Diaphorina citri, commonly known as the Asian citrus psyllid (ACP), is an insect pest that threatens citrus crops worldwide. This small, sap-sucking insect is a major concern as it transmits a devastating plant disease. Its widespread distribution in tropical and subtropical regions of Asia, and its subsequent spread to other citrus-growing areas, underscores its importance as an agricultural pest.
Identifying the Asian Citrus Psyllid
Adult Asian citrus psyllids are small, measuring 2-4 millimeters in length, with a brownish body that is mottled. A distinguishing characteristic of the adult psyllid is its feeding posture; it often feeds with its head down, nearly touching the leaf surface, while its body is lifted at about a 45-degree angle. Its transparent wings are often mottled with white or light-brown spots, and the insect may appear dusty due to a whitish, waxy secretion.
The life cycle of Diaphorina citri includes egg, nymph, and adult stages. Females lay almond-shaped, yellow to orange eggs on the tips of new growth or within the crevices of unfolding leaves. A single female can lay up to 800 eggs during her lifespan, which can last several months. Eggs hatch into nymphs, initially light yellow, progressing through five developmental stages to become brownish or greenish before maturing into adults. Nymphs are found on new shoots, characterized by slow movement and the production of waxy tubules that direct honeydew away from their bodies.
The psyllid feeds on plants in the Rutaceae family, including most commercial citrus varieties. Ornamental species like orange jasmine (Murraya paniculata) are also preferred hosts and can harbor large psyllid populations. Identifying these life stages and host plants is important for early detection.
The Link to Citrus Greening Disease
Diaphorina citri is a vector for the bacterium Candidatus Liberibacter asiaticus (CLas), which causes Huanglongbing (HLB), also known as citrus greening disease. This bacterium is phloem-limited, residing within the plant’s vascular tissue. The psyllid acquires the bacterium by feeding on an infected citrus tree. After an incubation period of around 21 days, it can transmit the bacterium to healthy trees as it feeds.
HLB causes severe symptoms in citrus trees, leading to decline and often death. Early symptoms include yellowing leaves, particularly an asymmetrical blotchy mottling pattern, a key diagnostic feature. Affected fruit are small, misshapen, and lopsided, often remaining partially green at the stylar end while the peduncular end turns yellow or orange. Premature fruit drop is also common.
HLB has a devastating impact on global citrus production. The disease causes significant reductions in crop yield and fruit quality, leading to substantial economic losses. Where HLB is prevalent, entire groves can become infected rapidly, sometimes within a year, though visual symptoms in mature trees may take up to three years to appear. This delayed symptom expression complicates disease management and highlights the importance of controlling the psyllid vector.
Strategies for Managing the Psyllid
Managing Diaphorina citri involves combining approaches to reduce psyllid populations and limit citrus greening disease spread. Cultural practices are fundamental. Removing infected trees, especially where disease incidence is low, helps eliminate pathogen reservoirs. Using certified disease-free nursery stock for new trees is also important to avoid introducing the disease. Proper tree care, including balanced fertilization and irrigation, contributes to overall tree health, potentially making them more resilient.
Biological control leverages natural enemies to suppress psyllid populations. The parasitic wasp Tamarixia radiata is a widely used biological control agent that lays eggs on psyllid nymphs, leading to their demise. Generalist predators like lady beetles, lacewings, and spiders are present in citrus orchards, but their impact on psyllid control varies. Ants, such as the Argentine ant, can interfere with biological control by protecting honeydew-producing psyllids from natural enemies.
Chemical control involves judicious insecticide use. Applications are timed to target new growth flushes where psyllids lay eggs and nymphs feed. Foliar sprays with pyrethroids or neonicotinoids are common, with winter applications often favored for effectiveness in colder temperatures and reduced impact on dormant natural enemies. Soil drenches with systemic insecticides like imidacloprid, thiamethoxam, or clothianidin also provide extended protection, particularly for young trees. Adhering to local regulations and safe pesticide use guidelines minimizes environmental and health impacts.
An integrated pest management (IPM) approach, combining these strategies, is the most effective way to control Diaphorina citri and mitigate HLB spread. This approach emphasizes monitoring psyllid populations and disease incidence, making informed decisions about interventions, and prioritizing environmentally sound, sustainable methods. The goal is to keep psyllid numbers low to reduce bacterium transmission risk, as there is currently no cure for citrus greening disease once a tree is infected.