Q. I purchased an Avocado tree at a local Arboretum three years ago and it's full of scale. I tried washing them off and spraying with Bulls-Eye (a bioinsecticide), but neither had any effect. I won't use chemical sprays. What should I do? I really love this little tree, and would hate to see it die.
- ---Jennifer in Marina Del Rey, CA
- ----Chris in Bucks County
- ---Bill in Lansdale PA
- ---Ewa in Stockholm, New Jersey
- --Sarah in Drexel Hill, PA
Adult scale don't move; they cluster on plant trunks and stems (and sometimes leaves and fruits) sucking sap and weakening the plants. Soft scale, like the notorious one infesting that magnolia in New Jersey, produce a lot of black sooty honeydew and so can easily be confused with aphids, another sap-sucking pest. The sweet honeydew of both pests attracts ants, who enjoy this nasty sugar source so much they will fight off beneficial insects to protect it. If your scale are honeydewing, use boric acid baits to eliminate those protective ants. (Most hard scale don't poop sticky black sugar; lucky us.)
There are thousands of different species of scale. Most are very distinctive; so you might be able to use their color, shape and markings to make a fairly positive ID. By definition, any scale you see is a female. Males don't have shells, look very different, and don't feed on plants. (As with mosquitoes and humans, all problems are caused by females, with the males simply being lured along by bad companions. Ahem.)
Anyway, the first thing to do is take proper care of the plants involved, as stressed plants are magnets for these pests. azaleas require a naturally rich, acidic soil; hollies suffer greatly when their soil is alkaline; all plants need good drainage; and chemically fertilized plants are especially popular targets of aphids, mealybugs, and scale. So if you have wood mulch piled a foot deep, prune your plants badly and at the wrong time of year, and douse them with Miracle-Gro, the scale are your fault. Research the needs of your specific plants, provide those needs organically and things will likely improve.
Common-Sense Pest Control expert Bill Quarles of the Bio Integral Resource Center notes that dead scale can remain on a plant for a long time, so scrape some specimens off with your fingernail or a pocket knife and examine your catch with a magnifying glass; you'll be able to tell if the insect underneath the shell is alive or not. Bill notes that some people think the problem is getting worse when most of the scale they're seeing are long dead and the situation is actually improving. If you see a little worm-like creature in there, raise a cheer; that's a parasite killing the scale. Leave the plant alone and let those beneficials do their job.
If your scale is hale and hearty, scrape off what you can reach. If some limbs or branches are more scale than plant, prune and burn or otherwise ritually dispose of them while trash-talking the little suckers with phrases like "Gee, why don't you just run away!?"
Armor-protected scale are invulnerable to pesticides, both chemical and organic ones like that Bulls-Eye (a brand name of one of the wonderful spinosad products that are safe and highly effective against many other pests, especially caterpillars, ants and thrips). But even scale have to breathe, and smothering products like insecticidal soap and horticultural oils can take them out.
When the problem is on a big tree or shrub outdoors, spray dormant oil in late winter or early Spring. Later in the season and indoors, use a lighter, vegetable-oil based 'summer spray'. Insecticidal soap is better for small infestations and houseplants; use a commercial product (not home made) and rinse it off after a few hours so the plant doesn't get stressed.
Remember; soap and oil only work by smothering; you must spray (not 'wipe') directly on the pest for them to work. (Note: Although dormant oil is petroleum-based, it's still a very safe and sane way to smother overwintering pests and their eggs. If you object to all things petroleum, use a vegetable-based horticultural oil in the winter as well.
And finally, beetles, wasps and other beneficial insect attack and control scale naturally. If you've been spraying your scale with chemical pesticides, you have likely only been killing their predators. Some of these helpful creatures can be purchased commercially for release outdoors or in greenhouse type situations, including ladybugs, the predatory beetles Chilocorus nigritus and Lindorus lophanthae , the parasitic wasp Metaphycus helvolus, and numerous types of "Chalcid" wasps.
There are lots of other natural enemies out there; look up your specific type of scale to see which ones like to attack it.