INTRODUCTION

During recent years, a number of San Joaquin Valley winegrape growers have decided to investigate the commercial feasibility of growing certain cultivars which have traditionally been more common in coastal vineyards. Two of these new varietals which have sparked interest among San Joaquin Valley winegrape growers are Cabernet Sauvignon and Chardonnay. Beyond the predominant question of how will such historically uncommon cultivars perform (i.e., yield/quality) under valley climatic conditions, relatively limited research has been conducted into the patterns of arthropod community dynamics (both pest and beneficial species) in such new vineyards.

The broad objective of this research is to determine how patterns of arthropod community dynamics in newly-planted Cabernet Sauvignon and Chardonnay vineyards compare and contrast with arthropod patterns found in a recently grafted Cabernet Sauvignon vineyard and a long-established and ecologically well-balanced Ruby Cabernet vineyard in the San Joaquin Valley.

STUDY AREAS AND METHODS

All four vineyards studied in this research project from 21 June 1997 through 25 January 1998 were located at E&J Gallo Winery's Ripperdan Ranch located in Madera County, approximately 30 miles northwest of Fresno, California. The two newly-planted (i.e., first leaf during 1997) vineyards included 1) a 42-acre Cabernet Sauvignon block (772CS), with 36 acres having been fumigated with methyl bromide, and the remaining  6-acre portion left non-fumigated; and 2) a 100-acre Chardonnay block (791CH, comprised of 64 acres fumigated and a 36-acre non-fumigated section). Areas between vinerows in the fumigated sections of the newly-planted blocks had essentially no cover vegetation, while some resident vegetation persisted in the  non-fumigated portions of these two first-leaf vineyard plantings.

Also studied during 1997 were a 145-acre block of Cabernet Sauvignon grafted over established Grenache vines (781CS), as well as a long-established 40-acre block of Ruby Cabernet (745RC). Row middles in the grafted Cabernet and established Ruby Cabernet blocks had cover crops which were periodically mowed, along with some bermuda grass and climbing weeds on the berms.

Arthropods in all four vineyards were sampled by pitfall traps (three each in grafted Cabernet and established Ruby Cabernet, with four traps each [two in fumigated/two in non-fumigated areas] in the newly-planted Cabernet and Chardonney blocks) set at soil level in the berms and checked at weekly or bi-weekly intervals throughout the season. Leaf counts for leafhopper nymphs (20 leaves examined per vineyard on each sampling date) were also conducted in all four vineyards. Unfortunately, volume in the sparse first-year canopies of the newly-planted vineyards was insufficient throughout 1997 to allow for the 10 canopy shakes per date sampling, which was performed in the two more well-established vineyards.

Spiders, among the most important beneficial arthropods monitored using both the canopy shake and pitfall trap sampling techniques, were ecologically classified into three distinct groups based on their preferred location in the vineyard (i.e., canopy, or floor, or both; see also Table 1 note): Group 1 - ground hunting spiders (mostly runners and ambushers, including wolf spiders, ground spiders, and crab spiders), Group 2 - canopy hunting spiders (ambushers, web/snare users, including funnel weavers, sac spiders, and antmimic spiders), and Group 3 - dwarf spiders found both in canopy and on ground (including cobweb weavers such as Theridion spp. known to prey on leafhopper nymphs and adults in the grapevine canopy.

RESULTS AND DISCUSSION

During the extended 1997 field season, a grand total of more than 1000 spiders were collected in pitfall traps and canopy shake samples from the four vineyards studied at Gallo's Ripperdan Ranch. Overall, the most common spiders collected were wolf spiders (family Lycosidae) and antmimic spiders (Trachelas spp.). In addition, a season grand total of more than 1400 beneficial insects (especially parasitic wasps, carabid beetles, staphylinid beetles, green lacewings, and various predatory hemipterans) were found in the four vineyards.

