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A barn owl nesting box at Omeg Orchards. Notice the metal underneath the box that’s used to prevent raccoons from climbing the pole to reach the next.

A barn owl nesting box at Omeg Orchards. Notice the metal underneath the box that’s used to prevent raccoons from climbing the pole to reach the next.

PHOTO BY MELISSA HANSEN

As growers put barn owls and other raptors to work in their orchards and vineyards for rodent control, care must be taken to ensure that any rodenticides used do not poison the very ­raptors being encouraged to hunt and roost.

Since installing barn owl and kestrel nesting boxes and raptor perches in his orchard, Mike Omeg, cherry grower from The Dalles, Oregon, has greatly reduced his rodent control costs. And while he believes he’ll soon see a day when his raptor populations provide complete control, until then, he must continue to supplement their patrol with other means, using rodenticides in bait stations and in burrows.

Careful selection and use can help growers minimize the rodenticide risk to the raptor allies.

Raptor risk from rodenticides is a subject Omeg has studied closely, reviewing pages of toxicity data of the U.S. Environmental Protection Agency. He’s also conducted his own trials with rodenticides and raptor habitat in orchards that Omeg farms with his father.

He shared his learning during a raptor workshop he helped organize last fall as part of a three-year grant he received from Utah State University’s Western Sustainable Agriculture Research and Education and the U.S. Department of Agriculture’s Natural Resources Conservation Service. The grant provided funds to install nesting boxes and perches in his orchards and to study his efforts to increase raptor rodent control in the orchards.

“I believe you can use rodenticides safely around raptors,” Omeg said during the workshop that was co-sponsored by the Wasco County Conservation District. “But I want to make sure that growers don’t poison the very barn owls and hawks they are encouraging to come to their place. “

Minimizing risks

With rodenticides, secondary poisoning (consuming a poisoned rodent) is the concern to raptors. If you’re trying to encourage raptors, then you want to choose one that’s relatively safe, if possible, he said.

Two types of rodenticides are commonly used in orchards—anticoagulants and nonanticoagulants, like zinc phosphide/phosphine materials. Anticoagulant materials labeled for orchard use include several Rozol products (chlorophacinone), manufactured by Lipha­tech, and Ramik Brown (diphacinone). Ramik is manufactured by HACCO.

Zinc phosphide/phosphine products react with gastric acids to create phosphine gas inside the stomach, which kills the animal. According to the National Pesticide Information Center’s Web site, “Zinc phosphide is not expected to pose a secondary poisoning hazard because of its rapid breakdown in the bodies of the animals that ingested it. However, experimental oral exposures to animals poisoned by zinc phosphide have led to some secondary poisonings in both cats and dogs.”

Rozol is a multiple-feed anticoagulant, requiring multiple feedings to ingest enough poison to kill. Its paraffin-coated pellets make the product useful if ground conditions are moist or wet. Zinc phosphide pellets react with moisture, which limits their outdoor use if weather is rainy.

Omeg believes that Rozol can be safely used by orchardists.

He cited a New York study that analyzed the blood levels of raptors found dead. Of 78 red-tailed hawks analyzed, more than 50 percent had anticoagulants in their blood. More than 80 percent of the 53 great horned owls analyzed had anticoagulant ­rodenticides in their blood.

Based on EPA risk data that combine weighted primary and secondary risks to birds and mammals for chlorophacinone, diaphacinone, and zinc phosphide, on a scale of one to ten, Rozol (chlorophacinone) would be considered the safest because it had the lowest risk index (1.95). Diaphacinone (Ramik Brown) was indexed at 3.01, and zinc phosphide had the highest index of the three materials at 4.63. The EPA index was cited by Liphatech in Rozol promotional material.

In the EPA study, Potential Risks of Nine Rodenticides to Birds and Nontarget Mammals, a Comparative Approach, published in 2004, the agency stated, “The available laboratory studies indicate that major differences occur among the rodenticides in their secondary hazard to birds, with D-Con (brodifacoum) displaying the greatest hazard and Rozol (chlorophacinone) and the ­nonanticoagulants the least.”