Taxonomic name: Diaphorina citri Kuwayama, 1908
Synonyms: Euphalerus citri Crawford
Common names: Asian citrus psyllid, Asiatic citrus psyllid, Citrus psylla, Oriental citrus psyllid, Psilideo de l'aranjeira (Portuguese), Psylle de l'oranger (French)
Organism type: insect
Diaphorina citri or Asian citrus psyllid is one of the most serious pests of citrus in the world. It causes damage through direct feeding and its toxic saliva, leading to leaf distortion and curling in young tender growth. In addition the copious amounts of honeydew it excretes causes sooty molds to grow which blemish leaves and reduce photosynthesis. However it is the ability of D. citri to vector the Asian and American forms of the huanglongbing (HLB) disease which makes this so damaging. HLB is caused by phloem-restricted bacteria in the genus Candidatus Liberibacter. HLB causes chlorosis resembling zinc deficiency, twig dieback, stunting of growth and reduced fruit size and quality. Trees usually die after several years and entire orchards may be devastated. HLB seriously threatens citrus industries worldwide. At present there are no curative methods for trees infected with the bacteria, so control methods have focused on reducing D. citri populations. Control is achieved through a combination of physical, chemical and biological methods.
Diaphorina amoena, Diaphorina auberti, Diaphorina communis, Diaphorina murrayi, Diaphorina punctulata, Diaphorina zebrana
Diaphorina citri is confined to the plant family Rutaceae, occurring on wild hosts as well as on Citrus, especially lemon and lime (EPPO 2005a). A preferred host is Murraya paniculata, an ornamental that is widely grown in southern Florida. Populations on this plant can be extremely high and therefore inspection of this plant may be the best way to survey for Asian citrus psyllids (Halbert 2006). Host preference is influenced by season, variety, flush morphology, abundance, frequency and duration of flushing (Beattie and Barkley 2009).
Asian citrus psyllids feed on citrus and other closely related plants of the Rutaceae family (Arakelian 2008). They consume large amounts of sap from the plant as they feed, and excrete copious amounts of sugary honeydew. Honeydew coats trees and causes sooty mold to grow. Sooty mold fungi can lead to blemishing of leaves and fruit and reduction in photosynthesis (Wang et al. 2001 in Yang et al. 2006).
As they feed psyllids inject a salivary toxin that inhibits terminal elongation and causes malformation of leaves and shoots. Grafton-Cardwell et al. (2006) report that “A single psyllid nymph feeding for less than 24 hours on a citrus leaf causes permanent malformation of the leaf. Overwintering adults aggregate on newly forming citrus leaf buds where they feed and mate. Often, initial infestations of Asian citrus psyllids are highly aggregated on individual trees within citrus orchards. This aggregation and feeding causes distortion of the leaf buds that provides improved oviposition sites. Citrus flush is often severely damaged, resulting in the abscission of leaves and shoots (Halbert and Manjunath 2004) or malformed mature leaves. Mature trees can tolerate this damage since the loss of leaves or shoots is only a small portion of the total tree canopy. Nursery trees and new plantings may require chemical protection.”
By itself D. citri is a relatively minor pest (Halbert and Manjunath 2006). The most serious aspect of D. citri is its ability to vector Asiatic (Candidatus Liberibacter asiaticus) and American (Candidatus Liberbacter americanus) forms of huanglongbing (HLB). HLB or citrus greening disease is an extremely destructive disease of citrus (Halbert and Manjunath 2006; Bove 2006). The symptoms of HLB include yellowing of shoots and mottling and chlorosis of leaves that resembles zinc deficiency. Infected trees are often stunted and sparsely foliated. Fruit fail to color properly, have a bitter taste and are small, lopsided and hard (Grafton-Cardwell et al. 2006). The tree usually dies within 5 to 8 years, and entire orchards can be devastated after just a few years (Yang et al. 2006).
