Pest description and crop damage There are two types of scale commonly encountered: hard scale and soft scale. Hard scale tend to be fairly small in size and lie flatter against the stems than do soft scale. Soft scale is often very rounded and will also produce honeydew with its associated black sooty mold. Scale may have only one or multiple generations per year. Scale often comes into nurseries on propagation material. This example shows the importance of inspections and sometimes, quarantine of any new plant material brought into the nursery. Early infestations can often be rogued or pruned out.
Biology and life history The life history varies with species, so species identification is needed to determine life cycle. Generally, scale overwinter as eggs or as an immature scale on twigs and branches. If scale overwinter as an immature, they resume feeding in the spring, and eggs are laid later in spring. If scale overwinter as eggs, these hatch in spring, and the young scale (called "crawlers") migrate through the foliage to feed on the leaves. Most scale insects are female. Mature females are wingless and often secrete a hard shell-like covering for protection. The males are rare, small, non-feeding, and short-lived but look more like other insects as they have wings. With a few notable exceptions, the first immature stage, or the first "instar" females are generally the only stage that disperses on plant material. All other stages remain attached to the plant surface, sessile. Females lay eggs or crawlers under their secreted scale covering or in a cavity under their bodies.
Scouting and thresholds Look for scale along stems and leaf undersides. Inspect twigs during the dormant season for scale. Pay particular attention to weak plants. The crawlers are best observed starting in spring with a 10X magnifying glass. Double-sided sticky tape can be used to check for emerging scale crawlers.
Check for presence of holes in the scale covers indicating parasitic wasp activity. Ants, fond of honeydew, will fight off scale natural enemies to protect the source. Control of ants may increase biological control. There are several naturally occurring lady beetles that specialize as natural enemies of scale including Chilocorus and Rhizobius. Larvae of green lacewings and other insects are aggressive predators of scale. However, biological control will not necessarily prevent significant scale infestations.
- Avoid use of broad spectrum insecticides to preserve natural enemies.
Closely inspect all incoming plant material and plants used for propagation. Pruning and rouging may be an effective tactic in the landscape or on a limited number of nursery plants. As with aphids, avoid excessive nitrogen fertilizer, as this favors population increase. Control associated ant species as they move scale and protect them from natural enemies.
Scale pests are best controlled at the "crawler" stage: the immature, active stage when the insects are most sensitive to insecticides. Spray applications should be timed to coincide with emergence of the vulnerable crawlers. Another tactic is application of systemic insecticides that are drawn into the plant, managing multiple feeding stages of the scale. Time may be needed for some plants to translocate the insecticide to infested parts of the plant. Systemic insecticides are used against many scale species but generally have been less successful with hard scales and pit scales.
- Dormant-season oil spray-Apply with enough water to cover the entire tree thoroughly.
For more information
PNW Nursery IPM: A Matter of Scale (http://oregonstate.edu/dept/nurspest/scale.htm)
WSU Extension Bulletin: EB1552E: Scale Insects on Ornamentals (http://cru.cahe.wsu.edu/CEPUBLICATIONS/EB1552E/EB1552E.PDF)