New Washington State University research shows that flowering cover crops can attract beneficial insects to vineyards, though more work is needed to move the beneficials from the cover crop to the grape canopy.
There have been few studies on the establishment of flowering cover crops in drip-irrigated systems for the Pacific Northwest; however, there is a growing interest in sustainable vineyard practices and increasing the biodiversity of vineyards in Washington State.
A three-year cover crop research project was initiated in 2005 to learn if certain cover crop species were better than others at attracting beneficial insects to the vineyard. In conducting the study, Dr. Mercy Olmstead, WSU viticulture extension specialist, collaborated with WSU entomologists Dr. David James and Dr. Tessa Grasswitz (now with New Mexico State University).
"It’s so important to know what your pests are in order to plant the proper cover crop that will attract those beneficial insects," Olmstead said, sharing preliminary results of the beneficial cover crop research during the Washington Association of Wine Grape Growers convention held in February. Data from 2008 is still being analyzed.
In Washington, primary vineyard pests are thrips, leafhoppers, and grape mealybugs, she said, noting that the beneficials for those pests generally include minute pirate bugs for thrips, parasitic wasps for leafhoppers, and ladybeetles and green lacewings for mealybugs.
Attracting beneficial insects to crops is not new in the Northwest. Many orchardists in Washington have planted hybrid tea roses and raspberries for years to encourage the overwintering of parasitic wasps, she said.
The study compared four cover crop treatments seeded in the fall of 2005 and 2006 in two vineyard sites, one organic and one conventionally managed. Cover crop mixes evaluated were resident vegetation (serving as the control), cereal rye, a flowering mix, and a medic mix.
The flowering mix consisted of California poppy, California bluebell, candy tuft, sweet alyssum, and dwarfing cornflower. The medic mix was supposed to be drought tolerant, but Olmstead said they had great difficulty getting it to establish and eventually dropped it from the study. Organic sweet alyssum, priced at $500 per pound, was considered too expensive to be part of the flowering mix, so it was replaced with flowering buckwheat, based on Grasswitz’s previous work in hops.
"The beneficials need floral nectaries and pollen for their food source, but they also need a certain level of pest populations for food or they’ll go elsewhere," Olmstead said. Sweep nets were used biweekly to sample for insects, and sticky traps placed in the canopy were analyzed biweekly.
In general, pest pressure in the conventional site was low in both years, with the organic site having much higher levels of both Western grape leafhopper (Erythroneura elegantula) and Virginia creeper leafhopper (Erythroneura ziczac). Mite populations were low at both sites in both years.
In 2006, sweep samples of the ground flora showed some significant differences between treatments for populations of some of the more important predatory insects, such as big-eyed bugs, damsel bugs, and minute pirate bugs. Predatory insect densities were generally higher in the flowering mix than in the other three treatments for at least part of the season. However, the differences were not sufficient to cause a significant reduction in leafhopper populations in the plots sown with the flowering mixture.
Preliminary analyses indicate similar results in 2007, with populations of minute pirate bugs and big-eyed bugs again being significantly enhanced in plots sown with flowering annuals (at least in the organic vineyard), but with no demonstrable effects on pest numbers.
Olmstead summarized the research by stating that in both the conventional and organic vines, sweep samples of the flowering cover crops found beneficial insects. "We were able to attract beneficials into the flowering cover crops," she said, but added that a spike in beneficials didn’t quite coincide with the build-up of pest populations, though the results are promising.
"For example, in 2007, we saw for a couple of weeks an increase in beneficials in the flowering mix from our sweep samples, but when we analyzed the corresponding sticky traps, we didn’t find the same insects up in the vine," Olmstead said. "More research is needed to try different cultural practices, like mowing alternate rows, to see if we can move the beneficials up into the canopy where we want them." She concluded that entomological studies are difficult because they need to be long term.
Larger plots and multiyear build-up of beneficials will likely show pest suppression associated with increased numbers of beneficials. The scientists hope to include in future research native Washington plants to see what beneficial insect species they attract.