Plague of dragonflies swarms resort island of Jeju. Experts say they're harmless, though.

Swarms of migratory dragonflies have descended on Jeju Island in the south in recent days, covering boats and people and causing widespread bemusement.

Tropical globe skimmer dragonflies cover a fisher on a vessel near Jeju Island in footage broadcast by JIBS on Tuesday. [SCREEN CAPTURE]

Swarms of migratory dragonflies have descended on Jeju Island in the south in recent days, covering boats and people and causing widespread bemusement.

Experts have been flummoxed by the appearance of globe skimmer dragonflies, a wide-ranging species that is present on every continent except Antarctica.

One theory is that the country’s recurring heat waves brought them to Jeju Island, though why they congregated there en masse remains a mystery.

In footage aired by the island’s local broadcaster JIBS on Tuesday, countless dragonflies can be seen swarming a fishing vessel sailing three kilometers (1.86 miles) offshore at dawn on Sunday.

“I have never seen so many bugs flying into the vessel,” the captain told JIBS. He estimated that he saw “thousands or tens of thousands” of dragonflies that morning.

He added he was “unable to properly conduct fishing operations because the dragonflies landed on his body and face” and “wreaked havoc.”

The JIBS reported sightings of dragonfly swarms on the island have increased since they migrated south at the end of summer on the peninsula.

Hundreds of globe skimmer dragonflies land on equipment on a ship operating in waters near Jeju Island in footage broadcast by SBS. [SCREEN CAPTURE]

However, experts have called their appearance in swarms “extremely abnormal.”

Prof. Kim Dong-soon from Jeju National University’s Plant Resources and Environment Department noted that previous dragonfly swarms “would have been documented if they had taken place on land before.”

But Kim added that people should not be too worried by the dragonflies as they are “harmless.”

“They consume flies and mosquitoes, so they should be considered beneficial insects,” he noted.

BY LEE SOO-JUNG [lee.soojung1@joongang.co.kr]