Why do bonsai get damaged by insects in winter?
Although it is winter, and many of our trees are dormant, and very little activity is shown outside, a lot of activity is taking place. The bonsai are adjusting to changes in temperature, but also insects are staying alive, trying to find their way through winter. And one of the ways ia by feeding on the roots and sap of our bonsai.
Of particular worry are the vine weevil and scale. The larvae of the first, and the adults of the second stay alive by feasting on plants, and may cause you to find a severely weakened, or even dead bonsai come spring. Vine weavil larvae live on the roots of plants. Of particular interest are taxus and olives, I have found. The scale insect may sit on any plant with an active sap flow, so mainly the evergreens, again, Yews are a common host.
If you are keeping your trees in winter in a greenhouse or similar location, there is a risk of other plagues. Particularly spider mites –although not insects!- like the wind-still often dry conditions of storage spaces. And as these places sare often several degrees above 0, snails are also active in them in winter.
What to do to avoid bonsai damage due to insects in Winter
Prevention is the key. In fall, once temperatures start to drop towards 10C verify that all your trees are free of scale. They may be hard to spot. However, in winter they do not spread that much. So if you can ensure your trees are clean now, chances are they will remain in the clear till springtime.
In order to avoid early spring problems from insects hatching from eggs, there are several products available, typically available under the name of winter oil or soap. These products are sprayed over the whole plant and suffocate any eggs and smaller insects present on the areas hit with the spray. Similarly, one can opt for spraying the tree with diluted Yin seal. This lime sulfur can be diluted by a factor ~20 and used as an effective winter spray. In both cases, spray till the tree drips, and do this in a period where you can keep the tree dry for a couple of days afterwards. Note that lime sulfur is bad for fungus, and avoid getting it onto the bonsai substrate.
In order to combact larvae in the soil, one can use a general purpose insecticide in water and drench the soil in fall, if presence of bugs is suspected. Typically, one will spot tell-tale signs of the adults around the plants during summer. If you do not have unexplained leave-edge damage to your plant, typically the roots will be fine.