For the first time, Asian giant hornets have been spotted in the United States, specifically in Washington state, scientists say. Beekeepers have reported piles of dead bees with their heads ripped off, an alarming sight in a country with a rapidly declining bee population.

At more than two inches long, they're the world's largest hornets with a sting that can kill humans if stung multiple times, according to experts at the Washington State University. The giant insects are nicknamed "murder hornets."

"They're like something out of a monster cartoon with this huge yellow-orange face," Susan Cobey, a bee breeder at the Washington State University's department of entomology, said recently.

In his decades of beekeeping, Ted McFall had never seen anything like it.

As he pulled his truck up to check on a group of hives near Custer, Wash., in November, he could spot from the window a mess of bee carcasses on the ground. As he looked closer, he saw a pile of dead members of the colony in front of a hive and more carnage inside — thousands and thousands of bees with their heads torn from their bodies and no sign of a culprit.

“I couldn’t wrap my head around what could have done that,” Mr. McFall said.

Only later did he come to suspect that the killer was what some researchers simply call the “murder hornet.”

With queens that can grow to two inches long, Asian giant hornets can use mandibles shaped like spiked shark fins to wipe out a honeybee hive in a matter of hours, decapitating the bees and flying away with the thoraxes to feed their young. For larger targets, the hornet’s potent venom and stinger — long enough to puncture a beekeeping suit — make for an excruciating combination that victims have likened to hot metal driving into their skin.

Researchers say the sting of a murder hornet is painful and packed with neurotoxins. Even if someone is not allergic to the hornet, multiple stings have the potential to kill. The nervous system can shut down, and an allergic reaction may occur and cause anaphylactic shock. The insects kill roughly 30 to 40 people each year in Japan, where they’re most common.

The giant hornets are primarily a danger to bees. Scientists are hunting for the insects, whose queens can grow to two inches long, in hopes of rounding them up before they become rooted in the United States and destroy bee populations that are crucial to crop pollination.

“This is our window to keep it from establishing,” said Chris Looney, an entomologist at the Washington State Department of Agriculture. “If we can’t do it in the next couple of years, it probably can’t be done.”