ST. PAUL — Minnesota officials reported success Wednesday in the battle to slow the spread of emerald ash borers, saying stingless wasps released in Great River Bluffs State Park near Winona two years ago are reproducing, which is evidence they’re attacking the destructive tree pests.

Since the emerald ash borer is not native to North America, it lacks natural enemies. The stingless wasps come from Asia, where emerald ash borers originated, and feed on ash borer eggs and larvae. The wasps don’t attack humans. They’re being tried in other states as well.

Minnesota Department of Agriculture staffers found wasp larvae in trees being sampled for emerald ash borers, or EAB, on Oct. 23 and Nov. 1. Federal experts confirmed they were the same species released in the area in 2011.

“This is a major step in our battle with EAB,” Monika Chandler, the department’s biological control program coordinator, said in a statement. “First, this confirms the wasps are attacking the ash borers and reproducing in the field. Second, we know the wasps are traveling. These wasp larvae were approximately a half mile from the nearest release site.”

Stingless wasps have also been released in Houston County in southeastern Minnesota and the Twin Cities in hopes of controlling other infestations.