The Asian cycad scale, or Aulacaspis yasumatsui, was brought into Miami in 1992 by an expedition funded by a botanical garden. Because these scale insects can hide in the roots and in between the leaf bases of cycads, they were brought in undetected at the time. However, it wasn't long before these insects were detected, yet no one knew what impact they would have on south Florida cycads.
The only genus of cycads that is native to Asia is Cycas, which includes the well-known king sago (Cycas revoluta) and queen sago (Cycas rumphii) "palms". Not surprisingly, the plants most affected in south Florida have been the king and queen sagos. This scale insect reproduces itself so rapidly that a 15-foot queen sago with a spread of over 20 feet will be totally covered in a matter of a few months. The scale will cover the stems and leaves so thick that it looks as if snow has covered the plant.
Scale insects suck juices out of the leaves of their host plants, so that if left uncontrolled, they will eventually kill the plant. The insects are not hard to kill, but because they reproduce so rapidly, homeowners have found that they must continually spray their plants. Most homeowners don't take the time to do this, and many have simply let their plants die and have had the plants removed. In the last eight years, the loss of queen sagos in Miami is quite notable.
Another factor that has made the eradication of this insect difficult is that they can become airborne. Even though they don't fly with wings, any time there is a strong wind the immature form of the insect can be carried as far as a half-mile away. This means that even if you have taken care of the problem in your yard, you can get the scale again the next time there is a storm.
Infected plants from Dade County nurseries have been sent to many areas of the United States by chain stores. People have seen infected plants in Alabama, California, Georgia, Hawaii, Louisiana, South Carolina, Texas, and many other areas in Florida. From what I understand, there are parts of Hawaii that have infestations just as bad as in Miami.
The early reports suggested that this scale could not survive where temperatures got near or below freezing. During the last year, I have performed experiments on plants that were kept separate from other cycads. I tested different chemicals for eradication/survival of the insects under varying growing conditions. This cycad scale has survived in the fuzz around the apex of some Cycas revoluta plants that were subjected to a freeze of 23 F, and nine hours below 32 F. The Asian scale has been known to crawl down to the roots of plants during the winter time. This means that they may survive much lower temperatures than the example above. People have thought that the scale problem would be confined to areas that don't have regular freezes. My findings show that most of the southern states are not safe from these insects.
The primary hosts of these insects are the Cycas species that come from Asia. This includes the king and queen sagos, as well as C. taitungensis, the emperor or prince sago. Botanical gardens in Miami have found that the Asian scale will also infect Stangeria, and, to a lesser extent, some of the Australian Cycas species. This scale has also been found on the cones of a few Ceratozamia species.
These are the cycads that this scale flourishes on, but sometimes scales can be found on species that are not known to be hosts. Asian scale has been found on the foliage, and on the cones of Encephalartos, Dioon, and Zamia. It appears as if they can survive for shorter periods of time on these plants, but die before they get a chance to spread. I know of a few cases where Dioon and Encephalartos plants were moved to a clean location, just to start an infestation on all the Cycas in the immediate area.
The Asian scale looks very much like the scale that is commonly called " Magnolia scale". The Magnolia scale has a shape more like a pin, instead of round, like the scales people are more accustomed to seeing. Another common name for the Magnolia scale is " snow scale" because it is white and it covers the plant to the point that it looks like snow.
Unless you know what you are looking at, the only way to tell if you have the Asian scale is the speed in which it multiplies, and the thickness it covers the plant. The infestation usually starts on the petioles near the crown of the plant, and works out from there. Usually within a couple of months, the plant will be totally covered. Another concern that some people have is that this insect may end up adapting to other types of plants. It is so close to the Magnolia scale, that there could be a time that this scale could be a problem for palms and dichotomous trees.
So, what can we do to eradicate these insects? Reports show that the most effective way to get rid of them is to spray infected plants with a combination of horticultural oil and Diazinon. Mature scales have a hard shell that protects them from direct contact sprays. The immature scales are called crawlers and do not have a hard shell. Crawlers will leave the protection of the parent scale and will move to the new growth when possible. Horticultural oil is used to cover the entire plant, which smothers scales. If the plant is not totally covered with oil, the application will not be as effective. A week after an application of oil, the Diazinon is used to kill the crawlers. This process is repeated every two weeks until the infestation is no longer present. In cities like Miami where there is widespread infestation, this is a constant job because new scales can be blown onto uninfected plants at any time.
