Larch Borer Damage

In keeping with the larch posts, I thought I would talk about what is perhaps the most infamous pest of Larix laricina – borers. Borers are insect larvae that chew on the living tissue of trees as they tunnel around inside the trunks and branches, invisible to the naked eye. The parent insect (usually beetles or moths) lay their eggs inconspicuously on the bark of the tree and, upon hatching, the little demon-spawn chews its way in.

One interesting thing about borers is they are usually only a problem on weaker trees with a subdued systemic defense. Strong trees with a heavy sap flow are better able to restrain a potentially disastrous borer infestation. What does this mean for bonsai growers? Borers are most likely to gain the upper hand on recently collected trees, or very mature and slow-growing bonsai. In other words, borers pick on the weak and the elderly, making them complete assholes. This source says that “the chewing of some [borer] species may be heard by someone standing near the tree”. Ok now that is just scary.

Most often, a borer infestation is not identified until the damage is done. Just one enterprising borer could potentially girdle a trunk or a branch, causing the sudden death of a huge part of your tree.

Nick Lenz is a veteran of the borer wars, and describes in detail his approach to dealing with them in his book Bonsai from the Wild (2nd Ed.). If you live in the North East and grow any native species, especially larch or cedar, you need this book! Essentially, Nick’s approach is to bag and fumigate every larch he collects, assuming it is infested with borers. I have not been this aggressive with my collected larches (yet), although I have been treating them proactively with a systemic insecticide (imidacloprid). Hopefully I don’t regret my complacent approach in the future…

What Nick does not talk about in his book is perhaps the most alarming cases of borer damage – that on mature and established bonsai. My friend and bonsai mentor has experienced this problem this spring in a big way. Here are some pictures of what you hope to never see on your larches.

Cracks in the bark? Curious...

Cracks in the bark? Curious…

    Bark flakes away easily revealing...

Bark flakes away easily revealing…

    Large dead sections where the borer has been snacking.

Large dead sections where the borer has been snacking. You can even see the borer’s tunnel. Sawdust is a characteristic sign of borers.

    Another old, established larch with some cringe-inducing borer damage.

Another old, established larch with some cringe-inducing borer damage.

 

    It is difficult to see in this picture, although the borer trail extends well down the trunk leaving a trail of devastation in its wake.

It is difficult to see in this picture, although the borer trail extends well down the trunk leaving a trail of devastation in its wake.

 

Fortunately these trees have not yet shown any signs of branch dieback due to the borer damage, although the scars alone are rage-inducing. He treated the infected trees according to Nick Lenz’s prescription outlined in his book.

While I have not personally seen borer damage on my larches, my friend’s experience here has definitely raised my level of vigilance and hopefully yours as well.

6 Responses

  1. crust

    So when do you think the borers from hell descend upon the bushes?

    May 12, 2013 at 1:43 pm

    • I really have no idea. I would just assume the worst and expect them all through the growing season.

      May 14, 2013 at 9:13 pm

  2. And that’s why I hate pest! Once the borer is in the wood, what can you do to kill it?

    May 12, 2013 at 11:16 pm

    • I think best is to prevent it, since it is so hard to know when they are present until it is too late.

      Preventative use of systemic insecticides (imidacloprid) is where I start, but I’m not sure how effective it is. Maybe I’ve just been lucky so far.

      May 14, 2013 at 9:15 pm

  3. Nigel Saunders

    I lost 40 percent of my trees in one day with these boring beetles. I first noticed a young maple tree wilt in the afternoon, it was well watered, so I exaimed it closer and noticed a small pile of sawdust at the base of the trunk and one tiny hole in the bark. I checked the rest of the trees, and just under half ( about 40 trees) had the same signs. They attacked almost every species I had, larches, ash, maple, apple etc. I tried injecting alchol into the holes with a needle, this caused many of the beetles to back out of the holes, some holes had two inside! The trees that got attacked all died, some quickly, others took a week. I had never had this problem in my 15 years of bonsai and have never had this problem in the last 7 years. I checked all the trees after they died and was amazed how long and intricate the passages were, and that the beetles carved them under the bark in just a few hours!

    May 25, 2013 at 4:00 am

Leave a Reply

Your email address will not be published. Required fields are marked *