Unaspis citri
(citrus snow scale)

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Identity

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Preferred Scientific Name

• Unaspis citri (Comstock, 1883)

Preferred Common Name

• citrus snow scale

Other Scientific Names

• Chionaspis annae Malenotti
• Chionaspis citri Comstock, 1883
• Dinaspis annae Malenotti
• Dinaspis veitchi Green & Laing, 1923
• Prontaspis citri (Comstock) MacGillivray, 1921
• Unaspis annae Malenotti

International Common Names

• English: orange chionaspis; orange snow scale; white louse scale
• Spanish: cochinilla blanca de los citrus; cochinilla blanca del tronco; escama de nieve de los cítricos; escama de nieve de los citricos (mexico); guagua nevada de los cítricos; piojo bianco; piojo blanco de los citricos (mexico); piojo harinoso de los citricos; queresa del naranjo
• Portuguese: cochonilha branca da aranjeira (brasil)

Local Common Names

• Brazil: cochonilha branca da aranjeira
• Germany: schneeweisse citrus-schildlaus

EPPO code

• UNASCI (Unaspis citri)

Taxonomic Tree

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• Domain: Eukaryota
•     Kingdom: Metazoa
•         Phylum: Arthropoda
•             Subphylum: Uniramia
•                 Class: Insecta
•                     Order: Hemiptera
•                         Suborder: Sternorrhyncha
•                             Unknown: Coccoidea
•                                 Family: Diaspididae
•                                     Genus: Unaspis
•                                         Species: Unaspis citri

Description

Top of page The adult female scales are mussel or oyster-shell shaped, brown or brown-black with a lighter coloured margin, moderately convex and often have a distinct longitudinal dorsal ridge. The scales attain a length of 2.25 mm. The male scales are smaller, white, felted, elongate oval in shape with three longitudinal ridges. The exuviae of both sexes are terminal and brownish-yellow.

Authoritative identification involves detailed microscopic examination of slide-mounted, teneral, adult females. Detailed morphological descriptions, illustrations and keys to Unaspis are provided by Balachowsky (1954), Ferris (1937) and Williams and Watson (1988).

Distribution

Top of page U. citri originated in Asia and has spread widely in other tropical and subtropical regions. It is now found in North, Central and South America, Africa and Oceania (see the computer-generated maps and CIE, 1962 for an exact list of countries). The pest was found in Portugal (Azores) in the 1920s, but there have been no records since.


See also CABI/EPPO (1998, No. 154).

Distribution Table

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The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

Last updated: 23 Apr 2020

Risk of Introduction

Top of page U. citri is listed in the European Community Plant Health Directive Annex IIAI and is a quarantine pest on plants of Citrus spp., Fortunella spp. and Poncirus spp. (other than fruit and seeds). U. citri appears on the quarantine pest list of Russia and is under renewed consideration by EPPO as a potential A1 quarantine pest.

U. citri is a more tropical species than the closely related U. yanonensis. In West Africa, it is confined to the humid tropical zone along the coast and does not occur where there is a dry season (Vilardebo, 1974). In Queensland and New South Wales, in Australia, U. citri is confined to the non-irrigated humid coastal regions and does not occur in the semi-arid irrigated citrus-cultivation areas inland (Maelzer, 1979).
U. citri also has less of a tendency to attack fruits than U. yanonensis.

Hosts/Species Affected

Top of page U. citri is polyphagous attacking plant species belonging to 12 genera in nine families. The main hosts of economic importance are Citrus spp., especially orange, but the insect is also recorded on a wide range of other crops, mostly fruit crops and ornamentals, including banana, Capsicum spp., coconut, guava, hibiscus, jackfruit, kumquat, pineapple, soursop, Spanish moss and trifoliate orange.

Host Plants and Other Plants Affected

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Growth Stages

Top of page Flowering stage, Fruiting stage, Vegetative growing stage

Symptoms

Top of page Infestations of U. citri usually occur on the trunk and main limbs of trees under ten years old. Heavy infestations spread to the twigs, leaves and fruit. This results in yellow spotting on the undersides of leaves which drop prematurely, dieback of twigs and weakening and eventual killing of branches. Heavily infested bark becomes dark, dull, and hard, appears tight and subsequently splits. Weakened limbs and twigs become infected with fungi and may be subsequently attacked by wood-boring insects.

List of Symptoms/Signs

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Biology and Ecology

Top of page U. citri is sexually reproductive and produces several overlapping generations a year. The active first instars are most abundant during late spring, summer and autumn and populations tend to peak in late autumn. The life cycle takes 10-12 weeks to complete during the summer but longer during cooler weather. Each female can produce up to 169 first instars (over a period of 146 days); the average number of offspring per female is about 80. For a short period after hatching, the first instars are attracted to light and move upwards towards the apical twigs or on to the fruit, especially if leaf fall has occurred. Once a feeding site has been selected the females and immature males become sessile (Hely et al., 1982).

