Scales spend most of their lives immobile, with their tubular mouthparts anchored to the plant tissue on which they feed. Eggs hatch beneath females into young mobile scales called crawlers. Crawlers are small (<1/32") and flattened, looking like dust on the plant surface. Since crawlers lack chewing mouthparts they must leave the scale by crawling out from beneath the adult female body. Scale infestations spread within a plant as crawlers walk to the site where they settle and feed. They move to new plants when they are blown by the wind to nearby plants or plant parts. In the landscape, scale infestations also be brought in on nursery stock. Adult males of all scales are winged and because they have no functioning mouthparts are rather short-lived.

After an armored scale crawler begins to feed, it becomes very flat and covered with a clear wax shell. As it continues to grow, it remains beneath its waxy armor. At each of the three successive molts, it produces a new shell that expands beneath the old armor. This waxy covering is difficult to penetrate with insecticides. Winged males crawl out from beneath their cover and mate with covered females who produce eggs. Females can produce about 100 eggs each.

Soft scales are not covered by a waxy shell and excrete large amounts of liquid excrement called honeydew. Crawlers hatch from eggs in mid-summer. In most species, crawlers go directly to leaves where spend most of the summer. They return to the twigs and bark where they spend the winter as settled second stage scales. They continue to grow on twigs in the spring until winged males mate with wingless females, who swell with up to 1,000 eggs.

The natural enemies you are likely to encounter are lady beetles, predaceous mites,and small parasitic wasps. The evidence left behind by each of these natural enemies can be easily recognized.

Predators
(click predator's name to view photo)
Chilocorus kuwanae, one of many small species of "twice stabbed" lady beetles that feed on scales.
Larva of C. kuwanae feeding on euonymus scale.
Larvae of some twice stabbed lady beetles like Hyperaspis binotata, protect themselves from ants by mimicking mealybugs.

Though predators are common all year, some - especially the coccinellids, are most abundant late in the season. Overwintering occurs in bark cracks and crevices as well as in leaf litter. Other predators not mentioned include earwigs, green and brown lacewings, and dustywings.

Parasitoids
(click marked text to view photo)
Circular hole left by wasp parasitoid emerging from an armored scale and a soft scale.
Larval, pupal, and adult stages of a parasitoid. Most adult parasitoids find and deposit eggs in hosts when male scales are flying and scales are in the crawler stage. Some parasitoids are solitary, leaving single holes in scales. Gregarious parasitoids often emerge in large numbers leaving multiple holes.

Predaceous mites
(click name to view photo)
Hemisarcoptes spp. are clear mites that feed beneath the cover of a euonymus scale. This predator is carried between scale infestations by ladybeetles. These mites are present beneath the scale cover of armored scales all year long.

1) Determine if natural enemies are abundant by looking for the natural enemies themselves or their remains. 2) Determine if scales are alive by smashing the cover or the scale itself. Live scales will bleed when crushed. Dead scales are dry. 3) Use horticultural oil to smother live scales only when client relations or plant health is compromised. 4) Limit use of horticultural oil to when natural enemies are less active. Use in dormant season and immediately after crawlers have settled. 5) Exclude ants from trees infested with soft scales. These ants actively defend scales from local natural enemies. Use sticky or mechanical barriers when possible. 6) Scale natural enemies thrive in naturalistic woodland habitats where shade moderates temperature and humidity, and flowers and nectar provide food for foraging natural enemies. Design landscapes that help them survive.

Links
Photo guide of common midwestern scale insects http://www.entm.purdue.edu/entomology/ext/targets/e-series/eseriespdf/e-29.pdf

Using flowers to attract and conserve natural enemies http://www.entomology.wisc.edu/mbcn/fea304.html

SCALENET, Taxonomic references
http://www.sel.barc.usda.gov/scalenet/scalenet.htm

Texts
Kosztarab, M.1996. Scale insects of northeastern North America: Identification, biology, and distribution / Michael Kosztarab. Martinsville : Spec. Pub. Virginia Museum of Natural History ; no. 3.

Cliff Sadof, Purdue University; and Larry Hanks, University of Illinois. Copyright and reprint information. (Last update September 18, 2000)