There's a population explosion of tent-building caterpillars in trees of the higher mountains of Southern Arizona, like this one in the Catalinas.


A "population explosion" of tent caterpillars - which spin slightly creepy-looking shelters on tree limbs - has left some mountain areas looking "almost like a science-fiction movie," scientists say.

"A lot of people are creeped out by it - but it's just caterpillars and a bunch of silk and caterpillar poop," said John Palting of the University of Arizona Department of Entomology.

Palting - a graduate student specializing in caterpillars and the moths they become - said, "There seems to be a population explosion of tent caterpillars this year."

"The caterpillars seem to have gotten the upper hand on the birds and parasites that usually keep them in check," he said.

The result has been an abundance of the silky caterpillar tents in the Catalina Mountains near Tucson and other mountain ranges in Southern Arizona.

"Most years you see a few tents," Palting said, "but this year there are so many of them in trees that it's almost like a science fiction movie in some places."

Adding to the potentially off-putting nature of the scene for some people: "The black stuff you see in the tents is droppings - caterpillar poop," Palting said.

"Group shelters"

Caterpillars hatch in the early spring from eggs produced by moths the previous year.

"They build tents in trees, sort of as group shelters," Palting said. "Around their mouth they have a silk gland - producing something similar to what a silkworm produces. It's spun by all the caterpillars together."

The caterpillars feed on the leaves of trees, and after eating their fill they form a cocoon where the transformation to a moth begins.

"It's quite a dramatic transition from a worm to a moth," Palting said. "It hatches within a few weeks and is alive only for 24 to 48 hours. The moth is really just the vehicle to mate and continue the species."


"When caterpillars hatch, they start feeding on new leaves," Palting said. "By the time they reach maturity, they pretty much strip every leaf" from host trees.

Caterpillars can defoliate trees, but the damage they cause usually isn't lasting in wild areas.

"I haven't seen it where they kill a tree," Palting said. "They take the first crop of leaves, and the trees leaf out again."


People who live and work in the Catalina Mountains are seeing lots of caterpillar tents this year - and some have mixed reactions.

"They are kind of cool and kind of creepy, all at the same time," said Pamela Selby-Harmon, officer in charge of the post office in the mountain village of Summerhaven.

"It's one of those love-hate relationships," Selby-Harmon said. "You love the caterpillars, but you don't like to see damage to the trees. But Mother Nature has a lot more going on than we do as far as planning."


Palting said the tent show will wind down in the coming month - and the caterpillar population could eventually return to normal levels.

"The tents will probably break down with the summer rains and be pretty much gone by midsummer," Palting said. Meanwhile, "the population explosion generally means that the parasites will also have a good year and take the population back down. Such are the checks and balances of nature."

where to see them

The silky "tents" built by tent caterpillars are visible at many sites in the higher mountain ranges of Southeastern Arizona. Among the sites:

• Along parts of the Aspen-Marshall Gulch Loop Trail in the Catalina Mountains north of Tucson. The trailhead is in Marshall Gulch, about a mile south of the village of Summerhaven.

• Along trails and roads in the Pinaleño Mountains near Safford. Caterpillars "are having quite a population explosion there," said caterpillar expert John Palting. "Aspen along the road are leafless and covered in the tents - an eerie sight when backlit in late afternoon."

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