Photo 2. Nymphs of the citrus psyllid, Diaphorina citri, showing the white waxy threads that are extruded from their bodies.
Photo 3. Adult citrus psyllid, Diaphorina citri, showing wing patterns and characteristic feeding position.
Photo 5. Leaves with symptoms of huangbinglong disease; note that most leaves show yellowing on one side of the leaf. The leaf in the top left corner is health for comparison.
Photo 6. Symptoms of huanglongbing disease on citrus leaves and fruits. Notice the uneven colour of the fruit on the right.
Asian (or Asiatic) citrus psyllid
Worldwide. Asia, North, South and Central America, the Caribbean, restricted in Africa (Mauritius and Reunion), Oceania. It is recorded from American Samoa, Guam, Northern Mariana Islands, Papua New Guinea, and Samoa. It is no longer found in Australia, where it was eradicated.
The Asian psyllid occurs on members of the citrus family, the Rutaceae. It occurs on ornamentals and wild hosts as well as on citrus. It especially favours grapefruit, kumquat, lemons, limes, mandarin, orange, pomelo and tangelo. Murraya paniculata, a plant often used for hedges, is also a host, as is the curry tree (Bergera koenigii).
Eggs are yellow-orange, somewhat oval (almond shape), laid near the tip of the shoots, in folds of the young leaves, on leaf stalks (petioles), stems, and buds (Photo 1). After about 4 days the eggs hatch producing orange nymphs which at first crowd together, feeding on the tender shoots. They produce honeydew and also a white thread-like waxy substance (Photo 2). These fall on the leaves below and sooty mould fungi grow on them turning the leaves black. The nymph stage lasts about 12 days and during that time the nymphs moult several times.
The adults have wings with brown patches, but their characteristic feature is the feeding position (Photo 3). The head reaches downwards, almost to the plant surface with the body held at about 40 degrees. They usually feed on the underside of the leaf, jumping or flying short distances when disturbed.
The psyllid reproduces best at temperatures between 25 and 28°C. Under these conditions, the female lays between 400 and 800 eggs and remains alive for about 50 days. From egg to adult is about 15 days, and this means populations increase rapidly. Eggs and nymphs can only be found when leaf flushes occur, but adults occur throughout the year.
Adults are poor fliers rarely flying for more than 100 m at one time. Spread of the psyllid occurs by wind and via the trade in plants moving within and between countries.
The psyllid damages plants in two ways. It sucks the sap from young shoots causing stunting, distortions and sometimes death (Photo 4). Badly twisted leaves fall prematurely. Leaves also become covered with honeydew and sooty mould. Young flowers fall and fruits drop. Twigs dieback when infestations are heavy.
The Asian citrus psyllid transmits greening disease, also known as "huanglongbing" (yellow dragon disease). It is caused by a type of bacterium called Candidatus Liberibacter asiaticus which occurs inside the plant, in the phloem of the vascular tissues. There are three strains of the bacterium.
Symptoms of the disease are difficult to tell from lack of nutrients; they look similar to zinc deficiency. However, leaves on one branch or part of a branch turn pale yellow, or show blotches of yellow and green; sometimes the green areas are only along the veins (known as "vein-banding") (Photos 5&6). Within a year, all the leaves of the tree turn yellow, some fall, and there is no fruit production, or the fruits are small, misshapen with uneven colour - the stalk end is orange and the bottom is green - and the taste is bitter. The colour of the bottom of the fruit gives the diseases is common name: "greening". Eventually, the trees dieback (Photo 7).
The psyllid feeds on infected plants and takes up the bacterium in under an hour. Eight to 12 days later it can infect other plants, after feeding on them for about 24 hours. Importantly, symptoms take at least a year to develop before they become noticeable; by then the disease has been distributed widely.
Look for the eggs, nymphs with their waxy threads, and adult phyllids on new flushes of citrus, every 14 days during the time when flower buds open. Look for the adults especially on the underside of mature leaves - they hold their bodies at 40 degrees from the plant surface. Look for sooty moulds on the leaves.
Countries not yet infested by the citrus psyllid should take special precautions against its introduction. They should follow the FAO/IBPGR Technical Guidelines for the Safe Movement of Citrus Germplasm (http://www.bioversityinternational.org/e-library/publications/detail/faolbpgr-technical-guidelines-for-the-safe-movement-of-citrus-germplasm/).
Note, it is unlikely that the psyllid would be eradicated should it be introduced; furthermore, once a tree has developed huanglongbing disease there is no cure. Invariably, 2 years or so after the introduction of the psyllid to a new location, huanglongbing disease occurs.
Consult your agricultural authorities before using pesticides for the control of the psyllid. Use pesticides during periods of flushing:
AUTHOR Grahame Jackson
Photos 1&3 David Hall, USDA Agricultural Research Service, Bugwood.org. Photo 2 Michael Rogers, University of Florida. Photo 4 Michael Rogers, University of Florida. Photo 5 JM Bove, INRA Centre de Recherches de Bordeaux, Bugwood.org. Photo 6 Jeffery W. Lotz, Florida Department of Agriculture and Consumer Services, Bugwood.org. Photo 7 HD Catling, Bugwood.org.
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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