Another approach is the use of systemic insecticides such as Orthene. Orthene is sprayed on the entire plant, and the leaves absorb the chemical. Anything that eats the plant will die because of the concentration of the chemical in the plant. Orthene will be effective on a plant for approximately three to four weeks, although it is suggested to use this chemical every two weeks. Unfortunately, many people have found that Orthene is not a long-term solution to the problem. Thus, you can see why people have not been able to control these insects, and have decided to remove their sagos from their landscape or replace them with cycads that are not affected by the scale.
Yet another approach has been the use of predatory insects that feed on this scale. These insects have been released in areas of Miami and have been found to be somewhat effective. A problem with this, like with other predatory insects, is that they leave the plant once the problem is taken care of, or move somewhere else before the job is finished. People have found that the weather has also altered the effectiveness of these predatory insects. I can see the use of these insects in an enclosed environment, but they are not the best answer when used out in the landscape.
The good news is that I think I have found a better, long-term solution to the problem. There are products that are available to nursery owners that are not available to the average homeowner in retail stores. I have found that products with the active ingredient Imidacloprid are very effective for long term control of the Asian scale. (For those of you with pets, this is the same active ingredient that is used in the Advantage system for dogs and cats.) These products are made by the Bayer chemical company and go by the names of Merit, Marathon, and Admire. Admire is labeled for use on crops, whereas the others are labeled for ornamentals. These products are applied to the soil, and then the roots draw the chemicals into the plant. These chemicals work best when they concentrate in the new growth of the plant.
Many people will cut off the leaves of infected cycads to make it easier for them to control the insects. In most cases, when a new flush of leaves is produced, the scales are attracted to the new leaves and the infestation continues. If a product with Imidacloprid has been used prior to the new flush of leaves, the newly hatched crawlers will move to the new leaves and die. In my tests, I found that these chemicals have been effective for up to five months on containerized plants and a little less on plants in the ground, depending on how fast the chemicals leach through the soil. I have observed a plant that had been sprayed with this chemical, where all the mature scales had died. A few weeks later, more scales hatched and started eating on the petioles of a new flush of leaves. I was amazed to watch them die and fall off over the next few days.
Marathon comes in two forms, a liquid and a granular, both of which are applied to the soil. Some people have used Merit and have found it to be not as effective as they would like. However, some nursery owners have found that the amount of product used made all the difference in the world; a higher concentration of product used in the same situation was very effective on scale. Bayer is now making a new product called Advanced Garden that can be found in retail stores. It also comes in a liquid and a granular form and is very effective on scale. Advanced Garden is available with many formulas, but the most appropriate product for cycads would be the tree and shrub control.
I have had many people ask me what I think would be the very best way to take care of the scale problem. One problem I see with using Imidacloprid is that it takes a few weeks to get into the system of the cycads, so there is a chance that the insects can move to another plant before the chemical takes effect. I think the best combination would be to apply Marathon to the soil and wash it in. Next, cut off all the leaves on the cycad, and then spray the stem with horticultural oil to smother as many of the scales as possible. By the time the new leaves are produced, the chemical will be in the system of the plant. If any new scales hatch, or if any other scales fly onto the plant, these insects will die once they start feeding on the new leaves. I would suggest another treatment of Imidacloprid approximately four months after the first application whether there are insects on the plant or not. Continued use the product every four months should keep the plants free from subsequent infestations.
The Asian scale is a problem that is not going to go away any time soon. If nurseries continue to ship plants that are infected by this scale, these insects will be a problem throughout the southern United States, and even in the warmer locations around the world. If people are made aware of the problem and the availability of long-term remedies, I think we have a good chance of getting rid of these pests in our landscapes.
Author's note: I do not have any interest in the companies who make the products mentioned in this article. I have made suggestions on using chemicals that have worked for me. In no way do I guarantee any results, or assume any liability for any unforeseen problems that may arise from the use of these products.