Field Investigation of Grapeleaf Sap Brix Levels and
Leafhopper Populations in San Joaquin Valley Vineyards

by
Mark A. Mayse  and  R. Green, and William A. O'Keefe

CATI Publication #971101
© copyright April 1997, all rights reserved

INTRODUCTION

Advantages of practicing integrated pest management (IPM) with a "Plant Positive" or "Plant Health" perspective rather than a "Pest Negative" approach are becoming increasingly clear. The Plant Positive perspective emphasizes cultural practices (e.g., fertilizer and water management) which produce optimal crop yields without creating "overgrowing" plants that may attract and/or support higher numbers of insect pests. The more traditional Pest Negative approach to IPM unilaterally focuses on the goal of reducing pest population densities, typically with little regard to crop production factors that may have stimulated development of pest outbreaks in the first place.

Adopting a Plant Positive perspective allows IPM decision-makers to approach pest population outbreaks with an emphasis on dealing with their basic causes, rather than simply treating the symptoms (i.e., high pest densities). On the other hand, we must be very cautious in presuming that simple cause and effect relationships can be delineated from single, although easily monitored, parameters in our complex agricultural ecosystems.

Recent research in the IPM Program at California State University, Fresno (CSUF) on the effects of fertilizer type and amount on leafhopper population dynamics in vineyards reflects a strong commitment to a "Plant Health" approach to IPM (Mayse et al. 1991). Results of a three-year field project showed that Erythroneura (i.e., variegated and western grape) leafhopper densities were significantly greater on grapevines fertilized with synthetic ammonium nitrate compared with vines receiving compost fertilizer (Roy 1991, Garcia 1993).

In another field study of the relationship between plant nutrient levels and insect herbivores conducted in a completely different type of ecosystem, Clancy (1992) showed that for the western spruce budworm (Choristoneura occidentalis Freeman) feeding on foliage of Douglas fir trees, the resistant trees had significantly higher levels of sugars in new growth foliage than did the nearby susceptible trees.

At CSUF we have further pursued the Plant Health line of research by investigating effects of vineyard irrigation practices on Erythroneura leafhopper populations. Higher leafhopper densities were found on vines receiving greater amounts of irrigation water (Trueta 1993). These results for cultural effects in IPM have been corroborated through similar research conducted by various University of California scientists (e.g., Drs. Kent Daane, Larry Williams, Ted Wilson, and Paul Trichilo).

While developing plans for our vineyard irrigation/leafhopper studies in 1991, we learned that certain organic agriculture consultants in California were promoting the use of Brix readings from leaf sap as a way to predict potential leafhopper problems. According to the information distributed, high Brix levels mean the plant is well-protected against sucking insects, while lower Brix levels indicate vulnerability to leafhopper attack (McMullin 1994).

A widely-circulated catalog (1993-94 Peaceful Valley Farm Supply Main Catalog, p. 54) stated that "a Brix reading above 12 indicates plant resistance to sucking insects." In the Natural Pest Management section of the catalog (PVFS 1993, p. 68), the reader was further assured that "You can specifically discourage sucking insects in certain crops by building up plant Brix (sugar content) to the point that the plant sap is too thick for the insects to extract easily."

Additional enthusiasm for the Brix/leafhopper idea emanated from a consultant's information sheet which included tips for using the refractometer to measure plant Brix. In this how-to sheet, it was stated that "There is a definite and significant relationship between the plant Brix and it's [sic] attractiveness to sucking insects.... This phenomena [sic] has been personally verified for the following insects: aphids, spider mites, leafhoppers, [along with five other arthropods]."

Naturally, the allegedly predictive Brix/leafhopper relationship being promoted by the various agricultural consultants would be quite intriguing to anyone supporting the holistic Plant Health perspective. Thus, as part of our CSUF vineyard irrigation/leafhopper study in 1992, M.S. student Antonio Trias Trueta gathered data for grapeleaf sap Brix levels as well as for Erythroneura leafhopper populations (Trueta 1993).

