We are fortunate that our bonsai are usually not heavily bothered by bugs. Generally, they are free of bugs, but what do we do when we find that our prized bonsai is covered with them?
It is important to examine your trees frequently. If you find that there is evidence of bugs, the first thing to do is to ask whether the bug was just passing through, or the bug has called in the whole family to set up housekeeping. If, for example, the problem was caused by a grasshopper who stopped by your place for lunch on his way to Hollywood to become a movie star, the best thing to do is nothing. Chalk up the couple of munched leaves up to the cost of growing your bonsai outdoors. Beyond encouraging them to leave, the occasional caterpillar or snail can be treated the same way.
If you find that the bugs have set permanent headquarters, then you need to act quickly because the insect infestation can grow rapidly, and the worse the infestation, the more difficult it will be to treat. The obvious solution to the bug problem is to make them go away, so the first choice is to simply hose them off. This approach works well for aphids.
When we do have bug problems, the frequent cause is not aphids but some species of scale, and these look like little white puffs of cotton. Unfortunately, hosing does not work because the little varmints stick tightly to the foliage. This brings us to the use of insecticides.
We have our choice of two different types of insecticides based on how they work. The first group consists of various soaps and horticultural oils. This group works on the outside of the bug by clogging the pores the bug uses to breathe, and so the insect dies from lack of oxygen. People, kids and pets do not breathe using this kind of pores, so this group of insecticides really doesn’t affect us much unless we happen to fall into a vat of the product. While anything can be toxic when taken the wrong way, this group is labeled nontoxic and you are pretty much able to use them any way you wish.
The second group of insecticides is labeled as harsh chemicals by the Master Gardener program and consists of products which act inside the bug. Most in this group act by jamming the place where nerves join muscles, and since impulse transmission is blocked, the bug dies of paralysis. Unfortunately, we have the same kind of muscle-nerve junctions, and if we are not careful with the product, we can have problems, too. This kind of product, which is available over the counter at local nurseries, has modest toxicity and can be safely handled with a minimum of care. Rubber gloves and a cloth mask will see you safely through a spraying. Malathion is an example of this group.
So we now have a bonsai with bugs and two choices of insecticide types. Which should we use? Each has strengths and weaknesses. The nontoxic group is safe for kids and pets but works by directly contacting the bug. Many bugs, and particularly scale, hide under the foliage and are missed by only one spraying. Further, the bugs may have laid eggs that hatch after you spray. You may need to spray several times weekly to totally eradicate the bugs.
The harsh chemicals leave a residue on the foliage and any bugs you missed by directly contacting them with the spray will be done in when they crawl across the leaf. Because of this, these products are more effective in dealing with a severely infested plant. The residue eventually gets washed off when you water your tree.
On the down side, the products must be used carefully because of their toxic properties. It is frequently noted that the harsh chemicals cause a lot of collateral damage by also doing in the “good” bugs as well as the “bad” bugs, and this is a justified claim. “Bad” bugs eat your plants and “good” bugs eat the bad bugs. When there are enough hungry “good” bugs in your plants they will solve the problem of insect control for you. It should be noted that the nontoxic group will also do in the “good” bugs if they get contacted with spray. If you choose to use a harsh chemical, the most environmentally friendly way is to spray only infested trees.
The use of insecticides can be a very emotional subject, but with a bit of care in the use and a bit of understanding in how they work, they are effective products. Severe bug infestations can be life threatening for your tree, so insect control is an important part of the culture of bonsai.