Laboratory studies of the population dynamics of U. citri showed that the net reproductive rate, intrinsic rate of increase and finite rate of increase were higher on orange than on lemon. The longevity of female scales on orange was approximately 13 weeks compared to 17 weeks on lemon (Fernandez and Garcia, 1988). Population studies in Colombia have shown that at any given time, 86.5 to 95.5% of U. citri are not feeding and that 43.9 to 79.3% of first instars are males which cease to feed after the second moult (Mosquear, 1979).

Like other diaspids, the main dispersal stage of U. citri is the first instar which as well as being mobile may be naturally dispersed by wind and animals. Once a feeding site has been selected the insect becomes sessile and is not naturally dispersed. However, it is readily carried on consignments of plant material and fruit.

Natural enemies

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Notes on Natural Enemies

Top of page Important natural enemies include the hymenopterous parasitoids Aphytis lingnanensis and Aspidiotiphagus lounsburyi and in Australia, a predatory caterpillar, Batrachedra sp. (for a complete list see Waterhouse and Norris, 1987). In Australia Batrachedra sp. causes spectacular reductions in the number of scales whenever pest populations become dense (Hely et al., 1982).

See Rosen (1990) for a discussion and further references to the natural enemies of Diaspididae.

Impact

Top of page U. citri is one of the principal pests of Citrus spp. in many of the citrus-growing regions of the world. It infests the trunk, branches and small shoots causing serious damage to orchards due to leaf drop and rapid dieback. Relatively small numbers of scales can cause damage.

Detection and Inspection

Top of page The small size, dark colour and sessile nature of the female scales makes them difficult to detect unless present in large numbers. In contrast the large white masses of male scales are conspicuous, hence the common name of citrus snow scale. On citrus fruit, the female scales can be confused with the common Lepidosaphes spp. or easily overlooked as dirt particles.

Similarities to Other Species/Conditions

Top of page U. citri should be distinguished from U. yanonensis which occurs throughout South-East Asia, Australia and France (Smith et al., 1992). Adult female U. citri have relatively few pygidial dorsal ducts, do not have marked divisions between the thoracic segments and have subjacent median lobes. Adult female U. yanonensis have numerous pygidial dorsal ducts, usually have marked divisions between the thoracic segments and have distinct median lobes.

On citrus fruit, the female scales can be confused with the common Lepidosaphes spp.. Adult female U. citri scales often have a distinct longitudinal dorsal ridge which is absent in Lepidosaphes spp.. Secondly, the male U. citri are white, felted, elongate oval in shape with three longitudinal ridges. Male scales of Lepidosaphes spp. are mussel or oyster-shell shaped, purplish or brown and similar in appearance to, but smaller than, the female scales.

Prevention and Control

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Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Regulatory Control (Plant Quarantine and Certification)

Importation of citrus plants for planting from countries where U. citri occurs should be prohibited. Fruits should be subject to requirements such as area freedom, place of production freedom or treatment.

Biological Control

Existing biological control agents include the hymenopterous parasitoids Aphytis lingnanensis used in Florida (USA), the Solomon Islands and Cuba, and Aspidiotiphagus lounsburyi in Cuba. Construction of field cages to enclose citrus trees has facilitated the release and establishment of A. lingnanensis in citrus groves in Florida (Brooks and Vitelli, 1976). U. citri increased greatly in numbers after 1963 in Florida and was not effectively suppressed by biological methods. High-volume pesticide sprays were required for control (Simanton, 1974). Browning (1994) provided an up-to-date assessment of biological control in Florida. Partial success has been obtained with A. lingnanensis and efforts are being made to make additional introductions.

Chemical Control

Chemical control is possible but the waxy surfaces, sessile nature, intermittent feeding and overlapping generations of U. citri make it difficult to control. In Cuba, the insecticides sulphur, carbaryl, dimethoate and malathion are commonly used (Castineiras and Obregon, 1986). In laboratory and field trials, mineral oil or a mineral oil/dimethoate mixture was found to be the most effective against the mobile stages and against the general population (Fernandez and Rodriquez, 1988).

References

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Balachowsky AS, 1954. Les cochenilles paléarctiques de la tribu des Diaspidini. Paris, France: Institut Pasteur.

Brooks RF; Vitelli MA, 1976. An easily erected tree cage for introducing insect parasites. Florida Entomologist, 59(1):67-70

Browning HW, 1994. Classical biological control of citrus scale insects. In Rosen D, Bennett FD, Capinera JL, eds. Pest management in the subtropics: biological control - a Florida perspective. Andover, UK: Intercept Limited, 49-78.

CABI/EPPO, 1998. Distribution maps of quarantine pests for Europe (edited by Smith IM, Charles LMF). Wallingford, UK: CAB International, xviii + 768 pp.

