Plant Pests and Bonsai

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.  

Most Common Pests

  • Mites - According to Wikipedia "Spider mites are members of the Acari (mite) family Tetranychidae, which includes about 1,200 species.They generally live on the undersides of leaves of plants, where they may spin protective silk webs, and they can cause damage by puncturing the plant cells to feed. Spider mites are known to feed on several hundred species of plants."
Greatly enlarged photo of a red spider mite

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.

  • Ants - Ants do not eat plants.  However, they can be damaging plant pests . . . they can cause pockets in your soil, spread to other pots and worst of all, they arrive for a reason Ants are most often attracted by aphids.

  • Aphids - are one of the most common problems for people growing plants in general. Many bonsai are susceptible to this ‘plant lice’ with piercing-sucking mouth parts. If that sounds scary, read more about Aphids here. 

  • Mealybugs - are a well known common pest.

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!

  • Scale - Scale is a sucking insect. It has many appearances but It often appears as a light brown bump or white spot on a leaf or stem. Many scale insects have a waxy covering. Some even look like small oyster shells.  They remain immobile for the most part and are easy to spot.

(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 - are truly horrible plant pests. One that is often missed!  Virtually all woody plants and trees are subject to borer attacks. If you're growing bonsai trees, you can have borers!

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!  

See these pictures of a juniper twig girdler (Periploca nigra) from the curator of the national collection in Washington, DC.

Not Too Common Plant Pests

  • Pit Scale - Every once in a while you may come across really odd plant pests, be sure to check it out! One example that rarely shows up is "pit scale".

Nashia inaguensis  (Bahama Berry)  is one species of plants Pit Scale is particularly attracted to.  It can be devastating.   

See a full description on Nashia page.

  • Witches Broom - This is a tightly clustered, abnormal growth of multiple branches on a tree or woody shrub. These growths are often caused by different undesirable organisms. Many are contagious to other plants!

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.

  • Vole - Another disaster in the making (other than "bugs") is a vegetarian critter called vole, no, not mole.  Guest Author Lee Squires wrote an article on how he lost many of his bonsai to an:  Invasion of the Voles!

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!

Where to Go From Here

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! 

Beneficials Page

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