It is widely-known among California agriculturalists that Brix values are generally obtained by using a hand-held refractometer which measures soluble solids within a liquid medium (e.g., plant sap). The refractometer is an instrument used extensively in the grape industry where Brix readings for grape berries give a representative value of the sugar content of the grapes. This simple yet critically valuable test enables growers, packers, and processors to accurately monitor fruit maturity and to plan the timing of harvest operations accordingly. However, Sugiura and Tomana (1983) showed that in the use of a refractometer, concentrations of other sap components including organic acids, amino acids, minerals, pectins, oils, and flavinoids will also affect measured Brix levels.

In his M.S. thesis research, Trueta (1993) found that although grapeleaf sap Brix readings stayed well below 12 throughout the season, leafhopper nymph densities nevertheless remained low ( < 5 per leaf) during the entire study. It thus became clear that it was possible to have relatively low Brix levels in leaf sap, but to also find correspondingly low numbers of Erythroneura leafhoppers in the vineyard.

Preliminary findings such as these appeared to contradict the proposed grapeleaf sap Brix/leafhopper theory. There appeared to be virtually no empirical evidence to support such a predictive Brix/leafhopper relationship, although a number of farmers and agricultural consultants have described corroborating anecdotal observations. Thus, in 1994 we initiated a two-year field study of the potential for an inverse, predictive relationship between grapeleaf sap Brix levels and Erythroneura leafhopper densities in San Joaquin Valley vineyards. Our primary objective was to rigorously evaluate the contention that high Brix levels in leaf sap (greater than 12) would lead to low leafhopper densities, and that low Brix values would be associated with high leafhopper numbers in vineyards.

STUDY AREAS AND METHODS

A total of eight vineyard field sites, including four different grape cultivars, were sampled for leaf sap Brix levels and leafhopper densities from mid-June to early October in 1994 and 1995. Vineyards studied in 1994 included the following (20+ acres each):

1. Barbera (organic) near Ripperdan in Madera County;
2. Grenache (organic) near Ripperdan in Madera County;
3. Carignane in Madera County; and
4. Barbera in Fresno County. The 1995 vineyard field sites (all in Fresno County) included:
5. CSUF organic (six acres);
6. CSUF conventional (six acres);
7. Thompson seedless (organic, 20 acres) at Soghomonian Ranch I; and
8. Thompson seedless (organic, 20 acres) at Soghomonian Ranch II.

Thus, five of the eight vineyard sites in this study were farmed using organic production techniques.

For leafhopper data collection, standard direct observation leaf sampling for detecting Erythroneura nymphal numbers on 20 leaves per experimental plot (e.g., vines at edge vs. interior of vineyard) on each date was conducted along with the Brix data collection. In 1994, leafhoppers were collected from the same vines which were used to gather Brix data, and during 1995 Brix data were taken from the actual leaves used for leafhopper nymph sampling.

Flaherty et al. (1992) indicated that highest first generation leafhopper nymph counts can be found on the second through sixth basal leaves, and that highest counts from subsequent generations can be found on mid-shoot leaves. Nevertheless, though second and third generation nymphs did begin to populate mid-shoot leaves, the greatest leafhopper nymph densities remained on the second through sixth basal leaves throughout most of this study. Therefore, it should be noted that Erythroneura leafhopper nymphs in this study were typically counted from older leaves (i.e., second through sixth basal).

Although leafhopper nymphs (small = 2nd-3rd instar, large = 4th-5th instar) were distinguished by species (i.e., variegated leafhopper Erythroneura variabilis and western grape leafhopper E. elegantula), leafhopper data are combined here for simplicity of analytical comparison.

Brix data were collected using temperature-compensating refractometers to evaluate soluble solids in plant tissues. The consultant's information sheet recommended that newly mature leaves or petioles should be sampled, indicating further that many growers take Brix samples between 1000 and 1200 hrs.

