Growing bonsai? Knowing about plant pests comes with the territory! Because we use many different kinds of plants to create bonsai trees, there are many different problems to watch for.
Not all bonsai are susceptible to the same diseases and insects. Each time you venture to another species, research the potential pests and diseases for that plant.
Spider mites are less than 1 mm (0.04 in) in size and vary in color. In the world of bonsai, mites rank pretty high on the 'bad bug scale.' The type of mites we see most often are red spider mites. They are barely visible, but can be easily spotted on a piece of white paper.
We often assume mites only attack our juniper bonsai, not so! They like everything from regular house plants to trees in the yard. Their feeding causes the plant to appear off-color and eventually turn completely brown. Another symptom is the webs they weave (where they lay eggs).
Mites can kill plants quickly. When you have a bonsai that has a gray caste to it or one that just doesn’t look quite right, suspect spider mites.
Mealybugs are soft-bodied insects that are often covered with cottony white filaments. You may be surprised at how many different kinds there are.
In this enlarged photo they are shown on asparagus fern. They are especially fond of foliage plants and will often show up on bonsai Ficus varieties. (Especially indoors.)
The mealybug is something you want to catch early, they are one of the causes of black sooty mold!
Another terrible invasion of plant pests can come from root mealy bugs! They are worse than the above ground kind, and very difficult to see!
(The soft bodied mealybugs (shown above) are also considered a type of scale. Often spraying a contact pesticide is not enough. Horticulture oils and systemics work best. )
Borers nearly always attack unhealthy or stressed plants or trees. Although you may think your tree is healthy, what about the injuries from cutting? These 'wounds' are perfect entry places for borers.
Not as common as many other plant pests, however, borers are always something to be aware of. Some bore deeply into the wood, however, many bore just under the bark.
Keep your eyes open for "frass." Frass is excrement from what larvae have eaten. When they are eating woody plants, the refuse left behind is much like sawdust. If you see any "sawdust" on the soil, draw a visual line straight up to the trunk or branch above. Look for a tiny hole, although sometimes they are barely visible. When you see these signs, find the soft area.
Try to gouge out the area as soon as possible. Don't stop digging around soft spots until you find something. A beetle and/or larvae is almost always still there! You may end up with a terrific shari or as in the ugly damage shown here, you may not be so lucky!
Nashia inaguensis (Bahama Berry) is one species of plants Pit Scale is particularly attracted to. It can be devastating.
Sporothrix schenckii fungus - This is a dangerous pathogen for people ! Linked to sphagnum moss and thorns such as those on Bougainvillea. Especially if you work with sphagnum moss, this is a must read article.
If you don’t know what’s crawling on your tree, or why your tree is doing something weird (like wilting or dropping leaves,) always ask! Each time you venture to another species, research the potential plant pests and diseases for that plant.
Remember, there are reasons for some plants to wilt or drop leaves. Never spray pesticides first!
Ready for some good news? There are more plant "beneficials" than plant pests. Some you may not recognize and others you may have wrongly decided are bad, could be beneficial bonsai care helpers!