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Enemy at the gates [Editorial]

Like the arrival of a Medieval plague, alien invaders are knocking on Baltimore's door. No, we are not talking about foreign armies storming the beaches or bug-eyed creatures from outer space bent on global domination. But it's almost as bad.

We are referring, of course, to the recent appearance in Baltimore of the emerald ash borer, a species of voracious Asian beetle that since 2006 has killed millions of white and green ash trees in its relentless march across North America. In June, city arborists trapped a couple of the critters in Druid Hill Park, a sure sign that more are on the way. If nothing is done, some 290,000 ash trees on city owned property could be at risk of being wiped out over the next few years.

Baltimore can't afford to let that happen, especially in view of Mayor Stephanie Rawlings-Blake's oft-cited pledge to attract 10,000 new families to the city over the next decade. Baltimore's tree canopy — a measure of the proportion of the city shaded by trees — is a critical element of the urban infrastructure that helps keep homes and businesses cool, cleans the air and water of pollutants and generally promotes a healthier, more pleasant environment for city residents. Without it, Baltimore's long-term viability as an attractive place to live and work is at risk.

That's why time is of the essence if the interlopers are to be repelled before they wreak havoc on our urban forest. As The Sun's Tim Wheeler reported recently, city officials are scrambling to develop a plan to protect Baltimore's ash trees before it's too late. Once large numbers of the stately trees are infested with the deadly pests it will be nearly impossible to save most of them.

In a nutshell, the city's current options range from bad to worse. It could try to treat newly infested trees immediately with chemical pesticides that kill the parasites before they seal their host's fate, or it could simply allow tens of thousands of mature ash trees to die and then replace them with a different species of younger trees that provide less shade but aren't vulnerable to the beetles.

Experience in other cities has shown that there's no practical way of completely eliminating the ravenous insects, whose larvae burrow beneath the tree bark and literally cut off the circulation of water and nutrients inside the tree. Treating the tree with chemical pesticides — either through spraying its leaves and bark or injecting them directly into the wood or soil — kills the larvae, but the different techniques and types of chemicals used in the process vary in effectiveness and cost.

Baltimore's chief arborist is currently weighing the various methods for controlling the infestation in order to present the mayor and City Council with a recommended plan of action by the end of the summer. But his task hasn't been made any easier by the fact that the city doesn't even have an accurate inventory of how many ash trees there are in Baltimore and where they are located — nor does his department have the funds to carry out such a survey.

As Mr. Wheeler reported, the cost of removing and replacing just the 5,000 or so street trees on city owned property could run into the millions of dollars. Treating them could cost considerably less, but questions remain about whether the chemicals might kill benign insects in the environment as well as the destructive borers and whether runoff from treated trees could threaten groundwater purity or leach through the soil as stormwater runoff to contaminate local tributaries to the Chesapeake Bay.

Advocates for treatment rather than removal and replacement argue that maintaining as many of the city's ash trees as possible will actually help reduce stormwater pollution because mature trees absorb a lot of water in their roots and branches, and rain intercepted by their leaves evaporates before ever reaching the ground. They also note that new trees planted to replace those that die or are cut down cast only a fraction of the shade of their predecessors.

These are all factors the city must take into account in determining the most effective response to the invaders, and there are serious risks and costs associated with whatever course the city eventually settles on. But doing nothing simply is not an option if officials wish to avoid the prospect of having a quarter million dead trees on their hands within the next three to five years.

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Copyright © 2015, The Baltimore Sun
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