A hand-held, temperature-compensating refractometer was used to measure grapeleaf sap Brix values in the field. A vise-grip type sap extractor was used to obtain adequate quantities of leaf juice from each 10-leaf replication. The sap extractor used throughout most of the study consisted of two flat squeeze plates (7x5 cm) mounted to a vise-grip tool. At the CSUF vineyard site (1995), a smaller sap extractor (6.5 x 2.5 cm squeeze plates) purchased from Peaceful Valley Farm Supply was used. Throughout the study, sampled leaves were stacked vertically and aligned along leaf venation (petioles removed), then rolled into a cigar-like configuration. The roll was then folded in half and placed between extractor plates with the "bend" exposed at the distal end of the extractor (approx. 3 cm). The bend of the leaf roll was cut with a sharp knife along the edge of the extractor plates to expose the interior of the leaf tissue. Sufficient pressure was exerted to force sap from the sample, which was then quickly dabbed onto the refractometer sample plate.

Brix values for the leaf sap samples were determined and then recorded. During peak heat periods of the day (not uncommonly 38o C), leaf sap extraction often became arduous, requiring multiple squeezes to provide enough sap for measurement. Although Brix levels were normally determined directly in the field, on occasion leaves were kept cool on "blue ice" (approx. 10o C) for two or three hours prior to sample measurement.

Among the numerous Brix-related variables investigated in this study were the following: leaf blade vs. petiole sap; new, mid-shoot, and old leaves; edge (outermost five vines) vs. interior of vineyard (20+ vines in from edge); and time of day for Brix sampling (0600-0800, 1000-1200, 1400-1600, and 1800-2000 hrs).

Brix and leafhopper sampling were conducted approximately every seven to 10 days, having been scheduled around worker availability and cultural activities such as irrigation. Effort was made to include sufficient sample dates per site to monitor leafhopper densities from first brood to just before grape harvest.

RESULTS AND DISCUSSION

Leaf Blade vs. Petiole
Brix data for plant sap extracted from leaf blades and from petioles in four vineyards during 1994 showed that leaf blades yielded consistently higher (i.e., up to double) values for Brix (Fig. 1). Also, the range between low and high Brix values for leaf blade sap was consistently about twice the low-to-high range of Brix levels detected in petiole sap samples. With few exceptions, petiole Brix levels ranged only from about 5 to 7 degrees throughout the four vineyards sampled in 1994. Thus, our data clearly suggest that the statement in the agricultural consultant's information sheet (Brix/leafhopper theory) telling users that "To standardize the process [of taking refractometer readings] ... sample [either] newly mature leaves or petioles" would be expected to produce substantial variability among sap Brix measurements.

Based on the 1994 petiole results, plant sap samples were taken only from leaf blades during 1995. Hereafter, data presented (both 1994 and 1995) are for leaf blade sap Brix values only. The noteworthy abundance of solutes in leaf blade tissue, along with a lack thereof in petioles, may at least partially be explained by the physiological process of phloem loading in which sugars are concentrated in photosynthetic leaf tissue (Salisbury 1985).

New, Mid-shoot, and Old Leaf Tissues
Although not so distinct a difference as was determined for leaves vs. petioles, Brix levels detected in sap from young leaf tissue were consistently higher than in more mature leaves for both the Cal-Jersey Barbera and the Cal-York Carignane vineyards in 1994 (Fig. 2).

However, differences in Brix levels among leaf types were not as evident at the organically grown Ripperdan Barbera and Grenache vineyard sites (Fig. 3); significant differences were demonstrated only for the Barbera, with older leaves yielding lower Brix values than both new and mid-shoot leaves.

During 1995, comparison of leaf blade sap Brix levels associated with new and old leaves in both the CSUF organic and conventional vineyards revealed only slight variation (Fig. 4). The range of Brix values for both leaf ages was also generally similar at the two 1995 vineyards.

Location in Vineyard
Sampling location in the vineyard (i.e., edge vs. interior) appeared to be a relatively unimportant variable with respect to leafhopper and leaf Brix data. Since Brix values in 1994 were only slightly lower (numerically) in edge vines for two of the four vineyards sampled, Brix and leafhopper samples were taken solely from interior vines during 1995.

