MINNEAPOLIS — Minneapolis officials are warning homeowners against pruning their ash trees.

Even with this late spring, conditions are warm enough for the beetles to attack and kill ash trees. The invasive pests are normally active from May through Labor Day. The city says it’s important not to prune ash trees now or move any parts like firewood or branches while the pests are active. That’s because moving the pests helps them spread to uncontaminated areas.

Emerald ash borers have killed millions of ash trees in 13 states, and ash trees make up about 20 percent of the tree canopy in Minneapolis. The Minnesota Department of Agriculture has imposed a quarantine on firewood, ash trees, and ash tree products in Hennepin, Ramsey, Houston and Winona counties to slow the spread.