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Hortiscope: Why emerald ash borers have a tree-specific fetish

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Editor's Note: We usurped Ron Smith exclusively this week to ask something readers have been wondering about.

As the emerald ash borer begins its march north, the water cooler talk has been: Why ash trees only? Are maples safe?

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For that matter, why do certain types of beetles cause Dutch elm disease, a fatal ailment that does not affect other specie, readers wondered. This is what Ron told us.

Some insects like the emerald ash borer evolve as a "species specific pest" - that is, they are genetically wired to be attracted to that one species for feeding and reproductive purposes.

No other tree species will do - it must be an ash species. If the ash were gone completely, the pest would likely disappear as well unless they mutated to be able to feed on something else in the plant kingdom. While the adult - really a beautiful insect - feeds on the foliage, that damage is negligible.

The larvae (the immature stage) feed on the inner bark of ash trees, disrupting the tree's ability to transport water and nutrients.

Emerald ash borer probably arrived in the United States on solid wood packing material carried in cargo ships or airplanes originating in its native Asia. Emerald ash borer is also established in Windsor, Ontario, was found in Ohio in 2003, northern Indiana in 2004, northern Illinois and Maryland in 2006, western Pennsylvania and West Virginia in 2007, and Wisconsin and Missouri in summer 2008, and this past year in Minnesota. Since its discovery, EAB has:

-Killed more than 40 million ash trees in southeastern Michigan alone, with tens of millions more lost in Ohio, Illinois, Indiana, Pennsylvania, West Virginia, Missouri, Wisconsin and Virginia.

-Caused regulatory agencies and the USDA to enforce quarantines (Indiana, Illinois, Maryland, Michigan, Ohio, Pennsylvania, West Virginia, Wisconsin, and Missouri) and fines to prevent potentially infested ash trees, logs or hardwood firewood from moving out of areas where EAB occurs.

-Cost municipalities, property owners, nursery operators and forest products industries tens of millions of dollars.

Obviously, not a pest to be taken lightly as it moves east, south, and west into our territory. There is 'good news and bad news' associated with this pest.

It is impossible to try and get a program going that would be economically and physically feasible to try and save the ashes in the forests - that's the bad news. The good is, that specimen trees can be saved in municipalities via injections similar to what they did with the DED (Dutch Elm Disease) - with the cost being a limiting factor when 100's or 1000's of trees are ash in any one community.

DED is carried by two species of elm beetles; the native and European. Both have the capability of infecting susceptible elms with this destructive fungus if they are carrying the spores on their bodies. The feeding activity of the beetles by itself is not lethal to the tree, as the elm beetle and elm have evolved over the eons without disastrous effects.

It is when the fungus arrived in America on some infected elm logs that the disease took off. The operational word here is "susceptible" - as the research that has been carried on found that some trees were immune to the pathogen and could be propagated asexually, while other research crossed different elm species to find a genetic combination that would be resistant.

In addition, the elm beetles seem to be a little more finicky in its selection of feeding sites - often preferring smaller "under-story" trees to the larger more "over-canopy" ones. Such is not the case with the EAB - it will attack the ash tree - period! Under-story or not; healthy or not, large or small!

To contact Ron Smith for answers to your questions, write to Ron Smith, NDSU Department of Plant Sciences, Dept. 7670, Box 6050, Fargo, ND 58108-6050 or e-mail ronald.smith@ndsu.edu.

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