Colorado is deploying tiny Chinese predator wasps in an experiment aimed at annihilating invasive beetles threatening one in six trees in the Denver metropolitan area.

The Oobius agrili wasps - which are bred in the US and don't sting humans - target the eggs of emerald ash borer beetles, an invasive species from Asia, which have killed more than 60 million trees in 25 states and have turned up in Denver.

"The emerald ash borer has become the most destructive forest insect to ever invade North America, both in terms of the number of trees that have died and in terms of the economic impacts to private landowners, communities, public lands, you name it," said entomologist Deborah McCullough, who estimates the economic cost over the next 10 years at about $20 billion.

The bug-on-bug style of warfare was developed in a joint program between the US and China.

"We cooperated very well with our colleagues in China," said team member US Department of Agriculture (USDA) entomologist Juli Gould. "It's a story of mutual respect and cooperation and getting something done. How often does that happen, right?"

The emerald ash borer came into the US probably in wooden shipping pallets via the docks of Detroit in the early 1990s and began devouring ash trees, burrowing into their bark and wood, robbing the trees of nutrients and killing them in two to three years.

The beetles' numbers didn't grow enough to be noticed until about 2002.

The US government response was to try and eradicate the beetles, but it was evident early on that they were already just too widespread and that eradication was not going to work.

So they turned to biological control. Scientists with the Forest Service and APHIS (Animal and Plant Health Inspection Service), both part of the USDA, worked with scientists at the Chinese Academy of Forestry, who began to look for natural enemies of the emerald ash borer beetle back in China.

"They were successful in not only finding the agents but also studying their biology and publishing a lot of peer review papers on the subject," Gould said.

They found three kinds of wasps that attacked the beetle at different stages of its life cycle. The wasps basically lay their eggs inside of the beetle's larva where they grow, consuming and destroying it, and eventually controlling the beetle population.

Before bring the predators into the US, however, they had to make sure they wouldn't attack anything else. That research was also done in China, and under quarantine in the US.

Once cleared, the USDA set up a breeding facility in Michigan to start growing populations of the wasps to eventually be released and set after populations of the emerald ash borer.

"These things have to be reared inside larvae that are eating under the bark of an ash tree," Gould said. "That's the only way you can rear an emerald ash borer." And the wasps have to attack the beetle under bark, listening for the vibrations of the larvae chewing and piercing their egg depositor through the bark and into the larvae.

The logs are then hung out in fields so the wasps can emerge naturally. "It's not an easy system to work with, that's for sure," Gould said. "You could rear a million of these tiny little wasps on one little tree."

As of 2014, 2 million wasps have been released in 19 states, Gould said.

Monitoring the effects of the campaign is not easy. Finding populations of the beetles is tricky. You have to find the eggs or peel the bark off trees to find the larvae. "But that's what we do," Gould said.

The emerald ash borer population appears to be coming down, not only because of the wasps but also because the ash trees are dying. The percentage of emerald ash borers being attacked by the wasps is also going up.

Contact the writer at [email protected].