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).