Influence Of Ground Covers On Vineyard Predators And
Leafhoppers
Michael J. Costello and Kent M. Daane
Introduction
Ecological theory states that vegetational diversity can lower
crop pest populations by enhancing populations of natural enemies.
This theory has been cited for the grape agroecosystem with respect
to predators of the variegated leafhopper, Erythroneura variabilis,
an important insect pest in San Joaquin Valley vineyards. Several
hypotheses exist which could explain higher numbers of predators
on grapes where cover crops are present: 1) the cover crop provides
additional habitat, 2) the cover crop alters that microclimate
such that predators are favored, and 3) the cover crop provides
alternative prey. Given these hypotheses, we conducted a series
of field experiments to determine the effect of cover crops on
vineyard predators, and ultimately on leafhoppers.
Materials and Methods
Replicated experiments have been conducted at four sites: a table grape vineyard in Reedley (1993-95), a raisin vineyard in Del Rey (1994-95), a wine grape vineyard in Woodbridge (1994-95) and a "juice" vineyard at the Kearney Agricultural Center (1993-94). Summaries of experimental designs are as follows:
1) Table grapes
Cv.: 'Ruby Seedless'. Training: bilateral cordon, spur pruned, 36 in. crossarm. Cover crop: sown in Nov., either purple vetch (Vicia benghalensis) and barley (Hordeum vulgare) or a mixture of common vetch (Vicia sativa), bell beans (Vicia faba) and winter peas (Pisum sativum), which were replaced by a complex of summer resident vegetation consisting primarily of bristletail (Setaria sp.), crabgrass (Digitaria sanguinalis) and cupgrass (Eriocholoa gracilis). Treatments: 1) cover crop disked in late March/early April, 2) cover crop mowed through spring and ensuing resident vegetation mowed through summer. Design: randomized complete block (RCB), five replicates, plot size 1.25 ac.
2) Raisin grapes
Cv.: 'Thompson Seedless'. Training: head, cane pruned, 24" crossarm. Cover crop: purple vetch and barley sown in Nov. Treatments: 1) cover crop disked in mid March, 2) cover crop mowed in March and disked in early June, 3) cover crop mowed in March and June and disked in early July. Design: RCB, four replicates, plot size 1.2 ac.
3) Wine grapes
Cv.: 'Cabernet Sauvignon'. Training: bilateral cordon, spur pruned, 36" crossarm. Cover crop: Perennial native grasses California brome (Bromus carinatus), meadow barley (Hordeum brachyantherum) and blue wildrye (Elymus glaucus). Treatments: 1) cover crop unirrigated, 2) cover crop irrigated weekly, 3) clean cultivation. Design: RCB, four replicates, plot size 0.35 ac.
4) Juice grapes
Cv.: 'Thompson Seedless'. Training: head, cane pruned, 24" crossarm. Cover crop: purple vetch and barley sown in Nov. Treatments: 1) cover crop disked in late March/early April, 2) cover crop mowed through spring and ensuing resident vegetation mowed through summer. Design: RCB, six replicates, plot size 2500 ft2.
Sampling for predators was conducted monthly by shaking vine vegetation
onto a ground cloth or into a cloth funnel, and by vacuuming cover
crop/ground cover vegetation. Leafhopper populations were sampled
each generation by counting individuals on 30 leaves per plot.
Temperature and relative humidity in the vines were monitored
for at least five days every month with hygrothermographs. Average
grape berry weight and soluble solids were measured at harvest.
Results And Discussion
Spiders were by far the dominant predator collected in the grapevines. For example, they comprised over 95% of the predators sampled in 1995. The most common spider species were in the families Clubionidae (sac spiders), Corinnidae (antmimic spiders), Theridiidae (cobweb weavers), Agelinidae (funnel weavers), Salticidae (jumping spiders) and Oxyopidae (lynx spiders). At the table grape, wine grape and juice grape sites, no pattern of spider abundance was found with respect to ground covers in any of the study years, and there were no season-wide significant treatment differences (ANOVA, P>0.05). However, significant differences for certain spider species on certain dates could be found. For example, in the 1995 table grape study, early-season lynx spider populations were higher in cultivated plots but in May and June orb weavers were more abundant with ground covers. The only time we found a treatment effect of cover crops was at the raisin grape site, where spider density in 1995 was significantly higher in plots with cover crop through early summer in June, August and September. At the raisin site cobweb weavers were more abundant in cover crop plots in June and July, and the funnel weaver in June and Sept.
We therefore find little support for the theory of predator enhancement by cover cropping in grape vineyards. Why this is so can be partly explained by a lack of support for hypotheses 1) and 2) (see intro). The spider community in the ground cover was very different than in the grape canopy, consisting primarily of species in the families Lycosidae (wolf spiders), Linyphyiidae (line weaving spiders) and Oxyopidae, which are not abundant in the canopy. This indicates that spider species are partitioned by arboreal vs. herbaceous habitat, and that cover crops do not provide habitat for vine-dwelling spiders. We also found little difference in microclimate between plots with ground covers and those clean cultivated. The only possibility remaining for predator enhancement by cover crops is that prey harbored by the cover crop move to the grapevines and increase survivorship/fecundity of canopy spiders. This may be the reason for higher spider numbers at the raisin grape site.
Leafhopper populations were indeed influenced by the presence
of ground covers. At the table grape site in 1993 & 1994,
third generation VLH density was over 30% higher under clean cultivation.
We suggest that VLH nymphal mortality was increased because of
lower host plant quality: ground cover vines had higher grape
berry sugars and lower brush weights in the ground cover treatment
indicate a greater amount of stress on these vines. This higher
stress level may be due to higher water use by the ground cover,
although this remains to be tested.
Cover Crop Research and Education Summaries