Home Gardening

Scale insects difficult to control
The Coast Gardener Newspaper and Web Column - May 25, 2002

There is one type of insect that troubles even the most experienced gardeners. Scale insects don't even look like insects. Have you ever noticed a whitish-looking growth on a branch or twig that looks more like a tiny bump on a log? If so, you have probably seen a type of scale.

Several scale samples have recently been brought into the Master Gardener office for diagnosis. I noticed recently that several hollies in downtown Gulfport were suffering severely from these dreaded pests. Indian hawthorn, camellias, magnolias, and many other broadleaf evergreen shrubs suffer from scale.

Correct identification is essential for control. First, you need to realize that scale insects are sucking insects rather than chewing ones. Scales produce a sticky, sweet substance called honeydew as they suck the sap from plant tissue. Aphids produce honeydew, too.

This honeydew provides just the right nutrients for black sooty mold to grow on it, which causes your plants to look like they have been dusted with a black sooty powder. The sooty mold is the first thing usually noticed. Many people believe that this is the problem, but the scales are really to blame.

There are two types of scales: hard and soft. Oak scale, tea scale, holly scale, euonymus scale, and pine needle scale are just a few of the hard scales. Many of the broadleaf evergreens suffer from the hard scale. Soft scales include wax and magnolia scale. These soft scales are even more difficult to control than the hard ones.

Scale insects hatch from eggs and the tiny "crawlers" move to tender plant parts when the weather warms in the spring. Here they attach themselves permanently to the plant and shed their six legs. As this process occurs, they develop a waxy shell-like covering over their bodies.

This shell is what makes scale insects so hard to control. Pesticides simply cannot penetrate the waxy coverings.

Scale insects are most often found on stems or the underside of leaves. You must direct your spray to these areas to obtain good control. Thankfully, there is hope for controlling them.

Insecticides containing carbaryl or malathion can be used in the springtime to kill the crawlers before they produce that waxy shell. Be very careful in your insecticide selection, as some products can harm particular plant types. For example, diazinon can injure gardenias and cygon will harm Chinese hollies such as Rotunda, Burford and Dwarf Burford cultivars. The label will tell you the precautions you should take for your situation.

A summer weight oil spray such as Oilicide, Volck, or Sunspray will smother the insects by cutting off their oxygen supply. Use this product if you failed to kill the crawlers or missed treating them altogether.

Pay very close attention, however, to the label of the product you choose to use. When temperatures are too high, damage to plant material can occur with some oil-based products.

In the fall, you should follow up with another oil spray. Follow the same temperature restrictions and precautions you used earlier in the season.

It is much easier to target your plants that have a history of scale infestation with preventive sprays each spring and fall. Preventing scale is much easier than trying to get rid of it.

These archived gardening columns were written by Chance McDavid, former Harrison County Extension Agent.