Citrus health worldwide faces a severe threat from the Asian citrus psyllid (Diaphorina citri) and the disease it carries, Huanglongbing (HLB), also known as citrus greening disease. This insect vectors the bacteria responsible for HLB. Understanding both the psyllid and the disease is fundamental to protecting citrus groves globally.
Understanding the Asian Citrus Psyllid
The Asian citrus psyllid is a small insect, 3 to 4 millimeters long, roughly the size of a grain of rice. Its body is mottled brown with a lighter head, and its forewings are broadest at the tip, featuring a brown mottled band along the outer edge. A distinguishing characteristic is its feeding posture: it feeds with its head down, almost touching the leaf surface, causing its body to lift at about a 45-degree angle.
Native to Asia, this insect has spread to parts of Africa, Central and South America, the Caribbean, and several U.S. states like Florida, California, and Texas. Females lay elongated, almond-shaped eggs, about 0.3 millimeters long, on new growing shoots and between unfurling leaves. Eggs are initially pale, then turn yellow, and finally orange before hatching.
Nymphs, yellowish-orange and resembling aphids, pass through five developmental stages, or instars, before maturing into adults. This life cycle, from egg to adult, takes anywhere from 15 to 47 days, depending on environmental conditions like temperature. Adults can live for several months, with females laying 300 to 800 eggs during their lifespan.
The psyllid causes direct damage by feeding on plant sap. Nymphs secrete a waxy honeydew, which can lead to sooty mold growth on leaves, blocking sunlight. Its primary impact, however, is transmitting the bacteria that cause Huanglongbing.
Huanglongbing The Citrus Greening Disease
Huanglongbing (HLB) is a severe and incurable plant disease affecting nearly all commercially cultivated citrus species and hybrids. It is caused by phloem-limited bacteria from the genus Candidatus Liberibacter, primarily Candidatus Liberibacter asiaticus (CLas), the most widespread and heat-tolerant form. These bacteria are primarily transmitted by the Asian citrus psyllid, which acquires the pathogen by feeding on an infected tree and then spreads it to healthy trees.
HLB bacteria reside within the phloem, the vascular tissue responsible for transporting sugars and nutrients throughout the plant. The infection triggers a systemic and chronic immune response in the phloem, leading to the production of reactive oxygen species (ROS) and the deposition of callose. This accumulation of callose and other cellular changes causes the phloem sieve elements and companion cells to become blocked and eventually die. The disruption of phloem functionality prevents efficient carbohydrate transport, leading to nutrient deficiencies and overall decline.
Visible symptoms often appear months to years after initial infection due to a long latent period. An early and characteristic symptom on leaves is an asymmetrical, blotchy yellow mottling, which differs from nutritional deficiencies that show more symmetrical patterns. Affected leaves may also become leathery with enlarged, corky veins. As the disease progresses, yellow shoots may appear on random branches, followed by progressive yellowing of larger portions of the tree canopy.
The disease severely impacts fruit quality and production. Infected trees produce small, misshapen, lopsided fruits that remain poorly colored, often green at the bottom, and have a bitter taste. Premature fruit drop is also common.
Over time, infected trees exhibit twig dieback, stunted growth, a thinning canopy, and a general decline in health, often leading to tree mortality within several months to years after infection.
Managing the Threat to Citrus Trees
Managing the Asian citrus psyllid and Huanglongbing disease involves a multi-faceted approach known as Integrated Pest Management (IPM). A foundational strategy is the use of disease-free nursery stock to ensure new plantings are not already infected. Regular monitoring of psyllid populations and visual inspections for HLB symptoms are also important for early detection and intervention.
Biological control plays a role, particularly through the introduction of natural enemies. For instance, the parasitoid wasp Tamarixia radiata has been released in various regions to target the Asian citrus psyllid. While generalist predators like ladybugs, syrphid flies, and spiders also contribute to psyllid suppression, their effectiveness can be reduced in areas with extensive pesticide use.
Chemical treatments are a primary management tool, especially in commercial groves, to suppress psyllid populations and slow disease spread. Insecticides, including pyrethroids, organophosphates, neonicotinoids, and diamides, are applied to manage adult psyllids and nymphs.
Winter sprays, from December through February, are significant because they target psyllids when populations are low and before new growth flushes appear. During the growing season, insecticide applications are timed with new growth flushes, where eggs are laid. However, reliance on frequent insecticide applications can lead to pesticide resistance in psyllids and may negatively impact beneficial insects.
Cultural practices also contribute to management efforts. Removing infected trees is a measure to eliminate sources of the HLB bacteria, especially when the incidence of diseased trees is below a certain threshold, such as 30%. For home gardeners, this means promptly removing any trees showing HLB symptoms.
Timing production activities that encourage new flushes, such as nitrogen fertilizer application and pruning, to coincide with periods when psyllids are least active, summer and winter, can help reduce psyllid infestations. Physical barriers, such as mesh netting, can be used in nurseries and greenhouses to prevent psyllids from reaching trees, although their use in larger orchards is still being evaluated.
Reflective mulches have shown promise in young trees by interfering with the psyllid’s ability to locate host plants. These combined strategies are important for mitigating the impact of this disease on citrus production.