Growing & Winemaking
Effects of Beneficial Cover Crop
In the summer of 2007, members of the Central Coast Vineyard Team (CCVT) staff met with David Gates and Caleb Mosley of Ridge Vineyard to discuss the importance, expense and effectiveness of planting a beneficial insect habitat as a vineyard cover crop. Seeds for these blends can cost upward of $20 per pound, which can be extremely costly to integrate into a cover program, yet they have immeasurable potential to aid in balancing pest populations. This discussion raised three questions:
• How do insect populations differ in a block where a beneficial insect blend is planted from a block where it is not?
• How effective is an insecta-flora blend cover crop at different planting densities?
• Do the beneficial insects that the insecta-flora cover attract make their way to the vine?
Research site
The research was implemented in fall 2007 at Ridge’s Monte Bello vineyard in the Santa Cruz Mountains. Winegrapes have been farmed at Monte Bello vineyard since 1886, and it is now home to 120 acres of Cabernet Sauvignon, Merlot, Petit Verdot, Cabernet Franc, Chardonnay and Zinfandel. Soils consist of decomposing Franciscan rock mixed with clay, laid over fractured limestone. The Santa Cruz Mountains have a cool, mountainous climate between the Mediterranean and Maritime zones, with annual rainfall ranging from 15 to 30 inches per year, averaging 22.64 inches annually.
The project area encompassed 7.1 acres on a south-facing slope of Cabernet Sauvignon, Merlot and Petit Verdot planted on Teleki 5C rootstock. Rows in this block are spaced at 8 feet, and vines are spaced at 4 feet and were planted in 1998. The Monte Bello vineyard has limited irrigation abilities, only using water to help establish young vines.
Experimental design
The beneficial habitat study had three treatments: habitat blend planted every fourth row, habitat blend planted every eighth row, and unplanted (Ridge uses a perennial grass cover program) using a Vari-Slice Positive Feed Native Grass Seed Delivery System seeder.
CCVT staff collected insect data monthly from June through August of 2008. Insect counts on the vineyard floor were gathered by pitfall traps: Clear plastic cups containing one ounce of isopropyl alcohol sunk into the soil so the lip of the cup was flush with the top of the soil (see figure 1). Ten pitfall traps were placed in each treatment and collected after a 24-hour period.
Insect counts in the vine canopy were gathered by placing yellow sticky traps within the trellis system (see figure 2). Ten of these traps were placed in each treatment and collected after a 24-hour period. Brian Henriott, a Cal Poly entomology student under the direction of Dr. David Headrick, performed insect identifications (keyed out to family) for both the sticky and pitfall traps.
| Grower discussion and conclusion
We decided to expand these cover cropping efforts to approximately 20 additional acres of our vineyards. The increase in beneficial insects was convincing enough to persuade us to expand, along with the fact that we have never considered some of the plant-feeding insects classified in the study as a threat to the health of the vine or the quality of the fruit. Parasitoid hymenoptera levels in the higher density plantings were encouraging as well. This insect has the potential to parasitize large numbers of malicious insects in vineyards, and is highly mobile and effective. Due to the extremely small size of seed in the habitat blend, we had significant difficulties calibrating the seeder when we began this experiment. In some instances, we prepared a seed bed and then spread the seed by hand to achieve more efficient distribution. Regardless of how the seed was spread, an impressive stand was established, and many of the species retained their blooming flowers late into the growing season. The most impressive feat of this habitat blend was its ability to withstand the intense drought conditions in 2007 and 2008. Our current stand is very full, and some of the alyssum and California poppies have begun to encroach into the adjacent rows. Not only does this cover cropping system provide significant benefits in terms of beneficial insect populations, it also helps to break up the species monoculture within a typical vineyard system. In the long term, a system with more species diversity (whether plant or insect) will stave off pest outbreaks and be less dependent on insecticides. It also significantly beautifies the vineyard. Visitors are always asking why we have flowers planted in the middle of our vineyards. This presents me with a great opportunity to explain our commitment to low input, sustainable farming practices. Our viticultural philosophy with all of our vineyards is to promote a natural balance within the vineyard. Utilizing this beneficial habitat cover crop is another great tool that we have to realize this goal. —David Gates and Caleb Mosley |
Results and discussion
With any management practice, it is important that economic input produces a desired ecological output. Our question was: How much economic input would be necessary to build a beneficial habitat sufficient to alter pest populations? At a planting density of every fourth row, 16% (0.2 acres planted over 1.25 vineyard acres) of the project area was planted with the beneficial habitat blend. With a planting density of every eighth row, only 8% (0.1 acres planted over 1.25 vineyard acres) of the project area was planted with the beneficial habitat blend. With seed costs at $14 per pound and a seeding rate of 20 pounds/acre, there is potential to gain similar effectiveness at a lower cost (saving roughly $28
per vineyard acre).
Above are the results for the pitfall trap and in-vine collection of insects in all three treatments. Beneficial insects found in pitfall traps consisted mainly of beetles (ground, rove, lady, click), spiders (tube, tarantula, wolf) and some small reptiles, including lizards and skinks. Pest species found in pitfall traps include aphids, leafhoppers, bark beetles and ants.
Vineyard floor insect populations showed a response to the three treatments. Beneficial insect populations were greatest when the habitat blend was planted every fourth row, decreased in treatment areas planted every eighth row, and declined further in the unplanted treatment. However, pest species did not follow the same trend. The unplanted area showed to have the greatest insect count for pests, followed by the every fourth row planting density, then every eighth row planting density.
In-vine (yellow sticky trap) beneficial insect species primarily included orb-weaving spiders, parasitic wasps and lacewings. Pests found in vine canopy traps included sharpshooters, mites, leafhoppers, thrips, aphids, whitefly and seedbugs.
Beneficial insect populations in the vine canopy follow the trend: Denser beneficial habitat planting equals a greater number of beneficial insects. Beneficial insect populations were more than double in treatments planted every fourth row than the area planted every eighth row. Also, a greater percentage of the beneficial insects found in the every fourth row treatments were parasitoid hymenoptera. Pest populations in the vine canopy were largely inflated by thrips, which accounted for nearly 90% of total collected pest species.
Gaylene Ewing is the project technician for the Central Coast Vineyard Team. A graduate of Cal Poly with a bachelor of science degree in earth sciences, she interned with CCVT in 2004, became outreach/education coordinator in 2006, and as project technician, is now responsible for design and oversight of replicated field trials and vineyard demonstration sites. To comment on this article, e-mail edit@winesandvines.com.
