CORVALLIS - Many farmers, orchardists and organic gardeners have long been interested in planting flowering plants in and around their crop plants to encourage beneficial insects, including pollinator and "natural enemy" species that prey on crop pest species.
An Oregon State University graduate student in horticulture tested the relative effectiveness of 11 flowering beneficial "insectary" plants in attracting natural predator species of hover flies and parasitic wasps in western Oregon.
"Seed catalogs and gardening books often publish lists of plants that supposedly encourage beneficial insects," explained Micaela Colley, who completed her master's degree in horticulture at OSU in April, with the guidance of John Luna, OSU assistant professor of horticulture. "Most of this information has been anecdotal, or word-of-mouth, not research-based. Our goal was to establish a scientific basis for making recommendations to growers who are interested in using plants to attract beneficial insects."
Colley and Luna planted 11 species of flowering insectary plants, four types of perennials and seven annuals, at the OSU Vegetable Research Farm in Corvallis and at two organic farms in the area. Perennials studied included: basket of gold, yarrow, Korean licorice mint and fennel. Annual species included alyssum, calendula, cilantro, mustard, Phacelia, marigold and buckwheat. All plants are reputed to provide pollen and nectar to natural enemies of crop pests.
Colley chose to study the relative attractiveness of her plantings to natural predators of aphids - hover flies and tiny parasitoid wasps. Hover fly larvae eat aphids and the wasps lay their eggs in aphids, thereby killing them. She and her OSU colleagues collected insect samples from her study plots of each type of flower through the summer of 1997 and counted relative numbers and types of hover flies and parasitoid wasps on each kind of insectary plant.
The researchers found that annual alyssum, cilantro, mustard and buckwheat were the most attractive to hover flies overall. To the parasitic wasps, mustard, buckwheat and Korean licorice mint were the most attractive.
"These aren't absolutes," said Colley. "The insect preferences for plants were relative. The most favored plants depended on what else was in bloom or not in bloom at the time."
Colley hopes that by learning more about insectary plants and the predator insects they attract, growers will be able to bolster natural populations of predator species of insects including hover flies and parasitoid wasps, thereby keeping pest species in check.
"For example, the larvae of many hover fly species are voracious aphid feeders," she explained. "They feed specifically on aphids, whereas a lot of other predators are more generalists, feeding on many types of insects. Hover flies have the potential to reduce populations of the cabbage aphid and green peach aphid, serious economic pests on broccoli crops in the Pacific Northwest."
Insectary plants provide essential nectar and pollen for many species of tiny parasitoid wasps that help provide biological control of many pest species of Oregon crops, added Luna.
By reducing pests biologically rather than chemically, Colley and Luna hope to help growers save money and reduce risks to human health and the environment.
"Natural predators aren't a 'silver bullet,'" Colley stressed. "There isn't an instant result like a pesticide. Biological control affects a whole cycle of life over time. It's a long-term solution."