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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. Any site about bonsai plant care is not complete without this discussion.

The biggest problem is: not all of them are susceptible to the same diseases and insects.

Juniper bonsai trees - such as this cascade by Ed Trout are susceptible to red spider mites.

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 good reasons for some plants to wilt or drop leaves. There are also many more good bugs than bad.  If you don't know, never spray pesticides first.

Most Common Pests

  • Ants - Ants do not eat plants.  However, they can be damaging because 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.

  • Mites - In the world of bonsai, mites also rank pretty high on the bad bug scale. They appear on temperate, tropical, indoor and outdoor plants. The type of mites we see most often are what we call red spider mites. They are barely visible, but can be easily spotted on a piece of white paper.
Enlarged mite illustration
by Eduardo Varona

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!

There are many pesticides to use – organic and chemical. One good choice is the Safer Brand Insect Killing Soap, Ready to Use Spray
  • 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 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; 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 are 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 something really odd, be sure to check it out! One example that rarely shows up is "pit scale".

Nashia inaguensis is one of the few species Pit Scale is particularly attracted to.  It can be devastating.

See a full description on that 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 are an importer of bonsai 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!

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! 

Leave plant pests and see helpers on the Beneficials Page.

Pests are not the only problem when growing bonsai,  go to Bonsai Care

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There are many pesticides to use – organic and chemical. One good choice is the Safer Brand Insect Killing Soap, Ready to Use Spray