PHOENIX -- No, it's not the end of the world -- it's just a massive swarm of caterpillars.
North Phoenix, especially around the North Mountain area, has been inundated with large, fat, yellow creepy crawlies.
The caterpillars are the precursor to the white-lined sphinx moth, which is actually quite beautiful.
"Adult moths have this really gorgeous pattern of brown and white lines and the hind wings are this bright pink, so when you see them hovering, they're very colorful, very fancy," said Dr. Googy Davidowitz, associate professor at the Department of Entymology at the University of Arizona.
Just like the rise in mosquitoes, the recent rains are to blame for this current population boom.
"The reason is simply that we've had relatively good rains, so there's a lot of foliage out there and this allowed the population to build up," he said. "Every few years, depending on the monsoon rains, you'll get these outbreak years."
He added that a third generation of the insects will most likely occur in late September or early October, if the weather does not get too hot.
"Now, the reason you're seeing all of them is ... they're now ready to pupate (become moths)," he said. "Because of the timing of the rain, this -- what we call "wandering behavior" -- is relatively synchronized. It happens over a two to three day period. They're wandering around the ground looking for a place to pupate."
But what researchers don't know is why the caterpillars are leaving the mountain and nearby nature preserves, instead of just staying in those habitats.
"The thought is if they are all spread out, predators won't be able to find all of them, because they won't be so highly-concentrated," he said.
The caterpillars are moving very fast and when adding to that their sheer numbers and grotesque appearance, many people are stepping on them or running them over -- and getting messy in the process.
When crushed, green goo leaves their bodies. Davidowitz said the goo is actually their blood, it just is not red like in other animals. But they can also emit a blue goo if they are frightened or surprised.
"If you scare them, what they're doing is they're just regurgitating, effectively, bile," he explained. "They're just vomiting on you."
Despite the apocalyptic-like scenario, he insists they are not only harmless but vital to the environment.
"The moths are actually very, very important pollinators for hundreds of species of plants...a lot of plants wouldn't exist without them," he added.
KTAR's Holliday Moore contributed to this report.