|Crop Knowledge Master|
Aspidiotus destructor (Signoret)
Jayma L. Martin Kessing, Educational Specialist
Ronald F.L. Mau, Extension Entomologist
Depatment of Entomology
Updated by: J.M. Diez April 2007
The coconut scale is a common pest of coconut and banana. It also infests many other trees and ornamental plants, some of the hosts include avocado, bird of paradise, breadfruit, ginger, guava, mango, mock orange, mountain apple, palm, papaya, pandanas, plumeria and sugarcane. See Williams and Watson (1988) for an extensive listing of hosts in the South Pacific area.
The coconut scale is common to tropical and subtropical regions worldwide, especially on islands. It is found in American Samoa, Fiji, French Polynesia, Hawaii, Irian Jaya, New Caledonia, Papua New Guinea, Solomon Is., Sri Lanka, Vanuatu and Western Samoa. It was first found in the State on Oahu in 1968 and has since spread to Kauai, Hawaii and Maui.
According to Taylor (1935) this scale disperses primarily with the aide of other creatures such as birds, insects and as is the case in Fiji, by bats. Accidental dispersal by human activities may occur through the transport of tropical nursery plants and goods made from plant material such as coconut leaf baskets (Taylor, 1935). Although little evidence exists, it is believed that another important mode of spread is by wind blown crawlers.
The coconut scale is among the most damaging of all armored scale insects (Beardsley, 1970). This pest is usually found in densely massed colonies on the lower surfaces of leaves, except in extremely heavy infestations where it may be present on both sides. It may also be found on petioles, peduncles and fruits. Mature scales are found on the older leaves. Infestations are typically associated with yellowing of the leaves in areas where the scales are present. The yellowing is caused by the removal of sap by the sucking mouth parts and the toxic effects of the saliva that kills the surrounding tissues at the feeding site (Waterhouse & Norris, 1987). This yellowing is distinct from and less mottled than that caused by another armored scale, Chrysomphalus ficus (Beardsley, 1970).
The coconut scale is classified as an armored scale. Unlike other scales, armored scales do not produce honeydew (Beardsley and Gonzalez, 1975). Armored scales feed on plant juices. Feeding sites are usually associated with discolorations, depressions and other host tissue distortions (Beardsley and Gonzalez, 1975).
Scale insects belong to one of two types, the armored scales or the soft scales. The coconut scale is classified as an armored scale. These scales are protected by a distinct, hard, separable shell or scale over their delicate bodies (Metcalf, 1962). The shell is made of entangled threads of wax exuded from the body wall of the scale and discarded cast skins (the old skin shed during molts). Armored scales lose their legs and antennae after the first molt. Females are always wingless and remain under their scale their entire life. Males have one pair of membranous wings, move about actively in search of females and do not feed during the adult stage. Reproduction is by eggs in most cases, but a few species birth live young. Eggs are protected underneath the scale or shell of the mother insect until they hatch. All armored scales have essentially the same life history (Metcalf, 1962).
Duration of developmental stages varies with temperature. Life history studies were conducted in Fiji by Taylor (1935) at a mean temperature of 79û F on seedling coconuts. Taylor found the total life cycle of females, from egg to the beginning of oviposition, required 34-35 days. Complete development of males required 30-35 days (Taylor, 1935). There are 8 - 10 generation per year in tropical regions.
Eggs are laid beneath the scale. Adult females shrink in size as they lay their eggs (Taylor, 1935). The female rotates as it deposits eggs so that eggs are arranged in concentric circles around the females. Eggs are laid in batches of 3 or 4 at a time, in moderately quick succession, with an interval of several hours between batches (Taylor, 1935). In life history studies conducted in Java on coconut, 65 to 110 eggs were laid by each female, with an average of 90 (Taylor, 1935).
The eggs are white when first laid and turn yellow after a few days. Because of the color change due to age of the egg, the eggs in the outer concentric rings are yellow and those in the inner and younger rings are white (Taylor, 1935).
