Unless it's early spring, and you're growing willow, it's almost always bad to see white fuzzy stuff on your tree. (willows make beautiful catkins that are fuzzy and white which is what is pictured above.)
Mealy bugs are soft-bodied insects that suck on plant sap like aphids. The females are what are normally seen - crawling around on plants covered in white fuzz. The egg masses that females lay can often look like mold growing on the bark of trees, or the undersides of leaves.
Ants farm mealy bugs, so if you see ants inspect hidden areas of your plants to see if there are insects that the ants are tending - for the purpose of collecting the waste they create.
Mealy bugs come in a few different varieties, including tiny ones that mainly inhabit the soil, rather than the tops of plants.
We propagate and sell a few varieties of plants that are well suited to indoor bonsai growing; among them is rough-bark dwarf jade (Portulacaria afra.) When kept indoors the natural predators of mealy bugs like birds, spiders, and many insects are absent, allowing the population of the pests to proliferate.
To kill mealy bugs, try drenching the plant in a soap spray (like Safer Soap) or by diluting 1/4 Tsp of dish soap (not detergent) in a quart of water. Soaking the entire rootball in soap, and drenching the entire top is the best way to knock back mealy bug populations. Repeat this exercise at 2 week intervals until you no longer notice any bugs.
Because mealy bugs are very good at hiding, dense foliage, dense twigs, and even the rims of plastic pots and similar spots make good protection. Do not assume that anything short of dunking the entire plant will reach all the crevices they are using for hiding spots.
Mold on the surface of Portulacaria afra bark is not a common occurrence - if some part of the plant has died due to frost or another issue, mold can get a foothold, but in normal conditions, any white on the trunk is more likely to be some sort of pest, either the insect itself or eggs.
Portulacaria are quite drought tolerant and like hot dry conditions alternating with enough water to replenish their internal stores. Because mealy bugs are not able to survive the hot dry conditions like portulacaria, another way to deter mealy bugs is simply to let the plant dry out for a week or two in a bright sunny spot. (Note that doing this with other bonsai species will result in plant death!)
Do you have a slam-dunk method for getting rid of mealy bugs? Leave a comment below!