**Psyllids** (order Hemiptera, superfamily Psylloidea), often called **jumping plant lice**, are small, highly mobile insects closely related to aphids and whiteflies. They are major agricultural and ornamental pests because they cause direct feeding damage and, more critically, are effective vectors of serious plant pathogens. The nymphs often secrete a waxy, sugary substance called **honeydew** and some species cause characteristic, unsightly **galls** or curled leaves on their host plants.
Taxonomy and Classification
Psyllids belong to the order Hemiptera (true bugs) and undergo incomplete metamorphosis. They are distinguished by their resting posture—appearing like a miniature cicada—and their strong hind legs that allow them to jump quickly when disturbed. Economically important species include the **Asian Citrus Psyllid** (*Diaphorina citri*), which vectors citrus greening disease, and the **Tomato/Potato Psyllid** (*Bactericera cockerelli*).
Physical Description
Adult Psyllids are tiny, $1/16$ to $1/8$ inch long.
- **Appearance:** Resemble small, winged aphids, often green, black, or mottled. They hold their wings roof-like over their bodies.
- **Vectoring Position:** The Asian Citrus Psyllid is often found feeding at a steep, head-down angle, sometimes nearly perpendicular to the leaf or stem.
- **Nymphs:** Flat, oval, and often waxy or covered with white, cottony secretions.
- **Damage Sign:** Damage includes distorted new growth, leaf curling, waxy or crystalline honeydew residue, and sometimes premature leaf drop.
Distribution and Habitat
Psyllids are distributed globally. Their habitat is the tender, newly developing growth of their specific host plants, which include many trees (e.g., boxwood, eucalyptus, sycamore) and vegetable crops (solanaceous plants like tomato and potato). They require mild temperatures and shelter for overwintering.
Behavior and Damage
Psyllids use piercing-sucking mouthparts to extract sap from the phloem tissue of the plant.
- **Toxicity/Disease:** Some species inject toxins while feeding, causing leaf burn or distortion (e.g., Tomato Psyllid causes **Psyllid Yellows**). The most severe damage is pathogen transmission (e.g., **Citrus Greening/HLB**), which leads to the death of millions of citrus trees globally.
- **Sooty Mold:** The honeydew excreted by nymphs provides an ideal substrate for the growth of black **sooty mold**, which interferes with photosynthesis and is unsightly.
- **Aesthetic Damage:** Gall-forming psyllids cause localized swelling and distortion of leaves or stems, which can affect the ornamental value of landscape plants.
Management and Prevention
Control is challenging due to their mobility and the low population thresholds required to transmit devastating diseases.
- **Monitoring (Key):** Regular inspection of new growth flushes, often using yellow sticky cards, is crucial to detect early infestations.
- **Biological Control:** Release of specialized parasitic wasps (*Tamarixia radiata* for citrus psyllid) and encouragement of generalist predators (lacewings, minute pirate bugs) are cornerstones of IPM.
- **Chemical Control:** Control often requires the use of **systemic insecticides** (applied to the soil or trunk) to protect new growth flushes from feeding and transmission, especially in high-risk agricultural areas.
- **Pruning:** Pruning and destroying infested plant tips can eliminate early-season populations.
Conservation and Research
Psyllids are managed as high-priority invasive and disease vector pests. Research focuses intensively on understanding the bacteria and viruses they transmit, developing disease-resistant host plants, and improving the effectiveness of biological control agents to reduce reliance on chemical sprays.