U.S. entomologists say a tiny Chinese wasp is proving an effective biological weapon against an invasive insect pest decimating ash trees in North America.

The emerald ash borer has killed tens of millions of ash trees throughout the eastern United States since it was first detected in 2002 in Michigan and Canada, they said, and has the potential to kill an estimated 7 billion ash trees in urban and rural forests.

Shortly after its discovery research began on its natural enemies in its native China, focusing on three parasitoid wasp species.

Writing in the Journal of Economic Entomology, the researchers said since 2007, when federal and state regulatory agencies approved the release of the wasps in more than a dozen states, the populations of these parasitoid wasps have been increasing and expanding in Michigan, suggesting they will likely play a critical role in suppressing the emerald ash borer in that state.

The tiny wasps, which do not sting humans, lay eggs into or on the borer larvae, killing them in a classic example of a biological control program, the researchers said.