- Fall 1997 "Update" Newsletter Article -
Study results conflict with Brix claims
From CATI Publication #971001
Copyright © 1997. All rights reserved.
To entomology professor Mark Mayse and graduate student Kip Green, it sounded a little too good to be true. Certain resource guides serving the agricultural industry had proclaimed that grape growers could count on fewer problems with sucking insect pests if the Brix levels in the vines were above, or kept above, a certain level.It was suggested that high Brix levels (which meant the sap in the leaves had greater viscosity) made it more difficult for sucking insects such as aphids, spider mites and leafhoppers to draw the sap out of the leaves and stems, whereas lower Brix made the sap more easily extractible.
"Although the theory has been widely promoted, we could find no empirical data in the technical or popular literature supporting the Brix/leafhopper relationship," said Mayse, who teaches and conducts research for Fresno State's Department of Plant Science.
From Mayse and Green’s curiosity about the Brix claims emerged a two-year research project, designed to detect a correlation between Brix levels and leafhopper populations. With Green leading the evidence-gathering phase of the project, the pair established eight experimental vineyard field plots in the Fresno, California area, featuring Barbera, Grenache, Carignane and Thompson Seedless cultivars.
For two summers the researchers collected data on Brix levels and leafhopper populations. The findings, which Mayse already has reported in some journals, "provide no consistent support for the proposed Brix/leafhopper relationship which has been widely promoted," he said. In fact, much of the results show patterns "either in direct opposition or essentially neutral to the original theory."
On the other hand, Mayse is quick to explain that this does not mean that Brix levels have no effect on pest populations. "Most farmers, agricultural consultants, and even academic researchers are beginning to recognize that grapevine nutritional status can definitely affect the population dynamics of leafhoppers and other pests," he said. However, the relationship is evidently more complex and involves more factors than simply leaf sap Brix levels.
More details of Mayse and Green’s study are available in a research bulletin to be published this fall by the California Agricultural Technology Institute. It will be titled "Leaf Sap Brix and Leafhoppers in Vineyards." The publication will be available for viewing in the Research Publications section of the Viticulture and Enology Research Center (VERC) web site. Anticipated completion date is Nov. 20.
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