Some experts are calling it a population explosion — of caterpillars, that is. In central Arkansas, extremely warm weather recently resulted in an infestation of variable oakleaf caterpillars that attacked red oak trees.

As the hordes of hungry critters stripped the oak trees of their leaves this summer, they did what comes naturally and, because there were so many of them, it sounded like a lightly falling rain. But the noise was really frass, or hard pellets of excrement, dropping everywhere, on people, cars, sidewalks, patios, decks and, of course, swimming pools.

“The caterpillar poop falls into pools and clogs pumps and filters,” said David Townley, vice president of Townley Pool and Spa in Little Rock, Ark. “People think it’s ‘balls of algae,’ and they bring in water samples, asking if that’s what it is. But, no, it’s from the many variable oakleaf caterpillars that get into oak trees near pools, devour the leaves and deposit tons of excrement.”

The infestation surprised even veteran pool professionals. “I’ve never seen anything like it,” Townley said, adding that his company’s longtime employees with 15 to 20 years’ experience also said it was a first.

These caterpillars are 1.5 inches long and vary in color from green to yellow, with a dark stripe running down their backs, said Jim Northum, a forest health specialist at the Arkansas Forestry Commission. He said they range from eastern Canada to southeastern states such as Arkansas and Missouri, and infestations have been known to cover millions of acres. This recent outbreak has been most severe in central Arkansas, especially the area that includes Conway, Mayflower and Little Rock.

The variable oakleaf caterpillar feeds on leaves of deciduous trees, including all species of oaks, but prefers white and red oaks. Usually the infestations subside after a couple of years, giving the defoliated trees time to recover. Authorities do not recommend applying chemicals to the trees, saying that parasites and predators will keep the insects in check.

Townley estimated that 60 percent of his customers were affected by the caterpillar invasion, no doubt due to the area’s abundance of trees. The situation had eased off by press time because the first generation had entered pupation, the inactive stage of development when they are not feeding but are in cocoons, transforming into moths.

Residents of the region are bracing for the appearance of the second generation, expected to arrive in September to dine on their favorite entrée, the oak tree.

While spraying pesticides on oak trees is frowned upon, homeowners wanting to make their pools usable during an outbreak could treat the water with a sanitizer. Townley said chlorine applied in the right amounts will break down the frass, and superchlorination is an option as well.

Meanwhile, some Arkansans took to the Internet to express their disgust over the caterpillar situation. “They are pretty active in Maumelle. My deck is a mess,” said Maxine Willmuth in an online forum. And Kay Southerland didn’t mince words: “My yard, patio, pool — everything is just nasty!”