Allamakee County was declared infested in May 2010, Des Moines County in July 2013, and Jefferson County in August 2013.
The emerald ash borer kills all ash species and is considered to be one of the most destructive tree pests ever seen in North America, the state agency said.
With four total ash borer finds in eastern Iowa, officials are considering a regional quarantine to slow the accidental movement of the pest by humans.
This regulatory action restricts movement of hardwood firewood, ash logs and wood chips out of the quarantined counties. "I think we're seeing the culmination of an EAB population that is finally large enough to detect, coupled with trees readily showing symptoms because of multiple stresses, including EAB, drought and floods occurring in recent years," said Robin Pruisner, state entomologist.
Pruisner said all Iowans are strongly cautioned not to transport firewood across county or state lines, since the movement of firewood throughout Iowa or to other states poses the greatest threat to quickly spread ash borer even further.
"Preventive treatments next spring - mid-April to mid-May 2014 - are available to protect vigorously healthy and valuable ash trees within 15 miles of the known infested area," said Mark Shour, an ISU Extension and outreach entomologist.
Ash is one of the most abundant native tree species in North America, and has been heavily planted as a landscape tree in yards and other urban areas. According to the USDA Forest Service, Iowa has an estimated 52 million rural ash trees and approximately 3.1 million more ash trees in urban areas. It is unknown how many public and residential ash trees are located in Mechanicsville.