WSU study on potato farming gives organic way a boost

If you want to grow a bigger potato, organic farming may be the way.

The balanced mix of insects and fungi in organic fields does a superior job of keeping pests in check, leading to larger plants, according to researchers at Washington State University in Pullman. Potato plants exposed to conditions typical of pesticide-treated fields fared more poorly in the research team's experiments.

The findings may help potato growers cut back on spraying and make more effective use of natural predators to control pests, said entomologist David Crowder, who led the study published Thursday in the journal Nature.

"The goal is to learn as much as we can about how these natural enemies are doing their jobs and what impact they're having, so we can incorporate their effects into management practices," he said.

Washington is second only to Idaho in potato production in the nation, and the state's crop is valued at nearly $700 million a year. But potatoes can be very vulnerable to pests. Washington potato farmers applied more than 19 million pounds of weed- and bug-killing chemicals in 2005, according to the latest data available from the U.S. Department of Agriculture.

Less than 1 percent of the state's potatoes are organically grown, and even many organic farmers use some type of chemicals or natural toxins to control pests. But farmers are under pressure from such companies as McDonald's — the nation's top potato customer — to green up their practices.

"People who buy a lot of potatoes are asking the growers to reduce insecticide use as much as possible, to document pesticide use, and include biological control as a consideration," said WSU entomologist William Snyder, a study co-author.

Snyder recently received a $2 million USDA grant to help potato growers shift their practices.

"We have some pretty progressive farmers who already spray much less, compared to the industry average," Snyder said. "It's kind of 'organic lite.' "

The Washington State Potato Commission also funds some of Snyder's research and hopes to translate the science into practical advice its members can use, said Andrew Jensen, the group's research director.

For the Nature study, the WSU scientists wanted to find out whether a balanced mix of insects could be beneficial. They examined bug counts from conventional and organic fields around the world, growing a range of crops.

Since many pesticides wipe out the majority of insects, it wasn't surprising to discover that the conventional fields were often dominated by only a few hardy species. In contrast, the organic fields had a much more even mix.

But would that mix provide any real-world advantage? To test that, Crowder set up 42 potato plots enclosed in fine mesh. He seeded each of his mini fields with Colorado potato beetles, one of the industry's worst scourges. Then he added varying numbers of insects, fungi and microscopic worms called nematodes that attack the beetles' eggs and larvae.

The potato plots with the most balanced mix of insects and fungi, typical of organic fields, performed the best: Pest numbers were 20 percent lower and plants were 30 percent bigger than in the plots with the lopsided insect mix typical of pesticide-treated fields.

The study didn't follow the potatoes to harvest, but plant size is closely correlated with potato size and yield, Crowder said.

Though it's not clear how the results would scale up, the study does suggest that farmers who reduce pesticide use might be able to rely on a mix of natural predators to take up the slack in controlling pests, he added.

The work also suggests a way to short-circuit the "pesticide treadmill" that forces farmers to use more and different chemicals as pests evolve resistance, said an accompanying article in Nature from researchers at the University of Zurich in Switzerland.

The results have broader ecological implications, Crowder said. Scientists have long focused on the number of species in an ecosystem as a measure of its health. The WSU experiments show that it's also important to have a balanced mix of species.

Research on organic farming has received short shrift in the past, said Jennifer Miller, sustainable-agriculture coordinator at the Northwest Coalition for Alternatives to Pesticides in Boise.

"Often the natural pest control that's happening on organic farms is overlooked," she said. "It's really great to see research looking at the value of this effect and cheaper ways of pest management that come with reduced pesticide use."

Sandi Doughton: 206-464-2491 or sdoughton@seattletimes.com