As might be predicted based upon the substantial ecological contrast of the newly-planted vs. the established vineyards, patterns of spider diversity and abundance were highly variable. It should be noted again that due to limited volume in the canopy of the new vineyards, spider data for those two vineyards came only from pitfall trap samples. On the other hand, both pitfall and shake data for spiders were compiled from the grafted Cabernet and the established Ruby Cabernet vineyards. In general, the two newly-planted vineyards proved to be quite similar with respect to distribution and types of spiders and beneficial insects. For these first-leaf vineyards, the fumigated section of the new Cabernet Sauvignon block had slightly more spiders.

Highest total spider counts, along with lowest leafhopper nymph counts and essentially no omnivorous leafroller larvae were found in the established Ruby Cabernet vineyard. Nearly half of all the canopy hunting spiders collected during 1997 were found in the Ruby Cabernets. A very small relative "peak" of leafhoppers (still under one nymph per leaf) was found during June and July in the Ruby Cabernets, followed by a subsequent peak in spider abundance in early August, particularly  for canopy hunting and dwarf spiders (especially Theridion spp.). The combined activity of these spiders  and other generalist predators in the established Ruby Cabernet vineyard may have been important in reducing leafhopper counts to virtually zero for the remainder of the 1997 season. Somewhat surprisingly, densities of carabid and staphylinid beetles, as well as tiny parasitic wasps which were relatively abundant in other vineyards, were relatively low in the established Ruby Cabernets.

The vineyards at Ripperdan Ranch in general had a wider diversity of spiders and insects of all kinds, as well as a much lower incidence of Erythroneura leafhoppers than did most vineyards monitored in related research projects on the California State University, Fresno Farm Laboratory during 1997. Some spiders and beneficial insects found at Ripperdan are quite uncommon in other vineyards. For example, gnaphosid spiders (prevalent in the established Ruby Cabernets), commonly known as ground spiders, are usually associated with houses and cellars. Although not much is known about their biology and habits, gnaphosids typically spin silken webs as nests under stones and debris.

Among the several types of gnaphosid spiders collected at Ripperdan, Marjorie Moody of Visalia has suggested that a larger and darker species is not a native spider, probably having originated in Italy. Two smaller species from Ripperdan belonging to the genus Micaria are both natives of California, with one having only been reported previously from Fresno and Inyo counties. Gnaphosid spiders tend to prefer cool, damp areas in their habitat, and the dense canopy of the established Ruby Cabernet vineyard at Ripperdan appeared to favor both this group as well as the delicate pholcid spiders.

A noteworthy spider find at Ripperdan during 1997 was a unique spider in the family Philodromidae, Ebo pepinensis, which had not previously been reported from any California vineyard. Known as running crab spiders, philodromids (i.e., Ebo) were found primarily in the newly-planted vineyards at Ripperdan, where the ground is more open and less shaded. Unlike other crab spiders in the closely-related family Thomisidae which lie in wait for prey, Ebo spiders actively seek their prey. Ebo spiders have been described in studies of apple orchards conducted in the eastern United States. Highest densities of this new California vineyard spider were found in the new Chardonnay block in early summer 1997, becoming rare after mid-September. Philodromids were not found in the established Ruby Cabernet block, and an intermediate number was collected from the grafted Cabernets.

Table 1 provides a comprehensive summary of spiders belonging to the three distinct ecological groups as they were collected from all four Ripperdan vineyards during 1997, presented both as season total numbers and as percentages accounted for by each of the three groups (see Table 1 note). Canopy hunting spiders (Group 2, including Trachelas and Cheiracanthium) comprised identical portions of season total catches (i.e., seven percent) for both of the fumigated sections of the two newly-planted vineyards. However, distribution by spider group varied dramatically in the two non-fumigated sections (i.e., 36%/32%/32% for Cabernet Sauvignon compared with 60%/22%/18% for Chardonnay). On the other hand, the pattern of spider group distribution was virtually identical for the grafted Cabernet (27%/49%/24%) and the established Ruby Cabernet (26%/50%/24%).

