Many years ago I discovered root mealy bugs in a bald cypress bonsai. At first, I thought it was mycorrhiza.
Mycorrhiza is a useful root fungus, helpful to many plants, especially conifers. However, this pest - Rhizoecus americanus (Pseudococcidae Family) - is related to scale. It is a soft-bodied, sucking insect that attacks the tips of roots (new growth). They are sometimes called “blind or soil” mealies.
Similar in appearance to the familiar above ground mealy bug, they both leave white waxy secretions in their wake -- like small bits of cotton.
Rhizoecus is primarily a warm climate pest. It is very common in Florida and other southern states. However, if shipped in plants, it continues to thrive indoors and in greenhouses.
These creatures are dangerous to your plants and are often ignored as insignificant or misidentified as mycorrhiza. They are neither!
If you are in doubt, the ideal way to identify them is through a microscope or magnifying glass.
Shown here is a live root mealy bug, greatly enlarged.
If it moves, you can bet it's root mealy bugs, not mycorrhiza.
An important part of taking care of bonsai trees is to observe them on a regular basis.
In this case, leaves may be pale (sometimes grayish) or wilted, despite regular fertilizer and watering. Maybe the plant growth has slowed down and/or flowering has ceased. In severe cases, the leaves may be misshapen.
To detect this pest in a well-established bonsai, gently lift the tree out of its container and look at the sides and bottom of the root mass. Many times you need to search no further.
Although they occur throughout the roots, they are most obvious along the edges. They can also appear on the inside walls of the container and sometimes even underneath the pot. Also, while pulling weeds, observe any soil that may come up with the weed roots.
I have seen bad infestations continue to develop out of containers onto old wooden nursery benches!
Spraying common chemicals or even repotting is often not sufficient to eliminate them!
When I found this pest on Bald Cypress roots, I knew what it was right away. I called the County Extension Service to find the current recommendation for treatment.
The Answer: Bayer Advanced 12 month Tree & Shrub Protect and Feed. I immediately sprinkled the granules on the affected bonsai plant. It also comes in liquid form. I rarely recommend chemicals, however Bayer Advanced is well worth the price.
If you only have one or two plants, you may find this valuable product a little pricey. Consider sharing the container with a friend.
When I posted this story, I received an email asking why "you didn’t mention the 'hot water technique'.”
Before I included this information, I researched the online mention of this technique conducted by the University of Hawaii College of Tropical Agriculture.
Their research found that "hot water dips alone will kill root mealys." I contacted Dr. Arnold Hara (the professor in entomology who wrote the article) for more details.
About water temperature, Dr. Hara responded: “A meat thermometer should work. Water at 120 F (49C) for 6 to 10 minutes is sufficient to control most insects.”
That sounded like really good news to me, so I asked about potential damage. Dr. Hara then told me “There may be some damage, but it should grow out of it; I recommend doing a test on a few plants before treating mass numbers. Some may be sensitive.” After this discussion, I would definitely consider “hot water” for stock plants.
(I would not personally use the process for bonsai.) However, if you are in a "do-or-die" situation from this pest, give it a try.
Because the root mealy bug is difficult to control:
More Bonsai Plant Pests