Jun. 25—A plane will not spray billowing streams of insecticides this year onto a 35-square-mile area near the Rio Chama because grasshopper populations are too small to warrant such treatment.

Federal pest-control managers, who’d considered the aerial spraying, called it off when grasshopper counts remained consistently below eight per square acre — the threshold deemed a severe outbreak that could deplete grasses essential for grazing cattle and harm the area’s ecosystem.

“The grasshopper density has remained below our minimum threshold for treatment,” William Wesela, national policy manager for the U.S. Agriculture Department’s Animal and Plant Health Inspection Service, wrote in an email. “Based on this data, USDA will not apply grasshopper treatments in New Mexico this year.”

It’s the second year in a row the agency canceled aerial spraying near Rio Chama. Last year, the proposed spraying was scrapped after it stirred an outcry from environmentalists, tribal advocates and political leaders.

Conservationists welcomed the news.

They worried the chemicals would kill bees, monarch butterflies and other insects vital to the ecosystem in this 22,500-acre expanse in Rio Arriba County. They also feared heavy rains could carry the insecticides into the river.

“It’s another victory; our landscape is safe,” said Terry Sloan, director of Southwest Native Cultures based in Albuquerque. “It’s really important to protect our landscape.”

If grasshoppers had reached the level of an outbreak, the agency had planned to spray diflubenzuron, which kills insects in the juvenile stage.

Last year, the agency was set to spray 670 gallons of carbaryl, a potent neurotoxin that’s “broad spectrum,” or indiscriminate in the species it can harm, including fish and birds that could eat the poisoned insects.

Carbaryl also is carcinogenic to humans, raising concerns about it falling on people rafting or hiking in the area. Diflubenzuron is not linked to cancer, according to the U.S. Environmental Protection Agency.

The switch to the less noxious pesticide appeared to come in response to the fierce opposition the earlier plan triggered, though USDA officials haven’t confirmed that was the reason.

Still, diflubenzuron, which doesn’t exterminate adults, can be deadly in a different way, killing butterflies while they’re caterpillars and bees while in the larval stage, said Sharon Selvaggio, pesticide program specialist for the Xerces Society.

Given its possible harmful effects, Selvaggio said she’s glad it won’t be aerially sprayed across miles of fields near a river this year, though she still thinks the agency has flawed pest-control policies it could improve.

For instance, the agency should work with land managers to increase vegetative cover across the land, she said, noting grasshoppers proliferate more on bare ground because that’s where they lay eggs. So creating more shade will help curb their growth, she added.

Heavy cattle grazing has left much of this Rio Chama area stripped of grass and other vegetation, she said. Managers should reduce grazing to allow the fields to rejuvenate and become more naturally resistant to grasshoppers, she said.

“You have to let the land heal,” Selvaggio said.

Xerces and a half-dozen other environmental advocates sent a letter to U.S. Agriculture Secretary Tom Vilsack, Interior Secretary Deb Haaland and State Land Commissioner Stephanie Garcia Richard accusing the federal pest-control agency of mishandling last year’s plan to aerially spray the area.

The agency never consulted with tribes, nor did it make its full finding of no significant impact available to the public, the letter said.

The groups contend the planned spraying would’ve been done when wildflowers are in bloom, violating a federal pesticide law. And the use of carbaryl would’ve come “perilously close” to breaching the Endangered Species Act because of the threats to wildlife.

Federal managers also overlooked carbaryl being linked to cancer, as well as how the spraying could affect a wilderness study area and the Rio Chama, which is designated as a wild and scenic river, the groups argued.

The letter called into question the agency’s methods for determining the grasshoppers’ densities, saying they were inconsistent.

The agency needs to address these deficiencies and not carry them into its next effort to spray pesticides in this sensitive area, she said.

Last year’s controversy led to the Bureau of Land Management, which controls much of the acreage, to cancel the spraying on its tracts to allow more analysis and public comment.

BLM managers this year were again skeptical about the proposed spraying.

They didn’t believe the USDA’s assessment satisfied the National Environmental Policy Act or that managers offered sufficient public outreach.

Sloan said federal managers should consider alternatives to pesticides, such as bringing in birds and other predators that eat grasshoppers. They also could explore cultivating grasses and plants that repel the insects, he said.

“Something that’s in tune with the natural environment,” Sloan said, rather than always turning to chemicals that can harm the landscape.