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CORVALLIS - In the world of mighty mites, Oregon has a classic case of good versus bad. And the good guys are getting the edge, with millions having been released in Oregon fields to rescue high-value crops.

As a result, many growers in the Pacific Northwest will eventually cut pest control costs in half, and considerably reduce their need for pesticides.

Here's the story:

The bad guys are spider mites that take a multi-million dollar toll each year on mint crops, strawberries, ornamentals and trees. The good guys are predator mites. Just about all they eat are spider mites.

So the world needs more predator mites. To make that happen, Brian Croft has a plan.

Croft, a professor in Oregon State University's Department of Entomology working with OSU's Agricultural Experiment Station, has studied mites for more than 15 years.

The good mites (Neoseiulus fallacis) occur naturally, Croft said. So growers have to minimize the pesticides that might kill the good mites. They probably also need an extra supply of good mites so they have the numbers to zap all the bad mites.

So far, the plan is working best in mint. Predator mites multiply like crazy in top-irrigated mint. "They like the high humidity," Croft said.

In the greenhouses of A.M. Todd Co., a mint handler near Jefferson, Ore., researchers have been multiplying mites on lima bean plants since spring.

Starting in late August, they began turning loose more than 10 million predators in mint fields around the wetter areas of Jefferson, Eugene and Junction City, and in the drier areas of the Pacific Northwest, such as central and north-central Oregon, the Columbia River Basin and the East Yakima Valley of Washington and Idaho.

"We should have had more predators in the fields by April, but our greenhouses weren't ready in time," said Joyce Takeyasu, an entomologist for A.M. Todd. "Now we are concentrating on post-harvest releases (August and September). Predator mites are good at over-wintering, so we should have enough in fields when spring rolls around if they disperse adequately."

Next April, mint growers should check their fields. If they average more than one spider mite per plant and if the ratio of spider mites to predator mites is too high, they will have to spray with pesticides or bring in predator mites or both.

Spider mite control is a big deal to Oregon's peppermint producers, whose $51 million crop is the biggest of any state's.

Currently, mint growers with spider mite problems apply Comite, often spending up to $35 an acre on the miticide and application cost. With some help from predator mites, applications can be reduced, significantly lowering pest control costs, according to Mark Morris, an OSU doctoral student who works for A.M. Todd Co. in research and development.

Morris said new miticides are even more expensive, so biological control could have even a greater payoff.

"When spider mites approach the treatment threshold, we figure one predator mite for every 10 spider mites will do the job," Takeyasu said. "In early spring if spider mite populations are below one spider mite per plant, we can start with a one to 100 ratio and the predator mites will hopefully catch up and deplete the population of spider mites."

Morris believes the pinpoint-sized predators "should eventually replace chemical pesticides as a first line of defense in growers' arsenals - although pesticides are needed as a backup in cases where biological control is not effective enough."

Morris said he won't inoculate a field with predators if a grower already has enough in a field. In those cases, natural biological control should control the spider mites.

Croft said predator mites are put into fields in one of three ways: - INOCULATION: "In strawberries, we inoculate plants in a new field that won't be harvested that year. If we inoculate a thousand plants, that's enough to get predator mite coverage of a 10- to 20-acre field by the next year."

- INUNDATION: "That means putting on a lot of predators and using them like a pesticide," Croft said. "This tactic is like releasing 10,000 coyotes to wipe out 100,000 rabbits, but is necessary and economical on high value crops."

In mint, Takeyasu said, biological control requires at least 1,000 predator mites per acre - at a cost to growers of $8 to $10. But this rate depends on initial spider mite densities and presence of natural predator mites or other generalist predators.

Even if more predators are needed, the price might seem like a bargain to mint growers whose crop is worth more than $1,000 an acre and who have seen spider mites wipe out all of a field's mint leaves - and their valuable oil. - PERIODIC RELEASES: The ploy is to scout fields for predator mites and to manage pesticide use accordingly. Mites are added from time to time when the predators are in short supply.

Croft said OSU extension specialists and Agricultural Experiment Station scientists are working with growers to encourage the natural spread of predator mites.

"It's a matter of multi-crop management," Croft said. "Growers need to integrate production practices' so natural predator control is encouraged."

Besides their value to mint and strawberries, Croft said predator mites can control spider mites on rhododendrons, virburnum, magnolias, azaleas and shade trees such as linden.