Misting your tree’s foliage with soapy water once or twice a month is a good preventative against insect problems!
Q: Are Bonsai more susceptible to insects if you leave them outdoors?
A: Not at all. I would say that the opposite is true. The tree outside is so much stronger that it is naturally much more insect resistant, and no house is really insect-proof. There is really no place where you can keep your tree completely isolated from insects.
Q: What is the best way to check for/monitor insects?
A: There is no substitute for looking at your tree often, monitoring it closely, coming in contact with it regularly to check for any signs of insects.
A good way to check for insects is to shake the tree foliage over a sheet of paper. If you notice tiny specks moving around on the paper, there are insect pests present on your tree.
Q: Will my indoor trees be infested more easily than my house plants?
A: No. But the same conditions that allow insects to thrive on your house plants will make them show up on you Bonsai as well. The common insects that bother house plants will also attack Bonsai, so your trees are always potentially at risk to a certain extent. Spider mites, aphids, mealy bugs, are all pests that infest Bonsai. Fungus gnats are another one.
Q: What can I do to minimize insects?
A: The best thing to do, as I said before, is to keep a close eye on your trees, and try to provide optimal conditions for them.
Misting is crucial, as is keeping the trees in a well-ventilated room. In fact, your misting bottle is in many aspects your number one Bonsai tool, even before a pair of scissors or a watering can.
It is inevitable that you will run into insects again and again over time, and it may be that sometimes you won’t notice an insect until damage has already occurred to your tree. That is why it is a good idea to give the occasional preventive spray, maybe even as frequently as once or twice monthly, with an organic insect controller.
One product that is widely available and has proven reliable is called Safer’s. There will be a white residue of the spray building up on the leaves; it is not dangerous, but neither is it pleasant to look at. So rinse your trees off – especially any larger leafed and tropical varieties – with a sink spray attachment or even under the shower now and then. They will be all the better for it. Many household sprays such as "Shultz-Instant" will also do the trick.
A lot of times you can actually use insects as a guide toward correcting a problem. Most insects that infest a tree will keep coming back if they are there in the first place because of poor conditions: not enough light, overcrowding, not enough air flow.
Some of our most successful Bonsai growers are people who have had insect problems and have brought their tree into our clinics on a regular basis, where we have coached them in what to do to keep the insect in check.