Scale Insects Difficult to Control
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 a 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 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.
Juniper scales are light gray or white in color, very small (about 2 mm) in diameter, and nearly circular. These insects become abundant on the needles, especially the undersides. The scales always attach themselves to needles rather than bark.
Juniper scale is a pest of ornamental junipers, especially the pfitzers. Badly infested plants show a yellowish brown foliage color and the overall appearance of the plant is similar to a plant badly in need of fertilizer or water. Portions of dead branches may be visible. Taking a closer look at the branches that show these symptoms may reveal the scale insects on the undersides of the needles. Plants decline over time and may be killed by severe infestations.
The scale insects overwinter as nearly full-grown individuals and mature in the spring. Under her scale covering the female lays eggs in the spring. She dies soon after egg laying but the scale cover remains attached to the needle, often for several years. The crawlers (immature scales) hatch from the eggs and leave the scale cover to search for a place to feed. The crawler stage may last for as short as 24 hours and after finding a feeding site the small, straw-colored crawler attaches itself to a needle and begins to feed. Soon it develops a coat of armor (the scale covering). The immature scales continue to feed and grow during the summer and during this growing period, the skin is shed several times. The shed skins make up the hard protective cover for the scale insect. By late fall the scales are nearly full grown and ready to overwinter. The cycle is repeated year after year.
A dormant oil or oil plus ethion may be used to help manage this pest. The oil should be sprayed on in mid to late April, but before new growth begins. This should be followed by two sprays of malathion plus methoxychlor or carbaryl (Sevin) spaced 10 to 14 days apart beginning in late June for crawlers. If these sprays are missed, one application of Metasystox-R can be made in late July against the adults.
Neolecanium cornuparvum (Thro)
The magnolia scale belongs to a group of insects described as soft scales. Scale insects are immobile for most of their life cycle and thus show little resemblance to the usual form of insects.
Often people do not realize the magnolia has a scale infestation until leaves and twigs turn black with sooty mold. Severely infested twigs and branches are weakened, growth stunted, and repeated infestations may result in the death of the branches or entire small trees.
Scale insects feed by inserting their syringe-like mouth-parts into the plant’s vascular system, sucking out sap and other vital plant fluids. Large amounts of these fluids are withdrawn, concentrated in the gut of the scale, and excreted as a clear, sticky liquid — honeydew. The honeydew drips onto the leaves and stems making them sticky as well as providing an ideal place for the black sooty mold fungus to develop. The honeydew also attracts ants and wasps which feed on it.
“Bumps on the twigs” is a phrase which can be used to describe these insects. The mature female scales are large, up to 1/2 inch (12.5 mm) in diameter, elliptical and convex in shape. Adult scales are permanently affixed to the branches of the host. They range from pink-orange to dark brown in color. Immature and mature females are often covered with a white waxy bloom. The presence or absence of the wax is the major field character used to distinguish between the magnolia and the tuliptree scale, both of which feed on magnolia.
Immature scales are much more flattened than the adults, but still elliptical in shape. The overwintering nymphs are dark bluish-black, about l mm long and clustered on twigs in incredible numbers.
The magnolia scale reaches maturity in August each year. The females give birth to living young which remain under the parent scale covering for a short time. These young (the first instar nymphs) are known as crawlers. Crawlers move from the shelter of the female and migrate to the undersides of young twigs where they spend the winter. Once the crawler settles and begins to feed, it remains in the same spot for the remainder of its life. As the insect ages, the exoskeleton hardens making it less susceptible to contact insecticides. In the spring two molts occur, one is usually in mid-May and the second in early June. The nymphs continue growing during July but at a slower rate, usually reaching maturity in August. There is one generation per year.
Most scale insects can be controlled by appropriately timed insecticidal sprays. Where severe infestations exist, pruning out heavily infested branches can also be helpful. In severe infestations, an insecticide program should be continued for two consecutive years.