Eggs hatch in the order in which they are laid (Taylor, 1935). Egg incubation is about as long as the egg laying period, such that the outer eggs are hatching as the last eggs are laid. This allows the larvae from the inner rings to crawl freely pass the remains of the older eggs (Taylor, 1935).
After hatching, the young larvae push their way out from beneath the adult scale. The larvae, called crawlers, have well-developed legs and antennae and a pair of bristles at the tip of the abdomen (Waterhouse & Norris, 1987). They crawl over the leaf surface until they find a suitable feeding site where they attach themselves to the leaf. Once a feeding site has been selected the scale will not move. The free living stage lasts from 2 to 48 hours, but usually does not exceed 12 hours (Taylor, 1935).
After the larvae have attached themselves to the leaf, they go through a period of rapid growth for 7-11 days before the first molt. All appendages are lost after the first molt. Up to the first molt there is no physical difference between the sexes, both are pale yellow.
In the middle of the second larval stage, males become reddish brown and elliptical in shape and the females remain pale yellow and circular. From this point, the development of the sexes differs. The second larval stage lasts for 5-8 days for males and 8-10 days for females (Taylor, 1935). Males are full grown at the end of the second larval stage and become pupae after their second molt. The male pupal period lasts for 4-6 days. Females continue to grow after their second molt for 8-9 days and do not change in body shape. When the female stops growing, she begins to lay eggs and is then considered an adult (Taylor, 1935).
The adult female is circular in shape and approximately 1/12 inch in diameter. Her orange-yellow body is visible beneath the milky-white, semitransparent, thin scale that covers her body. They are easily recognized because of their closely packed colonies and their resemblance to a fried egg, "sunny-side up" (Dekle, 1965). The female body color may also be greenish yellow depending on the food plant (Taylor, 1935).
Females produce an average of 90 eggs throughout her lifetime on coconut (Taylor, 1935). The oviposition, or egg laying period, lasts for 9 days (Taylor, 1935).
Adult, winged males emerge from the pupae by pushing out from beneath the larval skins. They do not emerge from the protective scale for 2 days. They are very minute two-winged yellowish insects, with antennae, eyes, three pairs of legs and a prominent long appendage projecting from the tip of the abdomen (Metcalf, 1962). Once out from beneath the scale the males crawl actively about and occasionally fly in search of a female to mate with. The adult male does not feed and is short lived.
Reproduction primarily occurs through parthenogenesis, or reproduction without fertilization, in which females are able to produce both male and female progenies. Occasionally a female may lay eggs that produce only male scales. It is therefore assumed that fertilized reproduction may occur in some instances, most likely influenced by food availability (Taylor, 1935).
Once the larvae have attached themselves to the leaf, scale formation occurs. Initially the scale appears as fine threads of silk. The larvae rotate as they produce silk until the matting of threads forms a thin continuous scale around the periphery of the larvae. The scale covers the entire larvae. This process takes about 12 hours (Taylor, 1935).
After accidental introduction to various Pacific islands, this scale became a serious pest of coconuts. With the introduction of parasitoids and predators, its pest status has been greatly reduced (Waterhouse & Norris, 1987). Largely because of efficient parasitoids and predators this scale is generally not a problem in Hawaii.