Time-of-Day Sampling Effects
Time-of-day sampling interval exerted relatively little impact on leaf sap Brix values in this study. In CSUF vineyard plots, Brix levels were consistently lower during the 0600-0800 hrs sampling period than for the 1000-1200 hrs interval (i.e., for all seven dates across two vineyards, Fig. 5), while at the Soghomonian vineyard sites this pattern was not consistently demonstrated (Fig. 6). The Soghomonian sites were under combination drip and furrow irrigation, while the CSUF vineyards were only flood irrigated. Differences in grape cultivar, age of vines, fertility, soil type, and other cultural practices may have had confounding effects on the comparison of time-of-day Brix measurements.

Although Brix values varied but minimally among time of day intervals during 1995, it should be noted that the physical chore of taking a refractometer reading in the hottest, driest part of the day was rather challenging. Indeed, whenever adequate sap amounts could even be extracted, Brix levels from the harsh 1400-1600 hrs interval were generally similar to data from the other three sampling periods.

Leaf Sap Brix Levels and Erythroneura Leafhopper Densities
Data for leaf sap Brix and Erythroneura leafhopper densities collected during 1994 and 1995 were analyzed and evaluated using several different approaches selected to elucidate any consistent and predictable patterns of relationship. Two of the most promising of these evaluative methods are used here for presentation of the Brix/leafhopper data. The first method simply involves constructing figures with seasonal dates along the X-axis and including Brix levels and leafhopper nymph densities as double Y-axes. Data compiled using this method comprise the upper, or "A" portions of Figures 7-14, with each figure representing a different vineyard field site for a given year.

According to the alleged Brix/leafhopper relationship, one would expect that the Brix and leafhopper lines would demonstrate a roughly inverse complementarity (i.e., the two lines in each A- portion of Figures 7-14 should approximate mirror images). One possible limitation to this technique of data presentation could involve leafhopper generation patterns, although charting data through several months in the various vineyards during this two-year study should minimize the visual challenges posed by such expected population peaks and valleys.

To further enhance the likelihood of detecting any valid predictive relationship between Brix and leafhoppers, an additional method of evaluating the data was selected. This second method for data presentation involves plotting leafhopper nymph densities (Y-axis) against the associated numerically sorted Brix values (X-axis), a technique which factors out the seasonal dates on which the correlative leafhopper and Brix data were collected. The lower, or "B" portions of Figures 7-14 illustrate this second method of data presentation.

The statistical methodology used in computing the average sorted Brix values as presented in this second data array (i.e., part B of each figure) involved grouping and blocking of data coordinates to enable factoring out the seasonal dates. Thus, the specific number of data points in part A of a given figure would not necessarily equate with the number of data points in part B.

Using the second method of data analysis, and based upon the inverse relationship alleged in the Brix/leafhopper theory, one would expect that the regression line formed in the B-portions of the figures would generally run from the UPPER LEFT to the LOWER RIGHT of each figure (i.e., from low Brix/high leafhopper coordinates to high Brix/low leafhopper data points; note that this pattern is roughly approximated in Fig. 8B).

Leafhopper and Brix data collected during 1994 are summarized in Figures 7-10. The pattern most consistent with the Brix/leafhopper theory in 1994 is found in Figure 8 (Ripperdan Grenache vineyard). Correlation analysis data (Table 1) indicate a very weak negative correlation (r = - 0.20) for Brix/leafhopper numbers from the Ripperdan Grenache vineyard. Although the general slope of the line illustrated in Figure 8B appears to support the theory, data in Figure 8A indicate that situations involving both low Brix/low leafhopper and high Brix/high leafhopper numbers were recorded, revealing a direct contradiction of the alleged predictive relationship.