Castineiras A; Obregon O, 1986. Toxicity of six pesticides used on citrus crops against Aspidiotiphagus lounsburyi. Ciencia y Tecnica en la Agricultura, Proteccion de Plantas, 9(3):73-79

Charles JG; Henderson RC, 2002. Catalogue of the exotic armoured scale insects (Hemiptera: Coccoidea: Diaspididae) in New Zealand. Journal of the Royal Society of New Zealand, 32(4):587-615; many ref.

CIE, 1962. Distribution Maps of Plant Pests, No. 149. Wallingford, UK: CAB International.

Culik MP; Ventura JA; Martins Ddos S, 2009. Scale insects (Hemiptera: Coccidae) of pineapple in the State of Espírito Santo, Brazil. Acta Horticulturae [Proceedings of the Sixth International Pineapple Symposium, Joao Pessoa, Brazil, 18-23 November 2007.], No.822:215-218. http://www.actahort.org

EPPO, 2014. PQR database. Paris, France: European and Mediterranean Plant Protection Organization. http://www.eppo.int/DATABASES/pqr/pqr.htm

Fernandez M; Garcia G, 1988. Population parameters of Unaspis citri (Homoptera: Diaspididae). Revista de Proteccion Vegetal, 3(3):198-200

Fernandez M; Rodriquez ME, 1988. Effectivity of five products in the chemical control of Unaspis citri (Homoptera: Diaspididae). Revista de Proteccion Vegetal, 3(1):45-52

FERRIS GF, 1937. Atlas of the Scale Insects of North America. Stanford Univ. Pr., Calif. ; London, H. Milford, 9 + [272] pp.

Hely PC; Pasfield G; Gellatley JG, 1982. Insect Pests of Fruit and Vegetables in NSW. Sydney, New South Wales, Australia: Department of Agriculture, New South Wales, viii + 312 pp.

Ministerio de Agricultura y Ganadería Servicio Fitosanitario del Estado (Costa Rica), 2009. Listado de Plagas (insectos y ácaros) en cultivos de importancia económica en Costa Rica (C). http://www.protecnet.go.cr/laboratorios/plagcul/cultivoc.htm

Mosquera P F, 1979. Some observations on the economic importance of the citrus snow scale, Unaspis citri. Revista Colombiana de Entomologia, 5(3/4):53-56

Mplzer DA, 1979. The current status of the biological control of insect pests of citrus in Australia. Australian Applied Entomological Research Conference. Queensland Agricultural College, Lawes, June 1979. Invited reviews and situation papers. Commonwealth Scientific and Industrial Research Organization. Canberra Australia, 236-240

Rosen D, 1990. Armoured Scale Insects: their Biology, Natural Enemies and Control. World Crop Pests 4B. Amsterdam, Netherland: Elsevier.

Servicio para el Agricultor, 1973. Control of the Citrus scale. Noticias Agricolas, 6(26):106

Simanton WA, 1976. Occurrence of insect and mite pests of Citrus, their predators and parasitism in relation to spraying operations. Tall Timbers Research Station: Proceedings Tall Timbers Conference on Ecological Animal Control by Habitat Management, No. 6, Feb. 28 - March 1, 1974, Gainesville, Florida. Tall Timbers Research Station. Tallahasee, Florida USA, 135-163

Smith IM; McNamara DG; Scott PR; Harris KM(Editors), 1992. Quarantine pests for Europe: data sheets on quarantine pests for the European Communities and for the European and Mediterranean Plant Protection Organization. Wallingford, UK; CAB International, ix + 1032 pp.

Stapley JH, 1976. Annual report of entomologist for 1976. Report, Ministry of Agriculture and Lands, Solomon Islands, 28 pp.

Vilardebo A, 1974. Citrus Coccoids in West Africa. Distribution and development in relation to climate. Bulletin SROP [Section Regionale Ouest Palearctique], No. 3:67-78

Williams DJ; Watson GW, 1988. The Scale Insects of the Tropical South Pacific Region. Part 1. The Armoured Scales (Diaspididae). Wallingford, UK: CAB International.

Distribution References

CABI, Undated. CABI Compendium: Status as determined by CABI editor. Wallingford, UK: CABI

Culik M P, Ventura J A, Martins D dos S, 2009. Scale insects (Hemiptera: Coccidae) of pineapple in the State of Espírito Santo, Brazil. Acta Horticulturae. 215-218. http://www.actahort.org

EPPO, 2020. EPPO Global database. In: EPPO Global database, Paris, France: EPPO.

NPPO of the Netherlands, 2013. Pest status of harmful organisms in the Netherlands., Wageningen, Netherlands:

UK, CAB International, 1962. Unaspis citri. [Distribution map]. In: Distribution Maps of Plant Pests, Wallingford, UK: CAB International. Map 149.

Williams D J, Watson G W, 1988. The scale insects of the tropical South Pacific Region. Part 1. The armoured scales (Diaspididae). Wallingford, UK: CAB International. 290pp.

Distribution Maps

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