Vegetable growers like Steve Mong of Stow, Mass., have controlled corn earworms, a significant pest for organic farmers, by using a new device designed by SARE- funded researchers that dispenses corn oil and Bacillus thuringiensis (Bt).
Vegetable growers like Steve Mong of Stow, Mass., have controlled corn earworms, a significant pest for organic farmers, by using a new device designed by SARE- funded researchers that dispenses corn oil and Bacillus thuringiensis (Bt). Photo by Ruth Hazzard

Insects and Diseases

"Organic farming requires more intensive management than conventional methods," said Joe Rude, the poultry farmer from Colo, Iowa, "because without access to a broad spectrum of pesticides and antibiotics, you have to understand the life cycle of the pests that are attacking your crops and animals. You have to understand the biological and chemical processes and work with the environment."

For organic farmers, this means employing strategies such as crop rotations, enhancing biodiversity, determining threshold levels of pest populations, introducing natural enemies and using good sanitation practices. Although certain sprays such as Bacillus thuringiensis (Bt) and rotenone are permitted, many organic farmers do not rely upon them exclusively. Rather, the key focus of organic pest control is prevention.

In California, where dry summers make it less challenging to grow organic fruit, a SARE-funded on-farm study comparing organic and conventional apples found that synthetic pheromones, biological control agents and sanitation successfully controlled codling moth mating in most locations. Disease control with sulfur and copper, when timed right, was as effective in scab control as the synthetic fungicides used in conventional systems.

In New York, where the wet humid summers pose enormous challenges for organic fruit production, Robert Pool, a Cornell University viticulture professor, found that organic grapes could be managed to be as pest-and disease-free as conventional ones. During a three-year, SARE-funded experiment, which compared organic and conventional grape production on three varieties, pheromone disruption and insect scouting allowed researchers to eliminate regular insecticide use.

"Going in, we thought the main problem would be diseases," said Pool. But instead, researchers applied, on average, less than one spray per year and found that the powdery mildew commonly observed on Concord grapes was far less destructive than predicted. The researchers also learned that pheromones successfully controlled grape berry moth and that scouting allowed them to control insects that emerged when the regular spraying was eliminated. They avoided an expected huge spike in grape leafhopper by releasing predatory wasps.

Other SARE-funded research has shown similar control:

  • In a study of potatoes in Idaho, researchers were able to control Colorado potato beetle with mineral and biological compounds.
  • In the Northeast, where sweet corn can be devastated by caterpillar pests, Bt and corn oil were used effectively to control corn earworm. Eight farmers from Vermont to Connecticut found that the oil controlled ear damage in 83 percent of their trial plots in 2000.

Many organic farmers have observed that, over time, pest populations seem to decrease. Results from a California on-farm organic tomato experiment showed the presence of 46 percent more predators and parasitoids and 43 percent more natural enemies on the organic farms, which could provide one possible explanation for reductions in pest populations. A SARE-funded study in Washington testing mowing frequency in pear orchards found that mowing only once a month rather than more frequently as done on conventional farms, creates alluring habitats, attracting beneficial insects that control pests.

"By reducing the frequency to once a month, we see a dramatic increase in natural enemies moving into the ground cover without a big increase in pests that feed on fruit," said David Horton, the ARS researcher testing mowing regimens.

Stone fruit grower Marilynn Lynn of Bridgeport, Wash., relies on living mulches to attract beneficial insects that prey on potential pests before they can harm her peaches, apricots and nectarines. "We mulch extensively," Lynn said during a satellite broadcast about organic production aired by Washington State University in spring 2003.

Calling their orchard grass, yarrow and clover covers a "bed and breakfast" for beneficial insects, she added: "They give a nice diversity to the floor of our orchard, providing food and water in the spring when they wake up."

For soil borne-disease control in organic systems, many growers use composts, long known as effective plant pathogen suppressants. Rotations also are important for decreasing pathogen populations, as most pathogens are plant specific. In general, rotating the crop, planting resistant varieties, and adding organic matter have all been shown to reduce the incidence of soil-borne diseases: In the SAFS project in California, a four-year organic rotation had lower incidence of corky root and red root rot than a two-year conventional rotation; an on-farm tomato study in the Central Valley of California showed that organically managed soils may be suppressive to the organism that causes corky root; and in North Carolina, another SARE-funded study showed disease was significantly reduced by organic soil fertility amendments and on organic versus conventional farms.