The California poppy may have potential as a cover crop to attract beneficial insects to eastern Washington vineyards.

The California poppy may have potential as a cover crop to attract beneficial insects to eastern Washington vineyards.

Washington State University scientists have found that flowering cover crops can attract beneficial insects and predators to orchards and vineyards, but more research is needed to determine the optimum flower mixture for the arid climate of eastern Washington.

Rose bushes are now being planted near tree fruit orchards to attract and help overwinter beneficial insects, thanks to U.S. Department of Agriculture research. But the concept of using cover crops as an insectary for biological control has only recently been studied in Washington, according to Dr. Tessa Grasswitz, Washington State University entomologist.

"The idea is to provide a host or alternate prey to the nonpest insects, attracting them early in the spring so populations at the ground level are ready when you need them later," Grasswitz said.

Some parasitic wasps and predator species need both pollen and nectar; some only use pollen. "But generally, they will live longer and lay more eggs if they have access to flowers."

When using flowers as insectaries, it’s important to have continuous flowering from the beginning of spring through the season, she said. A mixture of plant species is needed for continuous blooms. Flowers must be maintained during the season, which may mean that mowing is necessary. Strip mowing of alternate rows will help produce a "checkerboard" effect so that some rows are always blooming.

There are plenty of good bugs that growers want to invite into their fields, including lady beetles, pirate bugs, hover flies, parasitic wasps, lacewings, and predatory mites. Some of these can survive on pollen alone if prey is scarce, according to Grasswitz. Some will overwinter in the soil if they are managed properly during the off season.

"There are also hidden benefits of attracting insects," she said during a cover crop session at a recent wine grape conference. Rove beetles, ground beetles, and lady beetles spend most of their time on the ground during the day, but come up into the plants during the night.

Plant insectaries

Most of the research on plants as insectaries has been done in areas with higher rainfall than eastern Washington. The plants chosen depend on what will survive in your growing area, what seeds are available, and what you are trying to attract, she said.

Buckwheat trials in New Zealand, New York, and California vineyards have shown mixed results in reducing the number of grape leafhoppers, although egg parasitism was increased in the trials. In California, selective mowing of buckwheat helped force the beneficial wasps higher into the canopy, which reduced leafhopper populations.

In another California trial comparing a mix of vetch, barley, weeds, and flowers to bare ground, there was no difference in egg parasitism rates or general predator populations, she noted. However, the trial showed fewer leafhoppers in the cover treatments than in the bare ground treatments.

"In general, cover crops can have a negative impact on leafhopper populations, although the reasons are often unclear," she said. "And, you also have cumulative effects that build year upon year, increasing the resident population of beneficials."

Washington experience

Plant insectary trials have just started in Washington, with one year of data collected on grapes and two years on hops. The grape trial is in a drip-irrigated vineyard. Already, four plant species have been dropped from the trial. Four species showing potential include the drought-tolerant cornflower, buckwheat, California poppy, and California bluebell. Buckwheat can be sensitive to frost, especially late season.

Grasswitz reported that in the hop trial, poppy seeds germinated in October, and the plants survived the winter months. However, the best sowing time is still under study. Mixed results were observed from a fall sowing of poppies that resulted in poor stands, but the spring sowing resulted in a good stand. Reseeding of plants is being studied in the hop trial.

The bluebell, planted for the first time last year, did not achieve impressive establishment in the grape trial, but flowered in May, three weeks earlier than the other species, she said.

Grasswitz has not yet seen an impact on vine growth from the cover crops, but she noted that stands have been poor. More research on vine growth is needed.

"But the cover crops do attract beneficials," she said, noting that six groups of beneficial bugs were found in the hop trial. In hops, where the cover crop stands have been more established than in the grapes, they found lower numbers of spider mites throughout the season.

"So, yes, we can attract beneficials that we have here in Washington," she said. "But it will never be a stand-alone control. We will need to integrate it with other control tools."

The WSU research team will continue to study ways to build native populations of beneficials. Grasswitz encouraged growers to plant a few rows of flowers in their own vineyard and to share what they learn with researchers. •

For more information, contact Tessa Grasswitz at (509) 786-9274 or e-mail her at