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.
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.
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.
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.