The Asian citrus psyllid is a tiny sap-feeding insect that poses the single greatest threat to citrus trees worldwide. It carries the bacterium responsible for huanglongbing, commonly called citrus greening disease, which has no cure and eventually kills infected trees. Since arriving in the United States, this pest has collapsed Florida’s citrus industry and now threatens groves and backyard trees across the southern states and California.
Why This Insect Matters
The Asian citrus psyllid (Diaphorina citri) is a member of the plant louse family, roughly the size of an aphid. On its own, the insect causes only minor feeding damage. What makes it devastating is its role as a carrier of “Candidatus Liberibacter asiaticus,” the bacterium behind citrus greening. When a psyllid feeds on an infected tree, the bacterium enters its gut, spreads through its body into its salivary glands within one to two days, and remains there for the rest of the insect’s life. Every time that psyllid feeds on a new tree, it injects bacteria into the plant’s vascular system.
The bacterium doesn’t just live in the psyllid’s gut. Researchers have confirmed its presence in the salivary glands, blood-like fluid, fat tissue, muscles, and even the ovaries, meaning it colonizes the insect systemically. Both nymphs and adults can pick up the pathogen while feeding, and after a short latency period, they become capable of spreading it to every citrus tree they visit.
How to Identify the Psyllid
Adults are small, about 3 to 4 millimeters long, with mottled brown wings. The easiest way to spot one is its distinctive feeding posture: it tilts its body at a steep 45-degree angle, with its head down against the leaf surface and its rear end raised. No other common citrus pest feeds in this position, so it’s a reliable field identifier.
Nymphs are flat, yellowish-green, and found almost exclusively on new leaf growth (called flush). They produce waxy, white tubules that look like tiny threads curling from their bodies. If you see clusters of small, flat insects on the tender new shoots of a citrus tree surrounded by white waxy residue, you’re likely looking at psyllid nymphs.
Signs of Citrus Greening in Your Trees
Huanglongbing gets its name from the yellowing it causes, which typically starts on a single branch or shoot before spreading. The most diagnostic leaf symptom is called blotchy mottle: an asymmetric pattern of yellowing that looks different on the left and right sides of the leaf’s central vein. This uneven pattern distinguishes citrus greening from nutrient deficiencies, which tend to produce symmetrical yellowing.
As the disease progresses, you’ll notice vein corking (thickened, raised veins on the underside of leaves) and green islands, which are small patches of green surrounded by yellow tissue. Fruit from infected trees is often lopsided or oblong, smaller than normal, and ripens unevenly. A telltale sign is color inversion: the stem end turns orange while the blossom end stays green, the opposite of normal ripening. Cut an infected fruit open and you may find aborted seeds, a curved central core, and a bitter, off flavor that makes the fruit unmarketable.
There is no treatment that cures an infected tree. Once a tree has citrus greening, it will produce progressively less fruit, decline over several years, and eventually die.
Where the Psyllid Has Spread
The Asian citrus psyllid is established across Asia, parts of the Middle East, South and Central America, Mexico, and the Caribbean. In the United States, it has been found in Florida, Texas, Louisiana, Georgia, South Carolina, Hawaii, and Southern California, where it was first detected in backyard citrus in August 2008. Infestations have been documented in San Diego and Imperial counties in California.
Federal quarantine regulations restrict all movement of citrus plants and related species into California to prevent further introduction of either the psyllid or the disease. Eradication efforts are ongoing in Southern California, where the insect is established but citrus greening has not yet become widespread in commercial groves.
Economic Damage to the U.S. Citrus Industry
The numbers from Florida tell the clearest story. Citrus greening was introduced to the state around 2005, and the consequences have been staggering. Florida’s citrus-bearing acreage dropped from roughly 750,000 acres in 2001 to about 369,300 acres by 2021, a 51 percent reduction. Production volume fell even more sharply, declining 79 percent over the same period, primarily due to greening.
During the 2020-21 season, Florida produced 57.9 million boxes of citrus, a fraction of its earlier output. The total grower value of that fruit was approximately $785 million, with the broader citrus industry (including juice manufacturing and fresh marketing) contributing an estimated $6.9 billion to the state economy. Those figures represent what remains after two decades of disease-driven decline, not what the industry once was. The gap between those numbers and pre-greening production levels represents billions in lost economic activity.
Biological Control With Parasitoid Wasps
The USDA, working with state agriculture agencies, has been releasing a tiny parasitoid wasp called Tamarixia radiata to suppress psyllid populations. About the size of a pinhead, this wasp is one of the psyllid’s natural enemies in its native range. Female Tamarixia lay their eggs directly on psyllid nymphs, and the developing wasp larvae kill the nymph as they grow. Adult wasps also feed directly on nymphs, providing a second layer of population control.
Tamarixia targets only Asian citrus psyllid nymphs and poses no threat to other insects, plants, or animals. Releases are currently underway in Arizona, California, Florida, Louisiana, and Texas. While biological control alone cannot eliminate the psyllid, it helps reduce populations in residential areas and urban settings where large-scale insecticide applications are impractical.
Chemical Control Strategies
For commercial groves and homeowners dealing with active infestations, insecticides remain the primary tool. The two main approaches are foliar sprays that kill psyllids on contact and soil-applied systemic treatments that are absorbed by the tree’s roots and distributed through new leaf growth.
Soil-applied systemic insecticides are particularly useful because they move into the tender new leaves where nymphs feed, reaching immature psyllids that foliar sprays often miss. These treatments work best when applied during active root growth, typically June through September. Once applied, they take three to four weeks to move through a mature tree and remain effective for two to three months, depending on tree size and irrigation.
Foliar sprays provide faster knockdown of adult psyllids and have the longest residual activity when broad-spectrum products are used. The key challenge with chemical control is resistance management. Rotating between different classes of insecticides and avoiding repeated use of any single class within the same season helps preserve the effectiveness of available treatments. Losing effective chemical classes to resistance would be a serious setback, given that no cure exists for the disease the psyllid carries.
Protecting Backyard Citrus Trees
If you grow citrus at home in a state where the psyllid is present, regular inspection is your first line of defense. Check new flush growth for nymphs and waxy residue every time you visit your trees, especially during warm months when new growth appears. Look for the characteristic angled feeding posture of adults on leaves and stems.
Avoid moving citrus plants, cuttings, or budwood between properties, and never transport citrus material across state or quarantine boundaries. Even a single infected cutting can introduce the bacterium to a new area. If you notice asymmetric leaf yellowing, lopsided fruit, or other symptoms consistent with citrus greening, contact your local agricultural extension office. Early detection in residential trees has been critical to slowing the disease’s spread in states like California where eradication efforts are still active.