Douglas fir trees that survived the recent Frye Fire in the Pinaleño Mountains near Safford are prime habitat for endangered Mount Graham red squirrels — but those trees are threatened by tree-killing bark beetles.

What to do?

Bring on the pheromones.

Members of a national forest Burned Area Emergency Response (BAER) team plan to treat 300 acres of unburned Douglas fir forest in the fire area with MCH pheromone. MCH duplicates the natural pheromone emitted by male bark beetles — a pheromone that sends a signal to other male beetles that a tree is already taken.

The idea, said Dean McAlister of the BAER team, is to fool beetles into moving on and leaving the tree alone.

Sounds a bit complicated, but apparently beetles have fallen for the pheromone ruse in other forest areas, McAlister said.

“Previous success with MCH pheromone on the Wallow Fire (in Eastern Arizona) in 2011 indicates that the effects of bark beetles can be reduced by the use of this product,” McAlister said. “Based on the previous success, approximately 300 acres of unburned Douglas fir will be treated in the spring of 2018 within the area of the Frye Fire,” which burned more than 48,400 acres in June and July.

The area will be treated when temperatures on the mountain get to about 65 degrees and before the beetles become active for the season, McAlister said.

“The treatment process involves the use of bubble capsules containing the pheromone to repel beetles from uninfested areas,” he said. “It sends a signal of ‘no vacancy’ to other male beetles entering the area.”

The capsules, which release a cloud of MCH pheromone sufficient to affect beetle behavior, will be applied at a rate of approximately 20 per acre, McAlister said.

“The ideal place is about six feet off the ground on the north side of the tree,” he said. Each capsule is attached to a card that will be stapled or nailed to the tree.

“It’s a non-toxic product — not hazardous to humans or animals,” McAlister said.

He said monitoring planned for late summer of 2018 will determine the effectiveness of the treatment.

“A second treatment will most likely occur in the spring of 2019 to assure the maximum effect of the product on beetle invasion,” McAlister said, noting that there is $120,000 allocated to this multiyear project.

“The remaining trees are really important for the Mount Graham red squirrels” because only 590 acres of critical habitat for the squirrels remains in the aftermath of the Frye Fire, he said.

The majority of the fire area remains closed to the public, according to the Coronado National Forest.

A news release from the forest said that the Swift Trail, Arizona 366, in the Pinaleños will remain closed at Ladybug Saddle, Mile Marker 131, through the Labor Day weekend.

Tentative plans call for reopening the road in the first week of October, but that could change depending on conditions in the range.

Contact reporter Doug Kreutz at dkreutz@tucson.com or at 573-4192. On Twitter: @DouglasKreutz