|Crop Knowledge Master|
Hemiberlesia rapax (Comstock)
Victoria L. Tenbrink, Research Associate
Arnold H. Hara, Entomologist
Beaumont Research Center
Hosts of the greedy scale include acacia koa, apple, avocado, azalea, bamboo, chinaberry, citrus, coconut palm, Ficus spp., geranium, guava, peach, pear, persimmon, plumeria, rose. (Zimmerman 1948, Dekle 1965).
Greedy scale is found in all tropical regions of the world. An immigrant to the Hawaiian Islands, it was first listed in 1898 (Zimmerman 1948).
The first sign of greedy scale is the presence of armor on upper and lower leaf surfaces, fruits, and stems of plants. The female armor is round and convex, white or dirty white, 1.5 to 2 mm (less than 1/8 in) in diameter. When crowded the shape is irregular. The cast skins of the nymph form a large, yellowish to dark brown spot, near the center of the scale armor, called an exuvium (Zimmerman 1948, Dekle 1965). Male armor is smaller and oval rather than round. The armor must be pried off to reveal the insect attached to the plant by thread-like mouthparts that start at the middle of the body and are as long or longer than the body. The adult female insect lacks wings, legs, or eyes. Dead ones are dark brown and are dried rather than plump. Armored scales feed on plant juices and cause loss of vigor, deformation of infested plant parts, yellowish spots on leaves, loss of leaves, and even death of the plant (Dekle 1965, Beardsley & Gonzalez 1975). Since scales are spread by introduction of infested material they are a quarantine problem on exported potted plants, cut flowers, and cut foliage.
The number of days for each developmental stage and the number of generations per year depend on temperature, humidity, and rainfall (Beardsley & Gonzalez 1975). Based on a generalized life history of other tropical species, 30 days is the approximate time to complete the life cycle from egg to reproducing adult.
Eggs are laid under the armor of the female where they develop and hatch.
The first stage after hatching is the only nymphal stage with legs, so the insects are called crawlers. The crawler stage is short, and crawlers do not feed. Crawlers may stay under the maternal armor several hours until outside conditions, especially temperature and humidity, are good. After they leave the cover, they wander for a period ranging from minutes to days, but usually a few hours. At the end of the wandering period they flatten against the leaf or stem and begin to secrete their armor (Beardsley and Gonzalez 1975).
Newly settled nymphs insert their piercing, sucking mouthparts into plant tissue and start feeding on plant juices. Female nymphs shed their skin twice as they grow and develop. Males have a 5 stage development and do not feed during the last two stages (Beardsley and Gonzalez, 1975). The cast skins, called exuviae, are incorporated into the armor forming a dot near the center. The armor is non-living and is made of cast skins, threads, and liquid, all produced by the insect (Beardsley and Gonzalez, 1975).
Female greedy scales appear larvae-like. They remain under armor in one place throughout their lives to feed and reproduce. Males are very different in appearance and behavior from females. They are tiny, winged creatures with eyes and legs. When mature they emerge from the armor in the late afternoon. They do not feed, living only a few hours to mate. Mate-finding is probably aided by pheromones secreted by females (Beardsley & Gonzalez, 1975). Because of the late emergence and short life of males, they are rarely found in the field.
Since female armored scales are not capable of wandering once they have settled and started feeding, long range dispersal happens by passive transport of infested plant material. Short range dispersal happens as crawlers search out places to settle and feed (Beardsley & Gonzalez, 1975). It is the crawler stage that can be carried directly from place to place by people, animals, birds, ants, and wind currents (Dekle, 1965; Beardsley & Gonzalez, 1975). Wind is an agent of dispersal and also one of mortality, since crawlers dislodged by wind may not land on suitable host plants. Adult males are capable of only weak flight or wind transport. Living only a few hours, they do not feed. The last two developmental stages of males also do not feed. Female adults and feeding nymphs insert mouthparts into plant tissue and suck plant juices.
Since armored scales are spread chiefly through movement of nursery stock, only propagative material that is free of scales should be planted. Adequate plant spacing is important because armored scales seldom spread from plant to plant unless the crowns of the plants are in contact (Beardsley & Gonzalez, 1975). As plants grow, pruning maintains spacing and allows maximum coverage when using insecticides.
Biological control --Predators
Ladybird beetles, or ladybugs, have been introduced and become established in Hawaii to control armored scales. Some of these, such as Telsimia nitida Chapin, (Coleoptera: Coccinellidae) have become established on the major Islands. The adults and larvae of these beetles are carnivorous, eating soft-bodied insects. Scale covers that look chewed and have no insect underneath are signs that predators have been feeding on the scales.
Biological control -- Parasites
Tiny wasps lay eggs in developing scales. They are parasites that absorb food from the scale body. Instead of an adult scale under the cover, an adult wasp emerges. These can be very effective in controlling armored scales (Dekle, 1965). In Hawaii The wasp Aphytis chrysomphali (Mercet) (Hymenoptera: Aphelinidae) parasitizes greedy scale (Zimmerman 1948).
Scraping and scrubbing to remove scales from plants are effective mechanical control tactics. Removing scales is especially important on exported plant materials, since intact armor is a sign of scale infestation.
Insecticide mode of action and formulation are important because the armor covers and protects all stages but the crawler and the adult male. Contact insecticides target the crawler stage; systemics target adult females and feeding nymphs. Since scales have natural enemies, care must be taken to conserve these. Populations of other pests, such as white flies and other scales, may rise if their natural enemies are affected by chemical control. Spraying should be determined by presence of scales in the field rather than by the calendar. Scales are best detected by regularly inspecting all areas of the fields for scales. When detected, directing spray at hot spots rather than uninfested areas helps conserve natural enemies and also delay pesticide resistance.
In the packing house, insecticidal soaps can be used in the cleaning water to kill crawlers while scrubbing off adults. Dipping without scrubbing in a soap-pyrethroid solution for five minutes is only 70% effective against adults and nymphs (Hansen et. al. 1992). Even though scales are killed, it takes several days for the body to dry, so removal of the armor is required to assure inspectors that the plant material is insect free.
Beardsley, J. W. Jr. and R. H. Gonzalez. 1975. The biology and ecology of armored scales. Annual Review of Entomology. 20: 47 - 73.
Dekle, G. W. 1965. Arthropods of Florida Vol. 3, Florida Armored Scale Insects. Division of Plant Industry, Florida Department of Agriculture, Gainesville. 265 pp.
Hansen, J. D., A. H. Hara and V. L. Tenbrink. 1992. Insecticidal dips for disinfesting commercial tropical cut flowers and foliage. Tropical Pest Management 38: 245-249.
Zimmerman, E. C. 1948. Insects of Hawaii Vol. 5 Homoptera: Sternorhyncha. University of Hawaii Press, Honolulu. 464 pages.