Photo 1. Female citrus snow scale, Unaspis citri. The female is very different from the male; it is oval, brown-black and 2 mm long, whereas the male is 1 mm, and white.
Citrus snow scale, white louse scale
Worldwide. In the tropics. It is recorded from Australia, Cook Islands, Federated States of Micronesia, Fiji, New Caledonia, Niue, Papua New Guinea, Samoa, Solomon Islands, Samoa, Tonga, Vanuatu, and Wallis & Futuna.
Almost exclusively Citrus species, but the Pacific banana (fe'i, Musa x troglydytarum) is a host.
The male and female scales look quite different. The female is oval, oyster-shaped and brown-black; it is just over 2 mm long (Photo 1). The male is much smaller, 1 mm long, and white. Both have ridges running the lengths of their bodies. Numerous white males on trunks and branches give the appearance of snow, hence its common name.
Eggs hatch and the crawlers move to new sites on the same tree; they are spread further by wind, on birds, other animals and human beings. They eventually settle in crevices in the bark, start to feed and form protective scales over their bodies. Males moult twice and females four times, the last two are the pre-pupa and pupa stages with signs of adult features. The adult male is orange, with eyes, antennae and one pair of wings, but no mouthparts.
After the females are fertilised, the males die, usually within a few hours, and the females soon starts laying eggs. Unfertilised females also lay eggs and these develop as females. The entire life cycle is 2-3 months depending on temperatures.
The scale (Photo 1) occurs mostly on the trunk and large branches, except in heavy infestations when it spreads to the twigs, leaves and fruit. Heavy infestations cause leaves to yellow and fall, twigs to dieback and entire branches to die. The weakening of the trees in this way can lead to fungal rots and invasion by wood-boring insects.
Even after the scale has been brought under control, damage may still occur. The bark of previously infested trees does not expand properly as inner tissues grow, resulting in splitting and then invasion by other pests.
Populations are higher and damage is worse in dry times. It has been reported to be a locally important pest of oranges and lemons in Fiji, and a major pest and a constraint to growing West Indian limes in Samoa. In Solomon Islands, Waterhouse and Norris (1987)1 state that it is among the 10 worst pests of the country, "... causing widespread and serious damage every year". It is also important in Tonga.
Look for large white masses of the male scales, which are very distinctive. The darker oval female scales are more difficult to detect as they are small and do not occur in large numbers in any one place. Identification requires examination of the females under a microscope.
The management of the citrus snow scale is based on biological control, and in some countries the use of insecticides.
There are a large number of natural enemies of citrus snow scale. These fall into three groups: parasitoid wasps, predacious caterpillars and ladybird beetles. Of the many listed, two Asian wasps, Aphytis lingnanensis and Aphytis gordonae have greatest potential. However, different strains exist with different effectiveness against scale populations. Aphytis lingnanensis was introduced into Solomon Islands for control of citrus snow scale in 1976, but its establishment is unknown. In Australia, the caterpillar of a moth, Batrachedra arensella feeds on the scale. Many coccinellids eat the scale: Scotoscymnus prapawan has been released in Vanuatu. Chilocorus circumdatus is another successful biological control agent.
Nothing is reported, but see comments in the "Impact" section above for susceptible species.
Sulphur and malathion have been used, but the waxy coverings protect the scales from chemical sprays. Crawlers are more susceptible to insecticides than other stages, but they are difficult to see because they are so small. There are also overlapping generations, so adults are always present, and this means several applications of chemicals are necessary to obtain control. And adults are more difficult to kill because of their protective scales. (Note dimethoate, once recommended in Pacific island countries, is under suspension in Australia for use on many crops.)
AUTHOR Grahame Jackson
1Information from Waterhouse DF, Norris KR (1987) Biological Control Pacific Prospects. Inkata Press; and CABI Plantwise Knowledge Bank. (http://www.plantwise.org/KnowledgeBank/Datasheet.aspx?dsid=55685); and from Buckley CR, Hodges AC (2017) Citrus snow scale. UF/IFAS Extension Service, University of Florida.(http://entnemdept.ufl.edu/creatures/orn/scales/citrus_snow_scale.htm). Photo 1 (and Diagram) Gillian Watson, Senior Insect Biosystematist, Plant Pest Diagnostics Branch, California Department of Food & Agriculture, Sacramento, CA, USA.
Produced with support from the Australian Centre for International Agricultural Research under project PC/2010/090: Strengthening integrated crop management research in the Pacific Islands in support of sustainable intensification of high-value crop production, implemented by the University of Queensland and the Secretariat of the Pacific Community.
This fact sheet is a part of the app Pacific Pests and Pathogens
The mobile application is available from the Google Play Store and Apple iTunes.