Description
Scale insects are a large and diverse group closely related to aphids, whitefly and mealybug, all piercing sucking insects. Scale may easily be overlooked as they do not resemble other insects. Adult female scales of most species are circular to oval, and wingless. Adult males are rarely seen and are tiny, delicate, white to yellow insects with one pair of wings and a pair of long antennae. As adults the females are sedentary and covered with a hard shell or waxy covering. Some scales can severely damage their host, while others do no apparent harm. Armored scales and soft scales are the most common types, with Brown Soft Scale being the highest threat to indoor tropical plants.
Target Crops
Scale insects feed on a wide range of host plants and are common pests of many ornamental plants, trees and shrubs as well as indoor tropicals. Most species are restricted to specific host plants and some types of scale are serious pests to agricultural crops. Soft scale produce honeydew, which may attract ants and feed black sooty mold. Hard scale feed intercellularly and take up little plant sap, producing no honeydew. Infested plants appear water stressed, with yellowing leaves that may drop prematurely. Heavily infested sections of the plant may die.
Life Cycle
Scale insects hatch from an egg and develop through nymphal stages to adults. The first instar nymphs are called crawlers as they move around on the plant or are carried to other plants by wind, people or animals. Crawlers generally settle down and begin feeding within a few days, often staying in the same spot their entire life. The mature females hide the eggs under their bodies or under cottony or waxy covers. Females may produce eggs continually over a period of 3-4 weeks, with overlapping generations.
For more detailed information on different species of hard and soft scale, visit the resources listed below. The details on Brown Soft Scale are particularly important for tropical plant growers.
Beneficial Insect Control
Aphytis melinus are tiny parasitic wasps that attack hard scale (California red scale, oleander scale, San Jose scale, ivy scale, citrus scale) and are used in greenhouses and orchards. Rates for Aphytis are 1 to 6/ft². every two weeks, as needed.
Cryptolaemus mountrouzieri, the mealybug destroyer, eats mealybug but will also feed on all stages of soft scale pests. These small beetles are dark brown with orange heads and target soft scale like black scale, pulvinaria scale and cottony cushion scale. Rates are .5-1/ft² or 5 beetles/plant, or 1,000-5,000/acre.
Rhyzobius lophanthae is another small beetle that feeds on both soft and armored scale. The young larvae feed on scale eggs and crawlers and the adults feed on all life stages. They are used in orchards, landscapes, greenhouses and interiors. Rates are 2-6/10 ft² or 10-40 adults/tree.
Anystis baccarum or the Crazee Mite is a generalist predator that will feed on soft scale. Anystis prefers moist, warm conditions. Rates are 0.25/ft² for prevention or 1 - 2 mites/ft² for control.
Chrysoperla rufilabris and Micromus variegatus, the green and brown lacewings respectively, are also generalist predators. Rates for green lacewings are 1-5 larvae/ft² on hot spots, 5 eggs/ft2 or .25 adults/ft2. Rates for brown lacewing adults are 1-3/10ft².
Insecticide Options
Contact insecticides, such as soaps, oils and insect growth regulators, can control hard and soft scale larvae, nymphs, or “crawlers,” but they may be ineffective on adult scale. Repeated applications are necessary. The following products are labeled for scale control.
Pyganic is a broad-spectrum insecticide derived from Chrysanthemums (pyrethrins).
Suffoil-X suffocates and kills soft bodied insects.
Molt-X is a bioinsecticide that contains azadirachtin, an insect growth regulator.
M-Pede insecticidal soap works for soft bodied insects, mites and powdery mildew.
Ecotech Plus targets greenhouse pests with rosemary and peppermint oil as the active ingredients.
Cultural Control Tips
- Provide plants with good growing conditions and proper cultural care.
- Prune off heavily infested twigs and branches.
- Consider replacing problem prone plants.
- Control ants.
- Grow flowering insectary plants to provide food and shelter for natural enemies.
- Reduce dustiness that interrupts the activities of natural enemies (periodically hose off plants).
- Avoid the use of broad-spectrum insecticides which will kill both pests and beneficial insects.
Sources
Scale Insects – Wisconsin Horticulture
Scale up Your Efforts Against Scale – Raymond Cloyd
Brown Soft Scale – Colorado State University
A Matter of Scale – Oregon State University
