INTRODUCTION

The primary objective of this study is to investigate the abundance and diversity of arthropods associated with conventional and organic vineyard practices. The major targets of arthropod sampling include the following four groups: spiders, predatory insects, parasitoids, and Erythroneura leafhoppers.

STUDY AREA AND METHODS

The study is being conducted in a six-acre Barbera vineyard located on the California State University,  Fresno Farm Laboratory and divided into six experimental plots, each approximately one acre in size. The six plots were set up as three replicated blocks, each including an organic and a conventional treatment laid out in an alternating pattern. The Barbera vineyard block was originally established during 1992 and 1993, and two viticultural system treatments (organic and conventional) were first implemented in December 1994. Formal collection of field data began during the 1996 season.

Viticulture practices in the organic treatment included the planting of winter legumes during the first three years to build organic matter in the soil. Only "organically allowed" pesticides are used in the organic plots. Conventional treatment plots receive calendar-based, standard viticulture practices. Resident vegetation is allowed to grow between vine rows during the winter, and synthetic fertilizers and pesticides are applied according to conventional viticulture practices.

Four primary types of arthropod sampling techniques are used in this study: 1) leaf counts, 2) pitfall traps, 3) malaise traps, and 4) yellow sticky cards. Although corrugated cardboard bands were secured about vines to sample primarily Trachelas spiders, this method was found to be relatively ineffective at trapping adequate numbers for analysis and was thus discontinued after the 1995 season. Direct observation sampling   of leaf surfaces (i.e., leaf counts) involved 15 leaves per treatment plot at bi-weekly intervals throughout the 1996 season. During 1997, leaf count sampling intensity was increased to 30 leaves per treatment plot examined at weekly intervals.

Pitfall traps consist of plastic containers about 15 cm in diameter and 15 cm in height. The containers are buried so that the upper lip is level with the soil surface. A similar container with ethylene glycol for effective collection and preservation of arthropods is placed   inside the buried container. One trap was set up in each treatment plot, and pitfall trap samples were collected   bi-weekly during the growing season and monthly throughout the rest of the year.

Malaise traps are used primarily for collecting flying insects, especially parasitoids. Each trap consists of a 60-cm square frame of PVC pipe with fabric connected forming a pyramidal shape. A funnel-shaped chamber (modified two-liter soda bottle) is placed into the peak of the pyramid for collecting parasitoids and other flying insects.

Yellow sticky cards (10x20 cm) were hung from vineyard trellis wires about 30 cm above the grapevine canopy. With sticky material coating both sides of these yellow cards, samples were collected bi-weekly throughout the 1996 season.

RESULTS AND DISCUSSION

Leaf count results for 1996 revealed that variegated leafhopper (Erythroneura variabilis) nymphs predominated dramatically over western grape leafhopper (E. elegantula) nymphs. No significant differences (P>0.05) in variegated leafhopper nymph densities were detected between organic and conventional plots (Fig. 1). Theridion spider densities also were not significantly different between the two treatments during 1996.

In 1997, variegated leafhopper continued to be the clearly predominant Erythroneura species. As the season progressed, variegated leafhopper nymphs started declining around the same time spider densities began to increase slightly (Figs. 2, 3). Theridion spiders were by far the most frequently found spiders using leaf count sampling, with Cheiracanthium a distant second in terms of population density.

Spiders were numerically almost twice as abundant in the organic plots when compared with the conventional plots during 1997. Generally, spider densities increased until September, and then declined later in the season. Imidacloprin (i.e., Provado) insecticide application in the conventional plots resulted in very low leafhopper numbers from July through September. No statistically significant differences between the organic and conventional treatments for any of the arthropod groups sampled in this study were detected during 1997.

Pitfall trap results for 1996 (Fig. 4) indicated that among the three groups of natural enemies being monitored in the study, spiders were the most abundant, followed by predatory insects and then parasitoids. Season total spiders and season total predatory insects were significantly more abundant (P<0.05) in conventional plots than in the organic plots. Although parasitoids were numerically a bit higher in organic plots, the differences were not statistically significantly. It should be noted that pitfall trap sampling preferentially reflects those natural enemies generally found at the vineyard floor level, as opposed to the grapevine canopy zone. Pitfall trap samples for the 1997 season are currently being analyzed in the laboratory.

Malaise trap data collected during 1996 revealed that although parasitoids were numerically higher in organic plots, no clearly significant treatment effects were evident among any of the three natural enemy groups. Most of the parasitoids sampled using malaise traps were adults of the mymarid wasp Anagrus (an Erythroneura leafhopper egg parasitoid), and two distinct Anagrus biotypes were identified in the study.

Parasitoids sampled using yellow sticky cards were once again the most abundant natural enemy group during 1996, followed by predatory insects. However, no statistically significant treatment differences were detected among natural enemies using the sticky card technique. Predominant among the parasitoids was biotype I of Anagrus, which was collected at approximately 20 times greater densities than was Anagrus biotype II (a similar but morphologically distinct parasitoid).

Viticulture data obtained in 1996 indicated that conventional plots had significantly greater (P<0.05) cluster weights and nitrate-nitrogen in petioles, while organic plots showed significantly higher soluble solids and manganese in petioles. However, treatment differences among the 1997 viticulture data were not statistically significant. There was no clear evidence suggesting that variable arthropod numbers found among the two types of treatment plots contributed in any direct manner toward significant differences in overall grape yield during either 1996 or 1997.

CONCLUSIONS

Abundance and diversity of arthropods detected in vineyard treatment plots predictably depends heavily on the specific sampling technique being used for data collection. Arthropod densities associated with the organic and conventional treatment plots did not contribute to significant differences in grape yield, although various significant differences in viticulture data were detected during 1996 (although not in 1997). Results presented in this CATI Research Note reflect an ongoing research project, and results as well as conclusions should be considered preliminary at this time.

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{ CATI , VERC , VERC - Research Publication }

 

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CALIFORNIA AGRICULTURAL TECHNOLOGY INSTITUTE - CATI
College of Agricultural Sciences and Technology
California State University, Fresno