Integrated Pest Management
Oystershell Scale
HOSTS
- Ash
- Dogwood
- Maple
- Poplar (including aspen)
- Willow
- plus 130 other woody hosts
DESCRIPTION
Adult oystershell scales are enclosed in a shell made up of its own shed skins and waxy secretions in the shape of an oystershell. Under this covering is an immobile insect with no legs, eyes, or antennae.
BIOLOGY
Eggs are laid in late summer and early fall and overwinter under the female scale. Estimates of egg production range from 40 to 150 eggs per female. Egg hatch begins in late May or early June and continues for about two weeks. The newly-hatched, tiny white to light yellow nymphs are called "crawlers" and have functional legs. They move about over the bark for a short period of time, insert their long, thin mouthparts into the host plant, and begin to secrete the protective waxy coating. Once they settle to feed, the female scales remain in the same place for the rest of their lives. The functional legs are lost when the insect molts. The scale gradually takes on the "oystershell" appearance as it goes through successive molts and grows. In Utah, oystershell scales produce only one generation per year, but there may be up to two generations.
SYMPTOMS
- Scaly and cracked bark
- Loss of vigor
- Off-color and abnormally small foliage
- Tree die-back
- Tree death over a period of years
GENERAL MANAGEMENT
Oystershell scales are most susceptible to insecticides during their crawler stage--after the eggs have hatched, but before the nymphs have settled down and secreted the protective waxy coating.
Scouting
The crawlers can be detected by wrapping a piece of double-sided sticky tape around an infested twig around the middle of May. The tape should be checked every other day for the presence of crawlers. Replace the tape whenever it becomes wet or covered with dust and continue the monitoring process until egg hatch has been detected. Another method of detection is to hold a piece of white paper or a white pan under the branches and tap the branches with a stick. Any crawlers can be easily seen on the white surface. The color of the crawlers may be shades of white, yellow, orange, or purple. Heavily infested or sickly branches can be pruned from the tree. In some cases, heavy accumulations of scale can be removed from the bark with a stiff scrub brush or a plastic scrub pad.
INSECTICIDES
Timing
In northern Utah, crawlers may be active in mid-May and mid-July. Materials should be applied when crawlers are first noticed and reapplied in about 10 days to provide control of late-hatching nymphs.
Homeowner Options
Insecticides frequently recommended for oystershell scale control which can be used by homeowners include formulations of acephate (Orthene), carbaryl (Sevin), malathion, and oils.
Commerercial Options
Products for commercial use include most pyrethroids, aceamiprid (Assail), and dinotefuran (Safari).
Biological Control
Natural enemies of oystershell scales may provide some degree of control. These include beetles (such as the twice-stabbed and sevenspotted), parasitic wasps (Aphytis mytilaspidis and Encarsia citrina), and predatory mites.