All hemlock wooly adelgids in the U.S. are female because the males are unable to survive.

So how does the invasive species continue to spread?

In their native East Asian habitat, the invasive pest reproduces three times per year: twice asexually – meaning females lay eggs without mating – and once sexually with a male.

The first asexual generation produces only female crawlers. The second hatches into winged males and females.

Yet, the winged males and females die almost immediately after hatching because the trees they need to survive aren’t present in North America. The female crawlers feed off hemlock trees, but the winged males and females need tigertail spruce trees to survive and mate.

It’s a good thing that tigertail spruce isn’t present in the U.S., Colleen Teerling, an entomologist with the Maine Forest Service, said. The offspring produced by the winged hemlock wooly adelgid fly back to hemlock trees, meaning the pest would spread much faster if tigertail spruces were present.

Instead, the winged generation completely dies off. In the summer of 2021, a massive die-off of winged hemlock woolly adelgids left millions of fly corpses along shores and beaches from Maine to Massachusetts and stained beachgoers feet black, according to the Portland Press Herald.

“Adelgids as a group have the most bizarre life cycles you have ever heard of,” Teerling said. “Often, they (have) two complete, different generations that bear no resemblance whatsoever to each other. You would swear that they’re two different species.”