Season grand total spider distribution combining the four Ripperdan vineyards is presented, with respect to ecological groups, in Fig. 1a. Ground hunters and canopy hunting spiders each comprised 36 percent of the spider season totals, while dwarf spiders accounted for the remaining 28 percent. Pooled spider group data analyzed for each vineyard are presented in Fig. 1b. Combining fumigated and non-fumigated sections for the newly-planted vineyards produces nearly identical percentages of season total spider densities for the Cabernet (22 percent), Chardonnay (21 percent), and grafted Cabernet (21 percent) blocks. The remaining 36 percent of season total spiders was collected from the established Ruby Cabernet vineyard.

With respect to herbivorous pest insects, Erythroneura leafhopper population densities (almost entirely western grape leafhopper E. elegantula as opposed to the typically much more common variegated leafhopper E. variabilis) were consistently low in all four vineyards (i.e., generally averaging less than one nymph per leaf) throughout the 1997 season.

Omnivorous leafroller (OLR) larvae were found primarily in the grafted Cabernet vineyard and in the non-fumigated section (with resident cover crop) of the nearby newly-planted Chardonnay block. OLR life stages ranged from small early instar larvae through pupae and adult moths.  Following harvest (mid-September), OLR densities dropped substantially.  Numerous OLR larvae and pupae were returned to the VERC Entomology Laboratory for rearing out potential parasitoids. This activity yielded an estimate of roughly 50 percent parasitism of OLR larvae/pupae in the Ripperdan vineyards (cf. UC Grape Pest Management Manual suggestion that OLR parasitoids seldom account for more than 10 percent mortality, even when OLR populations are large and damaging).

Patterns of parasitoid recovery from vineyard-collected OLR larvae and pupae (Fig. 2) included the following: 1) tachinid flies accounted for 20 percent of total, emerging from OLR pupae collected in late September or OLR larvae pupating in early October;  2) braconid wasps comprised 35 percent; 3) ichneu-monid wasps caused about 30 percent of total OLR parasitization; and 4) polyembryonic (i.e., producing many parasitoid larvae from one egg laid in a single host) braconids and ichneumonids accounted for about 15 percent of OLR parasitism, occurring most often in early September but not occurring in October or November when the other parasitoids were more common.

An unusually large number of scelionid wasps (tiny parasitoids, most of which were wingless females) was collected in the grafted Cabernet vineyard. Although relatively little is known about the life cycle of these parasitoids, they are generally considered to be beneficial. Scelionid wasps accounted for nearly two-thirds of all beneficial insects found in the grafted Cabernet vineyard; this vineyard also proved to have the highest number of total beneficial insects. Lowest numbers of generalist predators (e.g., carabid beetles) were found in the established Ruby Cabernet vineyard, although this block did show the highest number of parasitic wasps in the family Ichneumonidae (i.e., mostly wingless members of the genus Gelis).

The newly-planted Cabernet Sauvignon vineyard produced twice as many beneficial insects in the fumigated section as in the non-fumigated area. Carabid beetles were most abundant in this vineyard, particularly in the fumigated section. This new Cabernet block also had the highest densities of Anagrus and related mymarid parasitoids.

CONCLUSIONS

General summary observations from the 1997 Ripperdan Project include the following:

1) Spiders and beneficial insects can be quite abundant in established, well-balanced vineyard ecosystems, but they may also reach relatively high densities in non-insecticide treated vineyards during the first-leaf growth season.

2) High numbers of relatively unknown natural enemies may be found in typical San Joaquin Valley vineyards when appropriate care is given to sampling techniques and procedures. Examples of these obscure predators and parasitoids include the previously unreported vineyard philodromid spider Ebo pepinensis, as well as various species of wingless scelionid and ichneumonid wasps.

3) Densities of important insect pests, such as Erythroneura leafhoppers and omnivorous leaf-roller larvae, may remain very low throughout an entire growing season coincident with natural enemy numbers at relatively high levels. For example, field parasitization rates for OLR may at times reach 50% (far above the more commonly reported 10% level).

4) From the relatively limited results of the first season of this ongoing research project, no evidence was collected which suggests any particular arthropod pest problems in the less traditional grape varietals being planted in the San Joaquin Valley.

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