Q. I recently started two dozen bonsai ash trees and am hoping you can offer some advice on overwintering them safely with the emerald ash borer so prevalent in our area. I plant all of my deciduous bonsai outside in their pots for the winter, but I'm concerned about keeping the ash trees safe from borer infestation. The trees are small enough that it would be easy to treat them, but is there anything organic that would selectively go after the borer? I'm guessing not, otherwise these trees wouldn't be facing extinction in the wild.
- ---Robert, who "Loves your show in Huntingdon Valley, PA"
A. Okay—first, a short class on the kind of bonsai he's talking about. In a nutshell, the art of bonsai involves pruning, restriction and training to keep what would become a full size tree much smaller. Some magnificent bonsai specimens are hundreds of years old and only a few feet high.
But there are two kinds of bonsai—indoor and outdoor. The indoor trees are varieties that can be grown as houseplants year round. The 'outdoor' ones have to experience the chilling hours and reduced daylight of winter, but as we have been warning about all potted plants, need to have their roots protected from freezing in the North. That means that knowledgeable growers with outdoor bonsai are busy burying their pots in the ground or planning to do so soon.
(I would personally do it now; if you 'plant' them in September, you'll get to watch their leaves turn color just like the big trees as they go naturally dormant.)
But, back to the topic: Ash trees absolutely are being decimated by this imported pest. And this question made me realize how woefully ignorant I was about the subject, so I asked Dr. Don Eggen, Forest Health Manager for Pennsylvania's Department of Conservation and Natural Resources to smarten me up.
He explained that the adult beetles emerge from infested trees beginning in late Spring and continuing through the summer. (An Ohio State Bulletin on the pests gives a 'first emergence' time range of early May in Southern Ohio to early June in Michigan.) Then they fly up into the canopy and feed lightly on the leaves of the tree.
This part was totally new info to me. I had thought of these pests only in their larval (borer) stage, but Dr. Eggen explained that it all begins with some minor-league leaf chewing by the adults right after they emerge. Now, this isn't where they do their damage; they're just nibbling a bit while looking for mates; but this is where a strictly organic control might be able to end the problem before they can get into your trees.
We'll provide the details on this exciting possibility in a moment; for now, let's get back to the basics: After mating, the females lay their eggs in the bark of an ash tree—presumably the one whose canopy they've been nibbling. The eggs hatch, and the larval stage of the beetle—the so-called "borer"—then feeds on the section that is the most important part of any tree; the area just under the bark that carries all the nutrients between the roots and the canopy.
At first, this just interrupts the flow of nutrients. Then it gets much worse. Dr. Eggen says that so many borers are typically carving out their distinctive 'S'-shaped galleries that they eventually 'girdle' the tree by destroying that precious layer under the bark in a complete circle.
And whenever that happens—whether it's down low from mulch piled against the trunk or higher up from borer damage—a tree that's girdled is a dead tree. And this borer is killing tens of millions of ash trees a year; what hope can a little bonsai have?
Actually, a lot of hope.
- Some outdoor bonsai are brought back indoors in the Spring—and it would seem easy to plan to do this before the pests typically emerge in your region.
- Even if the trees stay outside, their small size makes them easy to protect with floating row covers. Made of lightweight fabric, row covers allow air, water and light to reach plants, but keep insects off. I'd secure the covers completely around any trees that are still outdoors from early May through the end of July. The covers would prevent both leaf nibbling and egg laying.
But there is also a brand-new Bt that works against Japanese and other beetles, including some borers, and specifically including the Emerald Ash borer. Right now, the only place homeowners can buy it is from Gardens Alive, who sell it as "Beetle Jus". Our bonsai guy might want to hedge his bets by also spraying this new Bt on the leaves. Any adult beetles that feed on the sprayed leaves would die, ending the cycle before any of the damaging egg laying can occur.
This could be a whole new ball game. Yes, it's tough to repeatedly spray the canopies of mature trees with anything, but it's already being done on a large scale in many states with the old original strain of Bt—BTK—to try and limit the damage caused by the notorious Gypsy Moth caterpillar.
(Quick review: Bt stands for the basic organism, Bacillus thuringiensis, which occurs naturally in some soils. Different strains of Bt have different properties. The BTK strain kills caterpillars that eat the sprayed leaves. The BTI strain is used to prevent mosquitoes from breeding in standing water. And the new BTG strain only affects beetles that eat the sprayed leaves. Each Bt is very specific. For instance, BTK does not have any effect on mosquitoes, and BTI won't harm hungry, hungry caterpillars. And none of the Bts have any effect on 'good guys' like birds, bees, toads, pets, people or ferrets.)
And we repeat: the newest Bt—BTG—has been shown to be effective against the adult Emerald Ash borers while they feed up in the canopy. The pest is listed by name in the official EPA registration for the product.
Now: Dr. Eggen stresses that the only certain protection for a healthy ash tree is injection of a very specific pesticide directly into the trunk. The cost depends on the size of the tree, but it's in the hundreds of dollars; and it's a restricted-use pesticide that can only be applied by a professional. (He explains that the systemic pesticides that homeowners often buy are not nearly as effective, and can affect non-target organisms that visit the tree or the soil around it.)
Dr. Eggen adds that a treatment with the professional-only pesticide lasts three to five years; and only harms insects feeding on the inside of the tree. So I'm filing this under our "Common Sense Pest Control" category. Yes, it's a chemical pesticide, but one treatment lasts for several years and there doesn't seem to be much risk of it affecting non-target organisms.
Very important: Dr. Eggen stresses that you can't wait until your tree is under attack. Once attacked, a tree probably can't be saved by any kind of treatment. He feels strongly that the only certain way to save a mature ash is to have it treated before it falls under attack.
If you want to try the organic option, you'll have to have the canopy of your ash trees sprayed with the new BTG several times a season, beginning when the beetles typically emerge in your region. This should protect the sprayed tree and take a good number of these nasty beetles out of action before they can mate. Very exciting. If this treatment is adopted on a wide scale, it could wind up doing what our listener hoped for—saving ash trees out in the wild without chemical pesticides.
EPA Emerald Ash borer registration: