This is another insect that can be a problem on bonsai. Scale insects come in a bewildering array of shapes and sizes and can quite often go unnoticed until the infestation becomes quite established. The great thing about scale is, unlike the passion vine hopper they tend to stay quite still while you hit them. Scales are divided up into four groups; the armored scale, the soft scale, the felted scale and the naked scale. What they all have in common is they all do damage to your tree.
The scale you can see are the mature females. They build their protective cap under which they suck the carbohydrates from your tree and lay their eggs. These eggs hatch out into tiny mobile crawlers this stage has legs and will shoot off looking for a suitable site to settle down and build a home. This stage is so small they can be picked up in the wind and carried quite a large distance. After finding a suitable site the armored scale will go through a molt at which stage it will loose its legs and set up shop. Some of the other scale insects retain the ability to move until mature. Some of the larger armored scale can cause a dimpled effect of leaves and a heavy infestation can significantly reduce the vigor of your tree.
To control scale spray with an organic mineral oil this will kill the crawler stage and block the pores in the scales armor suffocating them to death. This won’t however kill the eggs so more than one application will be required. Check to make sure your tree will cope with being sprayed with oil, some trees won’t. The other method on small bonsai is to squash or flick them off with a small knife or finger, again more than one treatment will be required.