Beetle Borers and Bonsai-

One of the truly irascible jerks of the insect world (if you love trees, that is), borers will redesign your bonsai without even asking permission. Even worse, they’ll do it without you even being aware there is a major change in progress, since all their nefarious nibbling is done under the cover of bark.


Borer larva in its gallery under the bark, where it eats the phloem.

Most commonly it is flathead borers that cause problems for bonsai. The lifecycle is, for most species, one generation a year. Eggs are laid in the spring. Borer larvae nibble through the phloem of the tree in an ecstatic sugar festival over the summer. After girdling several branches or maybe even the trunk, they will drill deep into the heartwood and pupate there snugly and read trashy novels until spring. They then emerge transformed into the mature winged adult who is ready for the great epic poems and then, having learned nothing of aesthetics, ethics, or propriety, go on to lay eggs on your favorite bonsai. It’s a sad cycle.


Adult borer. There are more than 150 species of borers, some much more colorful than this one.

Flathead borers attack deciduous as well as conifer trees. We have one out here in Oregon (Northwestern USA) that will go after almost any conifer they can find. For the most part, if the tree is strong, the borers ignore it. Stressed trees, weakened by sun or drought, are the most commonly attacked trees. I lost a weakened juniper one year from a borer that girdled the trunk, but this summer lost the top of a relatively healthy hemlock, which gave me pause. Borers cause the majority of ‘naturally created’ jin and shari on collected pines and junipers.


Flathead borer larva that Bobby found in the top of one of our Mountain Hemlocks.

Keeping ridiculously vigorous bonsai does not seem like the best method of prevention, since most bonsai maintenance techniques are designed to slow down the metabolism of the tree—which makes them more susceptible to borer attack. Keeping healthy trees should certainly help, but prophylactic attention might be warranted in areas of high borer activity.

If borers are present where you live, you might consider a systemic like Safari. Befriending a woodpecker is another possibility, and while you’ll not have any more borer problems, you’ll still, sadly, have a tree full of holes.

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  1. You are a poet – thank you for another fine read 🙂

  2. Sharon says:

    So beautifully written! The teacher so enjoys reading your work, not to mention the information! Your teachers must be proud😉

  3. Daniel Dolan says:


    Thank you for this informative report on Borers. Luckily, in Chicago we have an ample supply of Ash trees to supply our principal borer with food for years to come.

    Now ……back to the rules of Bonsai.

    Most Bonsai practitioners appreciate that an occasionally violable precept of classical instruction is that secondary and smaller branching should be developed on the outer, convex curve rather than the inner curve of the larger branch.

    Though we accept that a certain tree might be improved overall if such a branch was maintained.

    In your experience, if a branch is removed from this interior curve of a conifer……is this an opportunity for Jin or in most cases not?

    Best regards,


    • crataegus says:

      Sorry to offer a vague answer, but it really depends…I would never use the word ‘most’ in this case, it ‘may’ however be appropriate. There are no rules with regard to jin. Some conifers seem really wrong with jin. Others seem to be begging for it. That is more of a gut decision. And then the issue of the interior curve—about the only generalization you can make is if you wish to represent a branch that died as a result of a lack of sun, then yes, and otherwise if you wish to have a jin that died as a result of external environment, then seek a jin to make on the outside of the tree. Hope that makes sense-

      • Daniel Dolan says:


        Thank you for your advice……..wasn’t sure is this was one of those….”Don’t do this” situations……like pinching Shimpaku.



  4. terry davis says:

    Old USDA bulletin reported that NEEM nails borers even inside the wood. It acts as an antifeedant, too, to repel attacks.

  5. Kevin Faris says:

    Oh man. I just had my first borer disaster on one of junipers. Is it likely if you do not find living borers but you have borer holes they have “flown” the coup per say? I spent a good long while reducing live vein and following the path of borer highway of food a.k.a borer devistation and much to my anticipation found no borer larva. I did look on other branches and in crotches of branches surrounding the borer sites but no borers emerged. Any ideas?



  6. crataegus says:

    Hola Kevin, Yes, to answer your question, they will leave the big hole long after all the damage is done. They were eating cambium and phloem last year. The large holes are from the adults emerging, who have chewed their way through your plant to the outside, leaving that oval hole. Sorry for the trouble!

  7. Kevin Faris says:

    Well that’s good news. Haha.

  1. […] as not to turn completely away from bugs today, do read Michael Hagedorn’s recent post on wood-boring beetles if you haven’t – it’s a good read about a pernicious […]

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