Purple tent-like devices designed to detect emerald ash borers will be a far more common sight around Minnesota this spring and summer as the state Department of Agriculture renews its efforts to track and head off the destructive, invasive pest.

The department will hang about 6,500 of the traps -- nearly 50 percent more than last year -- in trees around the state starting Monday.

The traps will be hung in ash trees and baited with a lure that smells like stressed ash tree to the beetle, which is commonly attracted to already weakened trees. A sticky layer on the outside of the trap holds the beetle until workers return to check for bugs.

The traps aren't an eradication tool, and only trap a few ash borers, if any, during a season. They are instead intended to alert the department to where the borers might be expanding their range. Citizens are asked not to disturb the traps.

The emerald ash borer is believed to have arrived in the United States in the Detroit area in shipping materials from China years before it was identified in 2002. By then it had already killed millions of ash trees in southeastern Michigan and has since been tracked across 15 states, as well as Ontario and Quebec.

Devastation elsewhere has left Minnesota with the most ash trees of any state -- nearly 1 billion. They comprise 7 percent of the trees in Minnesota forests and 15 percent of trees in cities, where they were a popular replacement for elm trees lost to Dutch elm disease. Ash borers were first detected in Minnesota near Hampden Park in St. Paul on May 14, 2009. Four counties where the beetles have been detected -- Ramsey, Hennepin, Houston and Winona -- are under restrictions limiting the transport of ash wood.

Emerald ash borers were found in Winona County last summer because of a positive find in a trap. The four quarantined counties will not be getting traps this year because the bugs have already been found there.

Minnesotans have been asked to help by watching for signs of borer damage on their ash trees. Potential signs include woodpecker damage, especially at the top of the tree; bark cracks or splits; S-shaped galleries under the bark, and die-back of leaves in the upper one-third of the tree branches.

Adult ash borers lay eggs in ash trees. The eggs develop into larvae, which drill into the tree and grow under the bark, usually emerging as adults the following spring, which is when they are most often detected.

Ash trees in North America have no resistance to ash borers, as they do in their native Asia. Researchers are slowly releasing several species of predatory wasps from Asia to contain the spread of ash borers, but it's expected to be years before they reach a balance with the ash borers.

Questions about the traps and the trapping program can be directed to arrest.the.pest@state.mn.us or 888-545-6684. For more details about emerald ash borer, visit the state Agriculture Department website at www.mda.state.mn.us/eab.

Bill McAuliffe • 612-673-7646