A tree care company reported finding larvae, and a USDA lab has made a preliminary confirmation that they're the young of emerald ash borer beetles.
No one is sure how the pests made it to St. Paul, but one possibility is firewood moved from an infested area. Minnesota Agriculture Department spokesman Michael Schommer said that's always a bad idea.
"Don't move firewood, regardless of whether emerald ash borer is in your immediate area," Schommer said. "It's not a good idea because many forest pests can spread that way."
Schommer said the green beetles have probably been in the trees for several years.
"Obviously, it's a time for folks to be watching their ash trees," Schommer said. "If they do see warning signs, it's appropriate to call their city forester of the extension service or the Department of Agriculture; we have resources we can offer. But also if you're planting new trees, it's certainly a good idea to consider species other than ash at this point."
Officials will survey the trees in the area to decide on other responses to the pest.
Last month, a similar quarantine was imposed in Houston County, because officials feared the beetle had crossed the Mississippi River from Wisconsin.
Emerald ash borers have killed millions of trees in ten eastern states. Minnesota has an estimated 900 million ash trees.