by Mike McLaughlin, Former Rainforest Manager
It's sneaky, it's deadly and it's everywhere. Many of the cycads in South Florida neighborhoods have yellow and brown leaves, and are encrusted with a white substance. You've never seen this before. What's going on here?
Miami-Dade County is experiencing the results of a surging population of an insect known as the cycad aulacaspis scale or Aulacaspis yasumatsui. It seems to affect only cycads, particularly favoring cycads of the genus Cycas, which includes the common Cycas revoluta (king sago) and Cycas rumphii (queen sago). If left unchecked, this scale insect can kill a mature Cycas. Other genera of cycads, such as Dioon, Encephalartos and Zamia can become infested, but usually the infestations are not as severe.
How do you know if your cycads have this bug? Once infestations become severe, the signs are pretty obvious. Catching an infestation before it gets out of hand, however, will take some close inspection. Take a magnifying glass and look carefully at the underside of your plant's leaflets and at the base of the leaves. It's easiest to find the adult females: look for round white dots about 1/16 inch in diameter. The female's body is actually under the white, waxy disk you see. The disk protects the female and gives the insect its name: scale. The males are about the same size and color, but are thread-like in shape. And if you look very closely, you might see tiny yellow specks moving about. These are the newly-hatched scale insects, called crawlers. This is the only stage of a scale insect's life in which it can move to new plants, carried by wind currents. Once the immature insects settle on a plant, they will stay there for the remainder of their lives. The scale is found throughout southern Florida.
If you determine you have infested plants, your next step will be to try to control this nasty pest. We hope to introduce natural predators from this scale insect's native habitat to reduce its population, but this process will take time. Meanwhile, we will have to rely on chemical warfare.
The best insecticide to use is still a matter of some debate. You'll hear many different recommendations about using common chemicals, but we're not convinced that the answer has yet been found and, in general, are reluctant to encourage the use of such toxic materials. We've been working with Dr. Bill Howard from the University of Florida's Research Center at Ft. Lauderdale to find a better control, one that will be easy and safe for homeowners to use. We are experimenting with various chemicals on containerized Cycas and on mature specimens in Fairchild's collections. When you visit Fairchild Tropical Botanic Garden, please forgive the appearance of some of our subjects!
If you are experimenting on your own plants, we have a few pointers for you. Be aware that dead scale insects adhere to the plant and are difficult to distinguish from live insects. To test, scrape them with your fingernail. If they are juicy, they're alive. If they are dry and flake off, they're dead. But even if you have killed the individuals on your cycad, continue to monitor your plant. Experience has taught us that they'll be back. If you are using sprays, be careful. Add a little dish detergent to your mix to reduce beading on waxy cycad leaves. Make sure to cover all plant surfaces thoroughly.
Because controlling these pests has been difficult, some people are giving up on their cycads. I hope that most will continue to fight for their plants. To lose these beautiful plants will alter the face of South Florida drastically, much as losing our coconut palms to lethal yellowing did a few years ago.
Update on Cycad Scale Control
by Chuck Hubbuch, former Director of Plant Collections
Fairchild Tropical Botanic Garden's horticulturists have been trying to find a control for aulacaspis scale that is relatively safe and effective for home use. Initial trials by Dr. Bill Howard of the University of Florida showed that insecticidal oils held some promise. The typical insecticidal oil is a petroleum product designed to kill insects without harming plants. It works by covering and suffocating the insects, not by poisoning them. It is less toxic to people and animals than most insecticides and the scale cannot develop a resistance to the treatment.
After about five weeks of insecticidal oil applications at Fairchild, Gillian Drake successfully brought the aulacaspis scale on Fairchild's cycads under control. Reinfestations of the scale continue, however, and the spray program must be renewed periodically. Gillian used insecticidal oils that are for summer use. Oils for warm weather use have the words, "summer", "lightweight" or "ultrafine" on their labels; or they may offer a summer rate on the back label. Because oil and water do not mix easily, you will need to add a spreader or spreader/sticker to the mixture.
To control aulacaspis scale on a cycad
Meanwhile, Dr. Richard Baranowski, at University of Florida, is working to find suitable insects that prey upon this scale. A beetle was released this year and another beneficial insect may be released in the near future. If successful, these predators will help us gain control of the aulacaspis scale without chemicals.