By Dan Gill, LSU AgCenter Horticulturist
Scale insects are one of the more common groups of insects that attack plants. You may find them feeding on trees, shrubs and even indoor plants.
When they first hatch out of their eggs, scale insects are mobile. At this stage they are called crawlers, and they are very tiny. Most gardeners never notice them.
Once they settle down to feed and pierce the plant’s tissue with their needle-like mouthparts, they never move again and become firmly attached to the leaves or stem. Scales also cover themselves with waxy material that protects and hides the insect. These two characteristics make scales look decidedly “unbug-like” and many gardeners don’t recognize that there is a problem until the plant is heavily infested and damage has occurred.
There are many types of scales, but here are a few of the most common:
• Tea scale insects are about the size of a hyphen and appear slightly fuzzy. White or brown, they are a major pest of camellias and some hollies.
• Florida wax scales look like waxy, white domes about the size of a nail head and occur on a wide variety of plants.
• Euonymus scales, on the other hand, attack euonymus and resemble tea scale.
• On magnolia trees, oyster scales look like small, white bumps on the leaves, and magnolia scales appear on the branches as yellow waxy blobs about one-quarter to one-half inch across.
• Soft brown scales occur on ficus, schefflera and other plants indoors.
• Fern scales appear as white dashes on the fronds.
As scale insects feed on the sugary sap of a plant, they excrete some of the sugar as a liquid called honeydew. The honeydew accumulates on the foliage and can cause it to look shiny and feel sticky.
This rich food source does not go unnoticed. Ants, wasps and other insects may be attracted to the sweet honeydew. Even more common are fungal organisms that produce an unattractive black coating on the leaves called sooty mold.
These fungi feed on the honeydew and do not attack or directly damage the plant, but the appearance of sooty mold is often the gardener’s first noticeable sign of trouble.
Be aware that other sucking insects such as whitefly and aphids also produce honeydew that can lead to sooty mold, and sooty mold does not occur with some scales, such as tea scale.
Scales spread from plant to plant as tiny crawlers that have legs and can move around. Crawlers can be controlled with contact insecticides such as insecticidal soap or Malathion, but because most gardeners never notice them, they miss the opportunity. Many scales produce crawlers in the spring. Once the crawlers have settled down to feed, they create their protective covering, and contact insecticides are largely ineffective.
Scales may not go away if you leave them alone, so control is usually necessary. The safest, effective way to control scale is with a horticultural oil spray. These insecticides contain oil in a form that will mix with water. When mixed and sprayed onto an infested plant, the oil coats the scale insects and clogs their breathing pores. The insects are suffocated rather than being killed by a toxic material. Brand names include Volck Oil Spray, Dormant Oil, Summer Oil and Year Round Oil.
For proper scale control, it is critical to apply the oil spray over every surface of the plant.
If the insects are on the underside of the leaves and the oil is only applied to the upper surface, it will have no effect on them. Because scale insects are difficult to kill, one or two follow-up applications should be made after the first one. Follow label directions carefully.
Oils also are effective against aphids, whiteflies, spider mites and the crawler stage of scales. And they are less harmful than other pesticides to beneficial predatory insects.
Oil sprays are best used when temperatures are between 45 and 85 degrees and should be applied only to plants that are not in stress. That’s one reason that the mild weather of spring is an excellent time to use them. Light, paraffinic oil, such as Year Round Oil, however, can be used during the summer.
An added benefit of oil sprays is that they also help clean the unsightly sooty mold from the plant. The sooty mold will not quickly disappear when the scale has been controlled. But as the food supply is exhausted, it will eventually weather off. Oil sprays help speed the process along.
Systemic insecticides are another option in controlling scale. These insecticides are sprayed onto the plant or applied to its roots.
The plant absorbs the insecticide into its tissue, and the chemical gets into the plant’s circulatory system and into the sap. When scales feed on the sap, they ingest the toxic insecticide and are killed.
Acephate, imidacloprid and dimethoate are three commonly used systemic insecticides that are effective against scale. Systemic insecticides provide an option for control when temperatures limit the use of an oil spray or when drenching around the base of the plant is more practical.
Always read and follow label directions carefully when using any pesticide.