Massive structures dubbed "super nests" each housing up to 15,000 yellow jackets have again begun appearing in Alabama — and experts warn many more of the car-sized units are likely to pop up throughout the state before the perennial invasion is over.

At least two "perennial yellow jacket nests" were spotted in the southern state in May. Researchers with the Alabama Cooperative Extension System (ACES) believe that, due to milder winters and an abundant food supply, wasps are able to survive and "enter spring with much larger numbers," resulting in the appearance of the nests.

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The first such "super nest" was discovered in June 2006, according to a news release from ACES. During that year, Alabama had at least 90 nests — which scientists say can contain thousands of the stinging insects, and can grow to the size of another "bug" — a Volkswagen Beetle.

“The most workers I have counted in a perennial nest is about 15,000 or about three to four times more than a normal nest,” said Charles Ray, an entomologist working with ACES. “However, one nest in South Carolina was documented with more 250,000 workers.”

Ray thinks Alabama will see many perennial yellow jacket nests this year, and said that in addition to the two already found, there are indications of a third.

If seen, Ray warns residents to not mess around with the nests.

“While these giant nests often appear less aggressive than smaller colonies, it is important that people do not disturb the nests,” he said.

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Experts advise that only licensed commercial pest control operators remove the massive nests.

The Associated Press contributed to this report.