|Crop Knowledge Master|
Howardia biclauis (Comstock)
Victoria L. Tenbrink, Research Associate
Arnold H. Hara, Entomologist
Beaumont Research Center
Recorded hosts of mining scale include acacia, allamanda, bougainvillea, cassia, ficus, ebony, gardenia, hibiscus, ixora, jasmine, kelumpang, lantana, lychee, mango, papaya, plumeria, poinsettia, pulasan, sapodilla, and sapote (Comstock, 1881, Dekle, 1965; Westcott, 1964; Zimmerman, 1948). The host range undoubtedly includes more than the ones recorded.
Mining scale is found world-wide, outdoors in the tropics and indoors in glasshouses in the temperate zones. It was first recorded in Hawaii from Kona, Hawaii in 1895 by Maskell. Besides the Big Island, it is found on Niihau, Oahu, and Maui (Merrill, 1953; Zimmerman, 1948; Nishida, 1992).
Mining scale feeds on plant juices. This causes loss of vigor, deformation of infested plant parts, loss of leaves, and even death of the plant (Dekle, 1965; Beardsley & Gonzalez, 1975). Quarantine problems arise if infested plants or plant parts are to be exported.
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. Mining scale infests the bark, leaves, and fruit of the host plant, burrowing beneath the epidermal layer (Comstock, 1881).
No information is available
The first stage is the only nymph stage of mining scale with legs. At this stage the insects are called crawlers. Crawlers may stay under the motherŐs armor several hours waiting for favorable temperature and humidity conditions. After leaving the mother, crawlers seek a suitable feeding site. This search lasts minutes to days, but is usually a few hours. When a feeding location is found, the insects flatten against the host and begin secreting armor The armor is non-living. It is produced by the insect and consists of the cast skins, threads and liquid which hardens as it dries (Beardsley and Gonzalez, 1975).
Nymphs insert their piercing, sucking mouthparts into plant tissue and begin feeding on plant juices. The insects shed their skins as they grow and develop. As a nymph develops, its cast skins, called exuviae, are incorporated into the armor forming a dot. The location of the dot is important in determining the species of scale. The dot of mining scale is located at or near the margin of the armor (Beardsley & Gonzalez ,1975).
Female mining scales are so-named because they are often partially concealed by the epidermis of the host plant. They are relatively large, up to 1/8 in (3 mm) in diameter. The color is variable: white, gray or yellow, but usually the color isn<306>t obvious because of overlaying host tissue. The scale armor is thick, round and mildly dome-shaped, with a dot near or at the edge of the scale, sometimes extending over the edge. As stated previously, the dot is composed of cast skins of earlier stages. The body of the female (under the armor) is spindle-shaped. Males have not been observed, so it is believed the species is parthenogenetic, reproducing without males (Comstock, 1881; Dekle, 1965; Zimmerman, 1948).
Since female armored scales do not move from place to place after they begin feeding, long range dispersal happens by passive transport of infested plant material. Short range dispersal occurs as crawlers search out places to settle and feed. (Beardsley & Gonzalez, 1975). It is the crawler stage that is easily carried from place to place by people, animals, birds, ants, and wind currents (Dekle, 19965, Beardsley & Gonzalez, 1975). Wind is not only an agent of dispersal, but also one of mortality, since crawlers may not land on suitable host sites and are poor travelers on the ground (Beardsley & Gonzalez, 1975).
Because armored scales are spread chiefly by movement of nursery stock, only scale-free propagative material should be planted. Within a field or shadehouse, adequate spacing is important, since scale crawlers can move from plant to plant when plants contact are in contact with one another (Beardsley & Gonzalez, 1975). As plants grow, pruning maintains spacing. Pruning also allows maximum coverage when insecticides are applied.
Tiny, parasitic wasps lay eggs in developing scales. As the parasite develops within the body of the host, it absorbs food, consuming the scale. In Florida, mining scale is heavily parasitized (Merrill, 1953). In Hawaii, Aphytus diaspidis (Hymenoptera: Aphelinidae) is one of the species of wasps that parasitizes mining scale (Zimmerman, 1948).
Ladybird beetles, also called ladybugs, (Coleoptera: Coccinellidae) have been introduced to the Islands to control insects. Some of these, such as Telsimia nitida Chapin, have become established on the major Islands. The larvae and adults of these beetles have chewing mouthparts. The beetles are carnivorous, eating soft-bodied insects. Damaged scale covers are signs that predators have been feeding on the scales.
Scraping and scrubbing to remove scales from plants are usually effective mechanical control tactics for adults and settled nymphs. Removing scales is especially important on exported plant materials, since intact armor is a sign of scale infestation and will cause shipment rejection. This procedure may be difficult when mining scales are partially covered by host epidermis.
A promising new disinfestation treatment for propagative material and cut flowers and foliage is hot water treatment. Water at 49ű C (120ű F) for 6 min killed all life stages of a related scale, the magnolia white scale, with no damage to bird of paradise foliage (Hara et al., 1993).
There are two approaches to chemical control of armored scales such as mining scale, depending on the life stage of the pest. The armor of nymphs and adults protects them from contact insecticides, therefore systemics will be more effective for these stages. Crawlers are not yet feeding and are not protected by armor, so contact insecticides will be effective against this life stage and systemics will not. Since armored scales have many parasites and predators that help hold populations in check, chemical control will be most effective when it is designed to conserve natural enemies. One way of conserving beneficials is to spray only when scales are detected in the field, rather than spraying on a calendar basis. Scales are best detected by regularly inspecting all areas of the field. When scales are detected, directing spray at hot spots rather than uninfested areas helps conserve natural enemies and delay the onset of resistance in the pest. Because of the constant changes in chemical availability and labeling, consult a University of Hawaii Cooperative Extension Service agent for the latest chemical recommendations.
In the packing house, soaps or detergents can be used in the cleaning water to kill crawlers while scrubbing off adults. Dipping in soap-pyrethroid solutions should follow scrubbing, since alone it is only 70% effective against adults and nymphs (Hansen et al, 1992). Another advantage of pre-scrubbing is that armor is removed. Removal is important because, though scales may be killed by insecticidal dips, it takes several days for the body to dry enough to appear dead. Removal of the armor is required to assure inspectors that the plant material is insect-free.
Beardsley, J. W. Jr. & R. H. Gonzalez. 1975. The biology and ecology of armored scales. Annual Review of Entomology. 20: 47-73.
Comstock, J. H. 1881. Report of the Entomologist of the United States Department of Agriculture for the year 1880. Washington: Government Printing Office. Pp 98-100.
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 & V. L. Tenbrink. 1992. Insecticidal dips for disinfesting commercial tropical cut flowers and foliage. Tropical Pest Management 38: 245-249.
Hara, A. H., T. Y. Hata, B. K. S. Hu, & V. L. Tenbrink. Hot water immersion as a potential quarantine treatment against Pseudaulacaspis cockerelli (Homoptera: Diaspididae). J. Econ. Entomol. 86: 1167-1170.
Merrill, G. B. 1953. A Revision of the Scale Insects of Florida. Bulletin 1. State Plant Board of Florida: Gainesville, FL. pp. 50-51.
Westcott, Cynthia. 1965. The GardenerŐs Bug Book, 3rd ed. Doubleday & Co.: Garden City, N. Y. 371 pp.
Zimmerman, E. C. 1948. Insects of Hawaii Vol. 5, Homoptera: Sternorhyncha.University of Hawaii Press, Honolulu.