Data for the other three 1994 vineyards (Figures 7, 9, 10) demonstrate essentially the opposite patterns from those predicted by the Brix/leafhopper theory (i.e., slopes in B-portions of these figures showed a general pattern from LOWER LEFT to UPPER RIGHT). Correlation analysis data (Table 1) indicate a moderately positive correlation (r = 0.47) for the Ripperdan Barbera vineyard. Also, a very weak positive correlation was demonstrated for Brix/leafhopper numbers from both the Cal-York Carignane (r = 0.25) and Cal-Jersey Barbera (r = 0.16) vineyard sites.

Data collected from four different vineyard field sites during 1995 are summarized in Figures 11-14. Patterns at the CSUF Barbera organic plots (Fig. 11A) appear to generally fit the Brix/leafhopper concept, although the sorted Brix data (Fig. 11B) reveal that the trend from upper left to lower right was interrupted at both low- and mid-range Brix levels. These inconsistencies are corroborated by the slight upward trend in leafhopper densities during September 1995 which coincided with the highest Brix values of the season (Fig. 11A).

In the CSUF Barbera conventional plots (Fig. 12), virtually all possible combinations of low and high Brix and leafhopper values were demonstrated. Thus the sorted Brix pattern in the conventional plots (Fig. 12B) revealed no clear support for the alleged relationship. The fact that the differential production system treatments (organic vs. conventional plots) at CSUF had been implemented for only two years may be relevant to the apparently high level of variability in patterns of Brix and leafhopper data from those vineyards.

Data for the two organic Thompson seedless vineyards sampled in 1995 showed patterns very similar to each other (Figs. 13, 14). Leafhopper counts generally declined from June to early July, while Brix values peaked in late June but then dropped sharply in early July (Figs. 13A, 14A). Sorted Brix/leafhopper data for the Soghomonian vineyards (Figs. 13B, 14B) provided no support for the Brix/leafhopper theory. In fact, these data are essentially the opposite of the allegedly predictive inverse relationship.

Correlation analysis data for the 1995 vineyards are not included in Table 1 along with the 1994 results because all four of the CSUF and Soghomonian vineyard sites yielded Brix/leafhopper numbers with correlation coefficient values near 0 (e.g., r = 0.08, - 0.09, etc.). Clearly, the 1995 field data provided essentially no support for the alleged predictive Brix/leafhopper relationship.

Overall Comparison of 1994 and 1995 Data
Grapeleaf sap Brix levels for 1994 were most commonly higher during later sample dates, showing a general increase of Brix over time. Furthermore, the highest leafhopper densities from three of the four 1994 vineyards were also associated with the highest Brix values. During 1995, leaf Brix values from CSUF vineyards were similar to the previous year, in that Brix levels showed a general increase over time. At the Soghomonian vineyard sites (1995), this general increase in Brix level over time was not detected, possibly due to fewer sample dates (i.e., a more restricted period of Brix and leafhopper data collection) for those sites.

Differences between 1994 leaf tissue age were significant, with new leaves having the highest Brix values at three of the four vineyard sites. Two of the study sites demonstrated a statistically significant descending gradient of Brix levels from new to mid-shoot to old leaves. Differences between tissue age were not as evident in 1995, with the CSUF vineyard sites showing no significant differences between leaf tissue types. Surprisingly, at one of the Soghomonian vineyards (site 1, 1995), Brix levels for old leaves were significantly higher than those for new leaves.

Time-of-day sampling had little impact on leaf sap Brix levels during the study, although a consistent trend was detected from time-of-day leaf Brix comparisons at the CSUF Barbera vineyard sites. Leaf Brix values taken there during 1995 in every case were lower during the 0600-0800 hrs sample period compared with the 1000-1200 hrs period. On the other hand, time-of-day effects were much less apparent in the Soghomonian vineyards (cv. Thompson seedless). For both the CSUF and Soghomonian vineyards, leaf sap Brix values commonly varied up to four points between sample periods. Although two different field research technicians worked at the CSUF and Soghomonian vineyard sites, considerable effort was made to maintain consistent sampling methodology in order to prevent adding sources of variation.

