation on Chardonnay. Horticulture and Crop Science Department, Cal Poly State University, San Luis Obispo. Poster Board presentation AAIE Conference.
Flaherty, D. L., & C. B. Huffaker. 1970. Biological Control of Pacific mites and Willamette mites in San Joaquin Valley vineyards. I. Role of Metaseiulus occidentalis. II. Influence of dispersion patterns of Metaseiulus occidentalis. Hilgardia 40(10): 267-330.
Flaherty, D. L. 1969. Ecosystem trophic complexity and Willamette mite, Eotetranychus willamettei Ewing, densities. Ecology 50:911-916.
Flaherty, D. L., C. L. Lynn, F. L. Jensen, and D. A. Luvisi, 1969. Ecology and integrated control of spider mites in San Joaquin vineyards. Calif. Agric. 23(4):11.
Frazier, N. W., and L. M. Smith. 1946. The Willamette mite on grapes. Hilgardia 17: 191-196.
McNally, P. S. & D. Farnham. 1985. Effects of Willamette mite (Acari: Tetranychidae) on Chenin Blanc and Zinfandel grape varieties. J. Econ. Entomol. 80: 947-950.
Found reductions in grape quality. (sugar content).
Welter, Stephen C., Phillip S. McNally, and Delbert S. Farnham. 1989. Effect of Willamette Mite (Acari: Tetranychidae) on grape productivity and quality: a reappraisal. Environ. Entomol. 18(3): 408-411.
Welter, S. C., D. S. Farnham, P. S. McNally, and R. Freeman. 1989. Effect of Willamette mite and Pacific spider mite (Acari: Tetranychidae) on grape photosynthesis and stomatal conductance. Environ. Entomol. 18(6): 953-957.
Welter, S. C., R. Freeman, and D. S. Farnham. 1991. Recovery of ‘Zinfandel’ grapevines from feeding damage by Willamette spider mite (Acari: Tetranychidae): Implications for economic injury level studies in perennial crops. Environ. Entomol. 20(1): 104-109.