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Rust mites are an alternate prey for predator mites.

Rust mites are an alternate prey for predator mites.

Orchardists used to tell Dr. Elizabeth Beers that they hadn’t sprayed for mites in 25 years or more.

Until recently, growers could rely on integrated mite control, meaning that predatory mites could be relied on to keep pest mites in check. But new orchard practices are changing that.

“Last year, there were mites everywhere,” said Beers, an entomologist with Washington State University in Wenatchee. “It seemed like a lot of people had mite problems. We’re seeing a breakdown of integrated mite control.”

With restrictions on the use of traditional insecticides, such as Guthion (azinphos-methyl) and Lorsban (chlorpyrifos), growers are using newer insecticides to control major orchard pests. Whereas the predatory mite Typhlodromus occidentalis had built up resistance to organophosphates, some of the new materials targeted at codling moth are disruptive of integrated mite control, preliminary studies indicate.

But it’s not just the new materials that are implicated. It’s also suspected that sulfur, a product used in orchards since early last century, is playing a role in the mite problem.

Studies that the Washington Tree Fruit Research Commission has done on chemical thinning strategies has shown that lime sulfur, particularly in combination with Crocker’s Fish Oil, is the most consistent chemical bloom thinner. Since registration of Elgetol was cancelled in 1991, there have been few good bloom-thinning options. Sulfur is also used for disease control in organic production. Beers said she’s seen some extremely high mite populations in cherries where sulfur has been used for mildew control.

While it’s always been known that sulfur was somewhat toxic to predatory mites, the product has never been used with the frequency and intensity that it is now, Beers noted. Sulfur is toxic to rust mites also but usually has a greater effect on predatory mites, allowing the pest mites to come back.

She thinks the breakdown in mite control results from a combination of a number of small disruptions, and even the postbloom thinner Sevin (carbaryl) might play a role.

However, Beers believes that sulfur and the new insecticides can be used in moderation. “I don’t want to come out and say, ‘Stop using sulfur entirely,’” she said. “It’s actually when you start putting them all together, these very small toxic hits on our predatory mite system, that I think you can end up with a problem. You start putting a combination of these materials together, and it’s hard for the predators to fend off everything.”


Beers and colleagues at WSU in Wenatchee began a three-year project in 2005 to look at the effects of the new codling moth pesticides Assail (acetamiprid), Calypso (thiacloprid), and Rimon (novaluron) on integrated mite control in commercial orchards.

In 2005, only one of the five orchards studied had a mite problem. It appeared that
all three of the new treatments caused mite problems in that orchard, but an organophosphate program did not.

In 2006, five out of six orchards studied had mite problems, and the one that didn’t was the orchard that was just added to the project that year. Those that did have problems were in the second year of the research, indicating a multiple-year effect.

Beers said chemical thinning records are being collected so that the role of thinning programs can be assessed.

In another project, Beers is studying the effects of a combination of sulfur, Sevin, and the new codling moth pesticides on mite populations. Mite problems were found where all three of those programs were used together but also where sulfur and Sevin were used with a standard organophosphate insecticide program.

Although the study needs to be repeated on a larger scale, Beers said the implications are that the chemical thinning program alone can cause problems. “That’s where I arrived at the conclusion that the thinning program might have a more prominent role than in the past,” she said. “Here’s our dilemma. We can’t not thin our orchards, and we can’t not control codling moth. We’re going to have to learn to cope with these new programs.”

Some of the new neonicotinyl pesticides have been associated with mite problems, but it was surprising to Beers that an insect growth regulator like Rimon would have an adverse effect on predatory mites.

Alternate prey

She hopes growers will be aware of the risk of mite problems. In some of the hotter growing areas, rust mites (an alternate prey for predatory mites) don’t survive as well and it’s harder to stabilize integrated mite control, she said. Where there’s a lack of rust mites for predators to feed on, outbreaks of spider mites are more likely.

Beers said growers can try substituting other thinning materials that don’t affect mites, such as NAA (naphthaleneacetic acid) for thinning or Intrepid (methoxyfenozide) or one of the traditional materials for codling moth control.

If they do have mite problems, there are plenty of effective and selective miticides to choose from, such as there are plenty of selective miticides to choose from, such as Zeal (etoxazole), Envidor (spirodiclofen), FujiMite (fenpyroximate), Kanemite (acequinocyl), Apollo (clofentezine), Savey (hexythiazox), Pyramite (pyridaben), and Agri-mek (abamectin).

“Integrated mite control is our flagship program,” she said. “We would be thrilled if growers said, ‘I haven’t had to use a miticide for 25 years,’ but this is not realistic for every grower in the state.”