Coffee green scale
Coccus celatus. Note there is another species of scale present on coffee, known as soft green scale (Coccus viride). In Papua New Guinea, both occur on the same coffee trees, but separation of the species requires expert examination of slide-mounted specimens. Coccus celatus is the more important.
Widespread. Coccus celatus is recorded from Southeast Asia, Africa, South America (Brazil), Oceania (Papua New Guinea). Coccus viridis has a wider distribution, and is present in many countries in Africa, Asia, North, South and Central America, the Caribbean, Europe and Oceania. It is recorded throughout Oceania, including Federated States of Micronesia, Fiji, Northern Mariana Islands, Palau, Papua New Guinea, Samoa, and Tonga.
Coffee green scale occurs mostly on plants in the coffee family (Rutaceae), but also on Calophyllum, Casuarina, citrus, guava, soursop, and more.
Nursery and young trees are weakened by the attack, and the leaves turn yellow. If the infestation is severe on mature plants (Photos 1&2), the berries dry and fall. The scale produces honeydew as it feeds on plant sap, and it is excreted in large amounts onto the foliage. Honeydew contains sugary substances favoured by sooty mould fungi, and their growth turns leaves and stems black.
The scales are attended by ants that are attracted to the honeydew. Surveys in Papua New Guinea have shown that the major ant species are introduced: Anoplolepis gracilipes (yellow crazy ant), Pheidole megacephala (African big-headed ant) and Technomyrmex albipes (white-footed ant). There are native ant species, but they are not as important in tending the coffee scale.
The eggs hatch within hours of being laid, and there are three nymph stages, all of which are mobile. By contrast, the adults remain in one place and become covered in an oval green scale, about 3.5 mm long. The nymphs and adults are more common on the undersurface of the leaves, and on the stems and berries (Photo 2).
There is no evidence that the ant species directly spread coffee green scale; their effect is indirect: it is to stop natural enemies from attacking the scales, and this increases the survival and spread of the scale. Long distance spread of the scale occurs on infested seedlings.
Surveys have shown that in Papua New Guinea, levels of infestation of coffee green scale are greatest at about 1500 masl.
In experimental plots on research stations, the presence of the coffee green scale causes yield reductions of up to 50%. Apart from its effect on mature trees, the scale causes dieback and death on seedlings, increasing the costs of planting and replanting. The effect on seedlings and young trees is greater when ants are in attendance as populations are higher.
Look for the adults; the females are about 3.5 mm long covered with a slightly curved oval green scale. Look for the green scale on the underside of the leaves and on young green stems. Look for sooty moulds and the presence of ants.
In Papua New Guinea, surveys in the 1980s showed that there were no hymenopterous (wasp) parasitoids and although there were several (ladybird beetle) predators, none were effective in controlling the coffee scales. Therefore, the (encyrtid) wasp, Metaphycus stanleyi, was introduced from Kenya in 1988. Unfortunately, control was only obtained in the absence of ants.
More recently, another encyrtid wasp, Diversinervus stramineus, is being considered for introduction from Australia. However, it is now known that in the absence of ants several native natural enemies exist, both parasitoid wasps, and predatory ladybird beetles.
In Papua New Guinea a fungus, Lecanicillium lecanii, is a natural enemy of Coccus celatus.
The use of pesticides against the coffee green scale is not recommended. It is likely that the wasp parasitoids and the coccinellid predators are more sensitive to them than the coffee green scales. The scales are protected by their waxy shells. If insecticides are required, do the following:
AUTHOR Grahame Jackson
Information from CABI (2015) Coccus celatus (coffee green scale) Crop Protection Compendium. (www.cabi.org/cpc).
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.
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