BOULDER — Colorado is deploying tiny predator wasps imported from China — 200 a week through mid-July — in a state experiment aimed at annihilating invasive beetles threatening one in six metro Denver trees.

The Oobius agrili wasps target eggs of emerald ash borer beetles, also from Asia, which have killed more than 50 million trees in 25 states and spread across 12 square miles within Boulder.

Wielding ovipositor points on their bellies, these wasps can pierce beetle eggs and lay their own eggs inside. Oobius larvae then devour beetle eggs, defeating borer beetles by preventing population growth.

“This is an experiment. We hope it is enough and that the wasps will establish themselves. The idea is they will reproduce on their own,” said John Kaltenbach, biological control specialist for the state Department of Agriculture.

But instigating bug-on-bug war to save an estimated 1.45 million metro Denver ash trees depends on keeping the wasps comfortable — and strong enough to make it through winter.

Kaltenbach and his team of state, city and campus arborists this week hung modified orange pill jars in trees that will launch wasps on the University of Colorado campus.

That’s a beetle stronghold where infected trees are dying and new beetles are boring into many of Boulder’s estimated 98,000 ash trees.

The arborists smeared honey on the jars, intended to fortify wasps as they crawl out and fly in search of fresh beetle eggs in tree bark crevices. Full-scale bug-on-bug battling — “biological control” — may take a few years.

Boulder foresters first detected wood-boring ash beetles here in September 2013. They probably entered as hitchhikers on imported firewood. Around 2002, the beetles migrated from Asia to Michigan in wooden packaging material and killed 30 million ash trees in Michigan, according to the U.S. Department of Agriculture. They’ve been confirmed in 25 states, including Colorado.

It’s hard to stop borer beetles because few native predators exist.

USDA scientists went to China looking for the beetle’s natural enemies. Working with Chinese foresters, they identified three wasps in China and, over the past decade, imported and raised them at a federal lab near Detroit.

In Colorado, state and local authorities have formed a response team. Team members set out more than 300 beetle traps, useful for detecting and monitoring the beetles.

They tried to kill the borers by injecting insecticides into infected tree trunks. They cut down infected trees. Last year, they introduced a larger imported wasp.

Yet more beetles are hatching this spring, leaving tiny D-shaped holes in bark as they spread.

Ash trees make up roughly 15 percent of the tree canopy in Colorado cities, state officials said. Healthy trees help retain stormwater, cool the air, sequester carbon pollution and provide beauty.

“If we don’t try anything, the emerald ash borer will continue to spread,” Kaltenbach said. “We’re definitely moving more pests around the world. It’s going to be a continual battle. This will never be done. Basically, we’re in a competition with insects for resources.”

Bruce Finley: 303-954-1700, bfinley@denverpost.com or twitter.com/finleybruce