A major swarm of wood-eating termites emerged from nests across the New Orleans area Monday night, swirling around lampposts, well-lit homes and trees and kicking off what is many New Orleanians’ least favorite season.

One expert said Tuesday that Baton Rouge could soon see its heaviest infestation ever.

Monday’s muggy conditions followed a weekend of heavy New Orleans rains, which are ideal for the moisture-loving Formosan subterranean termites, widely considered to be the most destructive variety.

Formosans, native to East Asia, were introduced to the U.S. mainland in the 1940s and ’50s through crates and cargo crossing the Pacific Ocean during and after World War II.

Military ships transported them locally to Camp Leroy Johnson — now the University of New Orleans’ East Campus — and the Algiers Naval Support Activity, from where they spread rapidly.

Formosans may reach an average of 10 million per colony, while native termites’ colonies number in the hundreds of thousands.

The first swarm of the two-month termite season starts each year around Mother’s Day, which is Sunday.

Major swarms will continue every eight to 10 days until the season concludes at the end of June, said Gregg Henderson, an LSU professor and veteran entomologist.

Those that fly are called alates, and they swarm in order to mate and reproduce, he said.

“They like to infest living trees, and often when they fly, they are coming out of the trees,” he said.

Even the tree-dwellers began their infestation from underground, and those that enter homes often do so by building mud tubes, called shelter tubes, in foundations, he added.

Though New Orleans and parts of St. Tammany Parish have seen swarms this week, Henderson — who has tracked Formosan termites for more than two decades — said there has not yet been a multiple-parish swarm.

He spent the better part of Tuesday counting those that fell victim to termite “light traps” he has laid around Baton Rouge — devices with fluorescent lights and buckets that attract and trap the flying insects.

Activity thus far has been light, he said, but he expects Baton Rouge will see the largest swarm recorded this year.

The termites have been on the rise in Baton Rouge since 2009, when Henderson first began his census.

He tracked them in New Orleans’ French Quarter for years before that, but he moved his work to Baton Rouge after an LSU AgCenter-administered program to abate the insects in New Orleans was terminated for lack of federal funding.

The program, Operation Full Stop, had a 14-year run and reduced termites in the covered area by 95 percent.

Annually, the bugs can cause up to $300 million in damages and repairs.

To avoid them, homeowners can invest in chemical barrier treatments or tent homes in cases of serious infestations. It’s also helpful to keep outside lights off or to invest in yellow porch lights, rather than white lights, Henderson said.

Follow Jessica Williams on Twitter, @jwilliamsNOLA.