Around 40 species of predators and parasites have been reported to attack the coconut scale in areas outside of Hawaii (Beardsley, 1970). Several of these natural enemies have been used to successfully control outbreaks of this scale (Sweetman, 1958). The coccinellid beetle, Cryptognatha nodiceps (Marshall), was introduced to Fiji and effectively controlled this scale (Taylor, 1935). Another coccinellid beetle, Chilocorus politus (Mulsant), has also been effective in Mauritius and Indonesia (Beardsley, 1970). Ladybird beetles have been the very effective throughout the Tropics, especially Pseudoscymnus anomalus, Cryptognatha nodices, C. gemellata, Rhyzobius satelles, Chilocorus nigritus and C. malasiae (the last four are non-specific predators) (Waterhouse & Norris, 1987). Cryptognatha nodiceps has a long adult life, high reproductive capacity, good dispersal abilities, and is a voracious predator with a preference for the coconut scale (Waterhouse & Norris, 1987). Pseudoscymnus anomalus is a specific predator and requires only 14 days for development to adulthood which is much quicker than the development of other predators (Waterhouse & Norris, 1987). Refer to Waterhouse and Norris (1987) for an extensive list of natural enemies of the coconut scale.
In Hawaii, two coccinellid beetles, Telsimia nitida(Chapin) and Lindorus lophanthae (Blaisdell), were introduced to control other scale pests and are the principal predators of the coconut scale (Beardsley, 1970). Other successful introductions include Chilocorus nigritis and Pseudoscymnus anomalus from Guam in 1970 (Waterhouse & Norris, 1987). Other natural enemies in Hawaii are the predaceous thrips, Aleurdothrips fasciapennis (Franklin) and two minute aphelinid wasps that are internal parasites of the coconut scale (Beardsley, 1970).
Chemicals used on scales are usually the same as those used on mealybugs and may include diazinon, dimethoate, formothion, malathion and nicotine (Copland and Ibrahim, 1985). As in the use of all chemicals, consult the label or a pesticide database, to determine what chemicals may be used on specific crops. Special care should be taken with chemical sensitive palnts (Copland and Ibrahim, 1985).
There is no listing for malathion, diazinon and dimethoate are not labelled as of April 2007.
Sprays are only effective on the crawler stage of scales. However, control is difficult on other life stages. Adults are firmly attached to the plant and remain so after their death that may give a false impression of the pest status (Copland and Ibrahim, 1985).
Chemical applications should be used only when parasites are not economically effective. The application of pesticides may kill natural enemies of the scale and result in a resurgence of the pest.
Beardsley, J. W. 1970. Aspidiotus destructor Signoret, an Armored Scale Pest New to the Hawaiian Islands. Proc. Hawaii. Entomol. Soc. 20: 505-508.
Copland, M. J. W. and A. G. Ibrahim. 1985. Chapter 2.10 Biology of Glasshouse Scale Insects and Their Parasitoids. pp. 87-90. In: Biological Pest Control The Glasshouse Experience. Eds. Hussey, N.W. and N. Scopes. Cornell University Press; Ithaca New York.
Dekle, G. W. 1965. Florida Armored Scale Insects. Florida Department of Agriculture: Gainseville. pp. 265.
Elmer, H. S. and O. L. Brawner. 1975. Control of Brown Soft Scale in Central Valley. Citrograph. 60(11): 402-403.
Metcalf, C. L. 1962. Scale Insects. pp. 866-869. In: Destructive and Useful Insects Their Habits and Control. McGraw-Hill Book Company; New York, San Francisco, Toronto, London. 1087 pages.
Swain, G. 1969. The Coconut Stick Insect Graeffea crouani Le Guillou. Oleagineux. 24: 75-77.
Sweetman, H. L. 1958. The Principles of Biological Control. Wm. C. Brown Co. Dubuque, Iowa. 560 pp.
Taylor, T. H. C. 1935. The Campaign Against Aspidiotus destructor, Sign., in Fiji. Bull. Entomol. Res. 26: 1-102.
Waterhouse, D. F. and K. R. Norris. 1987. Chapter 8: Aspidiotus destructor Signoret. pp. 62-71. In: Biological Control Pacific Prospects. Inkata Press, Melbourne. 454 pages.
Williams, D. J. and G. W. Watson. 1988. Aspidiotus destructor Signoret. pp. 53-56. In: The Scale Insects of the Tropical South Pacific Region Part 1: The Armored Scales (Diaspidae). The Cambrian News Ltd. 290 pages.