From Caitlin Lenahan at Tree Pittsburgh:
“The treatment option for emerald ash borer (an applied insecticide) can be expensive and may only be worth it for a large tree, but each person’s perception of what is “worth it” is different. The pesticide can only be applied by a tree health professional who is certified to use it, as it is very toxic. It is also either injected into a tree by drilling around it and injecting the tree with the pesticide, or it is added to water and the root zone is drenched (injection is a more sure-fire method, as you will know how much of the pesticide the tree actually gets)… You have to apply the pesticide BEFORE the insect ever gets to your tree. Time is running out for that option…
I do encourage anyone with a concern about the trees in their area to register for a Tree Pittsburgh Tree Tenders course… there are only two left in 2011… We cover pests and diseases extensively, as well as other tree care issues included utility line pruning.
I hope this helps… the best way to combat the ash borer, in my opinion, is to consider treating your tree for a few years if you like, but also replant an equivalent tree near it that will grow to similar size in time. That way, if/when the tree is lost (all living things do eventually expire) there will be another one nearby ready to shoot up in its place.”