Ryan Van Velzer

The Republic | azcentral.com

Bobbie Frink could not even step out of the car. Outside, thousands of fat, green caterpillars wriggled and writhed their way across the street near her home in Ahwatukee last week.

"It looked like the whole street was rolling." she said. "I couldn't believe my eyes."

The caterpillars are a common sight during Arizona's monsoon season, but this year's brood seems particularly large, said Nico Franz, curator for the Hasbrouck Insect Collect at Arizona State University's School of Life Sciences.

"Two years ago we had the same situation. We were in the South Mountain area and we were collecting insects there at night and caterpillars were all over the place," he said. "Last year we saw low levels of abundance and this year they seem to be peaking again."

The wet summer has provided a lot of food for the caterpillars, Franz said. Now, having eaten their fill of desert greenery, they are moving en masse, he said.

Although the mass movement isn't really understood, the prevailing theory is that the caterpillars are looking for a place to pupate, according to a report from Carl Olson, entomologist at University of Arizona. After appeasing their wanderlust, they'll burrow into soft soil, form a shell known as a chrysalis and begin their transformation into a white-lined sphinx moth.

Ugly, but harmless

The caterpillars of the white-lined sphinx moth are ordinarily lime green or yellow with black markings. They can grow as big as the width of your hand and have a yellow or orange horn with a black tip near on the rear end.

Despite the bright colors, squirmy demeanor and horns, the caterpillars are nothing to fear, said Aleks Woodroffe, entomologist with the Arizona Department of Agriculture.

They mostly eat desert plants, but could munch on tomatoes, apple leaves or ornamental plants in your back yard. However, they aren't considered pests and don't pose much of a threat to your garden, Woodroffe said.

"They're safe, not poisonous," she said. "They will go away pretty soon. They are not going to be around forever."

An important part of the desert ecosystem

As adults, white-lined sphinx moths (Hyles lineata) fly from flower to flower pollinating desert plants in much the same way as butterflies. These pollinators are important not just in Arizona's ecosystem, but throughout the U.S. and Central America.

The moths, sometimes called hummingbird moths, are named for the white line on their wings, but they also have a pink patch that's visible while they're hovering over flowers. They can have two-to-three inch wingspans and grow to be the size of a "small mouse," Woodroffe said.

"They're really cute and fuzzy," she said. "You might notice them. They are day-flying moths so you might actually see them flying around. They are very strong and are really good fliers."

Of course, what an entomologist describes as cute and fuzzy, others, such as Bobbie Frink, may find disconcerting.

When Frink found out that that the thousands of caterpillars she saw would be turning into moths, she began devising a plan:

"I think I'm going to buy some mothballs, I might event make a necklace I don't know."