A state quarantine has been issued on firewood, ash trees and ash tree products in two Minnesota counties after this week's discovery of emerald ash borer.

The quarantine took effect Friday in Hennepin and Ramsey County. An infestation of the tree pest was discovered the day before in a line of trees outside a row of St. Paul town homes.

Minnesota agriculture officials say the quarantine will help slow the ash borer's spread.

The state has an estimated 900 million ash trees.

A quarantine was previously announced for Houston County in southeastern Minnesota after an ash borer infestation was discovered across the Mississippi River in Wisconsin.