Plotting the 1994 and 1995 sorted Brix values with leafhopper numbers showed a tremendous amount of variability. A broad range of leafhopper densities was found throughout the range of sampled Brix levels. No significant correlation was evident between sorted leaf sap Brix and leafhopper nymph densities found in any of the eight vineyard data sets. Indeed, very weak positive and negative correlations occurred with nearly equal frequency throughout the study.

CONCLUSIONS

Results compiled during this two-year empirical study of eight San Joaquin Valley vineyards (including five organic operations) provide no consistent support for the claimed predictive relationship between grapeleaf sap Brix levels and leafhopper densities.

In only two of the eight vineyard field sites (Figs. 8, 11) were data patterns even remotely similar to those expected from the theory. More importantly, data patterns for the other six vineyard sites were either in direct opposition (Figs. 7, 9, 10, 14) or essentially neutral (Figs. 12, 13) with respect to the allegedly predictive inverse Brix/leafhopper relationship.

Although most farmers, agricultural consultants, and academic researchers are beginning to recognize that grapevine nutritional status can certainly affect the population dynamics of leafhoppers and other arthropod herbivores, we suggest that available empirical evidence for leaf sap Brix levels alone being useful in reliably predicting Erythroneura leafhopper population changes appears to be less than compelling.

ACKNOWLEDGMENTS

We wish to thank the Organic Farming Research Foundation (Santa Cruz) and the California Agricultural Technology Institute (CSUF) for their generous financial support of this research project. We also sincerely thank Greg Coleman and Joe Soghomonian for permission to conduct research trials in their commercial vineyards.

REFERENCES CITED

• Clancy, K.M. 1992. The role of sugars in western spruce budworm nutritional ecology. Ecol. Ent. 17: 189- 197.

• Flaherty, D.L., L.P. Christensen, W.T. Lanini, J.J. Marois, P.A. Phillips, and L.T. Wilson (eds.). 1992. Grape Pest Management (2nd Ed.). Univ. of Calif. Div. of Agric. and Natural Resources. 400 pp.

• Garcia, F.R. 1993. Effect of cultural practices on Erythroneura leafhoppers on grapes in central California. M.S. Thesis, Calif. State Univ. Fresno. 79 pp.

• Mayse, M.A., W.J. Roltsch, and R.R. Roy. 1991. Effects of nitrogen fertilizer on population dynamics of leafhoppers on grapes. In J.M. Rantz (ed.), Proc. Intl. Symp. on Nitrogen in Grapes and Wine. Amer. Soc. Enol. and Vitic., Davis, CA. pp. 295-299.

• McMullin, E. 1994. From the ground up. California Farmer, April. pp. 1-4.

• Peaceful Valley Farm Supply. 1993. Main Catalog (1993-94): Tools and Supplies for Organic Farming and Gardening. P.O. Box 2209, Grass Valley, CA 95945. 121 pp.

• Roy, R.R. 1991. Influence of grape fertilization on variegated leafhopper population dynamics. M.S. Thesis, Calif. State Univ. Fresno. 51 pp.

• Salisbury, F.B. 1985. Transport in the phloem. In J.C. Carey and H. Humphrey (eds.). Plant Physiology. Wadsworth, Inc. pp. 135-161.

• Sugiura, I.K. and T. Tomana. 1983. Use of refractometer to determine soluble solids of astringent fruits of Japanese persimmon (Diospyros kaki L.). J. Hort. Sci. 58: 241-246.

• Trueta. A.T. 1993. Effects of vineyard irrigation management on population densities of leafhoppers, grapevine productivity, and fruit composition. M.S. Thesis, Calif. State Univ. Fresno. 100 pp.

ABOUT THE AUTHORS:

Mark Mayse is Professor of Entomology in the Department of Plant Science and the Viticulture and Enology Research Center at California State University, Fresno.

Kip Green and William O'Keefe both earned their Master of Science in Plant Science degrees (Integrated Pest Management emphasis) at California State University, Fresno.

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