|Crop Knowledge Master|
Ischnaspis longirostris (Signoret)
|Black Thread Scale|
Victoria L. Tenbrink, Research Associate
Arnold H. Hara, Entomologist
Beaumont Research Center
Hosts include African iris, ixora, banyan, coconut and other palms, coffee, Ficus, litchi, mango, monstera, Pandanus, sago palm, star jasmine (Dekle 1965).
Almost cosmopolitan, this immigrant to the Hawaiian Islands was recorded from Oahu in 1932 and is found on all the populated Islands (Zimmerman 1948, Hawaiian Terrestrial Arthropod Checklist 1992).
The first sign of black thread scale in the field is usually the presence of armor on leaves, stems, and fruits. Their population density can reach 210 scales per square inch (Swezey 1945). Adult female black thread scales are about 3 mm long (1/8th inch). The armor is black, shiny, long and thin. The cast skins of the nymph forms a light spot, called a terminal exuvium, at the narrow end of the armor (Zimmerman 1948). The armor must be pried off to reveal the insect attached to the plant by hair-like mouthparts. 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. 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. Nymphs shed their skin twice as they grow and develop. The cast skins, called exuviae, are incorporated into the armor at the narrow end forming a dot. The armor is non-living and is made of cast skins, threads, and liquid, all produced by the insect
(Beardsley and Gonzalez 1975).
Female black thread scales appear larvae-like. They remain under armor in one place throughout their lives to feed and reproduce. Apparently, there are no males of this species (Dekle 1965).
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.
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, (Coccinellidae) have been introduced and become established in Hawaii to control armored scales. Some of these, such as Telsimia nitida Chapin, have become established on the major Islands. These beetle adults and larvae 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 at least two wasps, Arrhenophagus albipes (Encyrtidae) and Aspidiotiphagus citrinus (Aphelinidae) parasitize armored scales.
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. Contact insecticides target the crawler stage; systemics target adult females and 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 hotspots 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 38-2A9.
Swezey, O. H. 1945. Notes and Exhibitions. Proceedings, Hawaiian Entomological Society 12(2): 221.
Zimmerman, E. C. 1948. Insects of Hawaii Vol. 5 Homoptera: Sternorhyncha. University of Hawaii Press, Honolulu. p 404-405.