This is a yearly spring issue and can have several different causes.
First of all. Small prickles or dots appearing at leaves often is caused by aphids. Other bugs will eat the edges of a leaf.
Aphids and bugs in general likes tender new shoots and spring growth is a feast after winter dormancy. Leaves are tasty as the sugars are building up fast. Beech for example have tasty leaves, and you can eat them in a salad the first weeks after they open. Thereafter they turn bitter and insects will not like them. As you won’t.
As temperatures, rice bugs will reproduce rapidly. Look for them at the underside of leaves where they can hide.
This spring has been very cold and therefore bonsai also are weaker than normal, even if fed well last season. The awful cold unstable spring has set them back and make them more vulnerable to attacks.
Insecticides are one way to treat the attacks, and another method is flushing the leaves with water daily. Also from beneath to annoy and wash the insects away.
The advantage of the cold spring is that there will be fewer insects to attack our bonsai than normally. For the environment, this is worse, but that’s another story. On the other hand, fungus loves it.
The other part of the issue raised here is about fungus. This also thrives in this cold spring with a lot of humidity after several periods of rain.
Here treatment is trusted on fungicides. The only way to get that down when an attack is present. If it is powdery mildew low-fat milk sprayed daily will cure the mildew within two weeks or so. The milk makes the mildew break down and can be brushed off gently. Depending on the weather changes or not; else you have to continue the treatment as new attacks might occur. A very environmental friendly cure.
Or you can find products online or at garden centres. Other fungal diseases can be cured with Limesulphur diluted 1:10 with water and sprayed on the attacked areas. Avoid it going into soil and roots by covering the surface when spraying.
To avoid both types of attacks, aphids and fungal diseases, we have to look at how we grow our trees. How we arrange them in the garden or elsewhere.
Healthy trees are less vulnerable to any attacks. A bad spring like this year will set back some trees health anyway though. But else, it is important to keep trees strong growing and healthy. This will lower the attack rate. Some species are more prone to attacks of aphids than others though. So watch out especially for those trees.
Many trees have a natural defence mechanism build in. Healthy trees will react to attacks changing their sugars that are extra tasty in the spring, so they are less tasty when experiencing an attack. But bonsai are not as strong growing as trees in nature, so they are more likely to be attacked and their ability to withstand attacks is not as good as big trees. Just the small amount of foliage makes them vulnerable because only a small group of insects can rip off the leaves and fungus will quickly spread over a small area.
Therefore we need to protect the best possible. One way is placing bonsai where they receive optimal light so they grow strong. Next is to place the bonsai where it is a little windy. Not that they should blow off the shelves of course, but if the air is not moving humidity will be high and fungus will love it. So will bugs. Some air circulation is necessary for a healthy bonsai environment. Maybe the most important aspect of placing your trees is having some air circulation. Both to protect against insects and fungal attacks.
So the answer is all of this and better weather. 🙂 Hope this helps a little. good luck with the trees.