HLB affects almost all citrus cultivars, and causes substantial economic losses to the citrus industry by shortening the lifespan of trees and making fruit inedible (Das et al. 2007). Gottwald et al. (2007) report that “almost 100 million trees have been affected and destroyed in many countries of South and Southeast Asia, Indonesia, Philippines, India, Arabian Peninsula, and South Africa, compromising the local citriculture (Aubert et al. 1985; Bove 1986; Halbert and Manjunath 2004; Toorawa 1998). Since 2004, more than 500 thousand trees were officially eliminated in Brazil due to HLB and it is estimated that an additional 300 to 400 thousand trees were unofficially eliminated by commercial citrus growers.”
Interactions between D. citri and the HLB bacteria are not well characterized, but the psyllid is thought to acquire the bacterium after around 30 minutes of feeding (Roistacher 1991 in Halbert 2006). HLB is thought to multiply in the vector, and adults are able to transmit the pathogen after an 8-12 day latent period (Roistacher 1991 in Halbert 2006). There are conflicting results on whether HLB is able to be transmitted transovarially [transmission from mother to egg/larvae] (Buitendag and von Broembsen 1993; Roistacher 1991; van den Berg et al. 1992 in Halbert 2006).
Native Range: Tropical and subtropical Asia.
Known introduced range: Central and South America, the Caribbean basin, Saudi Arabian peninsula, Reunion and Mauritius to the east of Southern Africa. In the United States D. citri was first discovered in 1998 in Florida. It has now spread to Hawaii, Louisiana, Alabama, Georgia, Mississippi, South Carolina and California.
Introduction pathways to new locations
Horticulture: Plants for planting of citrus material (budwood, grafted trees, rootstock seedlings) can carry eggs and/or nymphs over long distances. The rutaceous plant
Murraya paniculata, frequently used as an ornamental bush or hedge, is one of the best hosts of this psyllid (EPPO, 2005a).
Ignorant possession: The psyllid is often transported from place to place by people moving infested plants (Plant Protection Service Secretariat of the Pacific Community 2003).
Military: It is possible that Diaphorina citri could arrive at new locations through passive transport of adult psyllids, which are strongly attracted to light, in commercial and military aircraft (the latter being a possible explanation for the recent detection of D. citri in
Guam) (Beattie and Barkley 2009).
Natural dispersal: Beattie and Barkley (2009) report that one likely source of D. citri into Australia could be through "air movements (e.g., cyclonic and jet streams) carrying psyllids from the Indonesian
Archipelago, and New Guinea".
Smuggling: Beattie and Barkley (2009) report that one likely invasion pathway for Diaphorina citri is through "illegal introductions of budwood from South Africa, Brazil, Asia or Florida by growers seeking commercial gain over other growers".
Local dispersal methods
Natural dispersal (local): It is not known how far Asian citrus psyllid vectors can fly, but informal estimates suggest that they might fly several kilometers (Halbert and Manjunath 2004).
Diaphorina citri feeds on the sap of plants in the Rutaceae family.
Adults live for about one to two months and their lifespan is influenced by temperature and host plant (Liu and Tsai 2000 in Grafton-Cardwell et al. 2006). The abudomen of a female becomes bright yellow-orange after she becomes gravid. Number of eggs laid depends on host plant. A female may lay up to 800 eggs in her lifetime (Halbert and Manjunath 2004). At a temperature of 25°C eggs hatch after about 4 days, and take about 16 to 17 days to become an adult (Tsai and Liu 2000 in Grafton-Cardwell et al. 2006).
Diaphorina citri has three developmental stages: egg, 5 nymphal instars and adult. "The eggs are orange-coloured and almond-shaped, 0.31 (long)–0.15 (wide) mm. Eggs are laid singly inside half-folded leaves of the buds, in leaf axils and other suitable places on theyoung tender parts of the tree. The nymphs pass through five instars. They are light-yellow to dark-brown, bearing well-developed wing pods. Nymphs will move away when disturbed but normally lead a sedentary existence clustered in groups. Adults are 2.5 mm long with yellowish-brown body and greyish-brown legs. Wings are transparent with white spots or light-brown with a broad, beige, longitudinal band in the centre. Adults are very active and jump on the slightest disturbance" (EPPO 2005a).
Compiled by: IUCN SSC Invasive Species Specialist Group (ISSG) with support from the Ministry of Agriculture and Forestry (MAF)- Biosecurity New Zealand
Last Modified: Thursday, 26 November 2009