From the article:..."
The buzz on American bees: too much inbreeding
According to the WSU research team, the root cause of the U.S. honey bees' vulnerability to varroa is a dwindling gene pool that has left them short on genetic traits that help honey bees resist varroa elsewhere in the world.
"Honeybees aren't native to America," Cobey says. "We brought them here. But the U.S. closed its borders to live honey bee imports in 1922, and our honeybee population has been interbreeding ever since."
WSU has monitored the genetic diversity of honeybee queens in Washington and California since 1994, showing a steady decline. Dr. Brandon Hopkins, the team's expert in freezing and thawing bee sperm, likens honeybee breeding to a poker game played with an incomplete deck of cards. "There's no way to get a four-of-a-kind if there aren't four aces in the deck," Hopkins says."