Alyssum attracts predator insects into broccoli fields


CORVALLIS - Oregon State University researchers in the Department of Horticulture recently conducted the first research in the Pacific Northwest that documents the effectiveness of planting "insectary" plants to enhance natural enemy populations in a commercial field crop.

The researchers observed an increase of beneficial predators of aphids, in a commercial broccoli field interplanted with alyssum to attract aphid predators, hover flies and parasitic wasps. Green peach aphids and cabbage aphids inflict millions of dollars of damage on broccoli crops in the northwest.

By researching biological control of broccoli pests, the OSU scientists hope that someday growers may be able to save money and reduce the impacts on human health and the environment by reducing the use of chemical pesticides.

"Many broccoli growers spray chemical insecticides to combat these aphids," explained Micaela Colley, a recent M.S. graduate of the OSU Department of Horticulture. "Natural predators may prevent a pest outbreak above the economic threshold, or reduce them so a 'softer' insecticide regime may be effective."

Colley, working with John Luna, assistant professor of horticulture at OSU, conducted her experiments on Stahlbush Island Farm near Corvallis in 1997. She compared the relative number of hover fly predators on pure broccoli stands to those interplanted with patches of alyssum or cilantro, known attractants of hover flies. She also looked at aphid populations in stands of broccoli only compared with interplanted broccoli.

Hover fly adults are known to feed on the pollen of cilantro and alyssum and other flowers. Many species of hover fly larvae feed on aphids. Scientific evidence that the planting of flowers attracts hover fly adults exists for other regions including the United Kingdom, New Zealand and California, said Colley.

"We focused on hover flies in this study because they have the potential to control aphids," explained Colley. "The larvae of many hover fly species are voracious aphid feeders. They feed specifically on aphids, whereas a lot of other predators are more generalists, feeding on many types of insects."

Colley and Luna found alyssum to be the most effective attractant to adult hover flies in their experiments. They found more adult hover flies and hover fly eggs in plots with alyssum than in the plots or plots interplanted with cilantro. Also, parasitism of cabbage aphids by a tiny parasitic wasp was nearly doubled in alyssum plantings compared to the pure broccoli stands. But they found no statistically significant differences in aphid numbers between the pure broccoli plantings and the plantings with alyssum or cilantro.

"Hover flies lay eggs in response to the presence of aphids," said Colley. "It was a light aphid year. Aphids appeared late in the season, and hence, so did the hover flies. I think that the timing of predation by the hover flies was too late to result in measurable differences. The timing might be different in another year. If there were more aphids earlier, we may have gotten a more measurable hover fly response."

"Insectary plantings aren't a 'silver bullet,'" stressed Colley. "There isn't an instant result like with using a pesticide. You don't measure the effects the next day. Biological control affects a whole cycle of life over time. It's a long-term solution."

The research still requires several more years of work before specific recommendations can be made to growers wishing to use this approach to pest management, stressed Luna.

"Like with most new research projects, we have more questions than answers," said Luna.

Broccoli grower Bill Chambers, owner of Stahlbush Island Farm and collaborator with Colley and Luna, strongly believes in the intercropping concept.

"We think intercropping to attract beneficial insects makes a tremendous amount of sense," said Chambers. "The amount we have spent on interplanting has been minimal compared with the amount we would have spent on increased use of pesticides. And we are enhancing biological diversity."