The Asian citrus psyllid (Diaphorina citri) is a small sucking insect no more than 4 millimeters long, and if its damage were limited to sucking sap it would not need a whole article.
But its real danger is that it is the main vector of citrus greening disease, known by its Chinese name Huanglongbing (HLB), which is currently the most serious disease of citrus in the world, and has no known cure.
Pest: A small insect that sucks citrus sap and transmits the bacteria that cause citrus greening (HLB), considered one of the most serious citrus diseases.
Danger: The disease cannot be treated once transmitted, so eliminating the vector is a top priority.
Sign: Twisted new leaves mottled with yellow, and an insect tilted at an angle on the tender growth.
Timing: It is abundant with new growth flushes, so monitor during every new flush.
Rule: Regular monitoring and early control of the insect, certified seedlings, and reporting to the agricultural authority when the disease is suspected.
Asian Citrus Psyllid Overview
The insect originated in South Asia and has spread to Southeast Asia, the Americas, and parts of Africa, the Middle East, and the Arabian Peninsula.
A tree infected with greening does not die in a single day. It goes on for years, gradually weakening, producing small, misshapen, bitter fruit that drops early, until it falls out of production. Major producing regions around the world have lost a large part of their orchards to this disease. That is why dealing with the psyllid is the first line of defense for the whole orchard.
The psyllid lives on two main groups of plants:
- Citrus of all kinds: Lemon, lime, orange, mandarin, grapefruit, bitter orange, and kumquat.
- Orange jasmine (Murraya paniculata): An ornamental shrub from the citrus family grown as a hedge in gardens and streets for the fragrance of its white flowers. It is one of the psyllid's favorite hosts and can become a large reservoir for it next to citrus trees. Curry leaf (Murraya koenigii) is also recorded as a host.
If your garden or farm has an orange jasmine hedge near lemon trees, inspect the new growth on the hedge with the same attention, and include it in the control program.
Signs of Damage: What to Look For
There are two kinds of signs: direct damage caused by the insect itself on the new growth, and the symptoms of the greening disease it transmits, which are the most serious and the slowest to appear.
Direct Damage From the Insect
- Nymphs: They feed on the tender new shoots and inject a toxin, so the new leaves twist and crinkle, and the growing tips may die. A notch may appear on the leaf after it grows, where the old feeding occurred.
- Twisted white waxy threads: They build up on the terminal shoots, and they are secretions of the nymphs.
- Honeydew and sooty mold: On leaves beneath nymph colonies.
- Adults: They feed on larger leaves and green stems. Their direct damage is limited, but they are the main vector of the disease.
Symptoms of Greening Disease on the Tree
- Yellowing of one branch or part of the tree: While the rest stays green, hence the Chinese name "yellow dragon disease."
- Asymmetrical mottling of the leaf: Irregular yellow and green patches that do not match on either side of the midrib. This is an important point to tell it apart from zinc, iron, or manganese deficiency, which give symmetrical yellowing on both sides of the midrib.
- Small, misshapen, lopsided fruit: They stay green at the end away from the stem while the stem end colors, the reverse of normal coloring.
- Bitter or sour, unpalatable taste: And dark brown aborted seeds.
- Early fruit drop: Sparse foliage, dieback of branches from the tips, and weak roots.
The most dangerous thing about this disease is its long incubation period: one to two years or more may pass before symptoms appear on a mature tree, and during this time the tree is a source of infection. A tree that looks healthy is not necessarily healthy.
If you suspect an infection, confirmation requires laboratory testing (usually by PCR) at a specialized authority, and it is advisable to notify the local plant protection authorities, because it is a quarantine disease in many countries.
How Does the Disease Spread?
The cause is a phloem-dwelling bacterium called Candidatus Liberibacter asiaticus. The psyllid picks it up when feeding on an infected tree, then transmits it to healthy trees when feeding on them, and a small number of carrier insects is enough. The disease is also spread by grafting with branches from infected trees.
Another species, the African citrus psyllid (Trioza erytreae), transmits another form of the disease in parts of Africa and southwestern Arabia.
Life Cycle and Appearance
The psyllid has incomplete metamorphosis: egg, then nymph, then adult, with no pupal stage. The cycle is completed in about 14 to 50 days depending on temperature, and overlapping generations can reach 9 or more per year in warm climates, depending on the availability of new growth.
Egg
- Shape: About 0.3 mm long, almond-shaped with a pointed end, yellow turning orange just before hatching.
- Distinguishing sign: Laid almost exclusively on very new growth, inside the small folded leaves at the shoot tips and between the buds. Do not look for them on large old leaves.
- Number: A female may lay up to about 800 eggs in her lifetime.
- Hatching: They hatch in 3 days at 28 degrees Celsius (82 F), and may need 10 days at 15 degrees Celsius (59 F).
Nymph
- Duration and size: It passes through five nymphal instars over about 11 to 40 days depending on temperature, and its length ranges from about a quarter of a millimeter in the first instar to nearly 1.5 mm in the last.
- Shape: Flattened and wingless, yellow to orange, with wing buds visible in the later instars.
- Distinguishing sign: Each nymph excretes a twisted white waxy tube with a small bead at the end, a sign specific to the Asian citrus psyllid.
- Where it lives: It completes its development only on tender tissue, so it stays clustered on new shoots.
Adult
- Shape: About 3 to 4 mm long, the male smaller than the female, mottled brown, with wings spotted brown and white and a dark band along the edge of the forewing.
- The most important distinguishing sign: The feeding posture. It lifts the rear of its body off the leaf at an angle of about 45 degrees with its head down, like a small thorn stuck in the leaf. No other common citrus pest stands this way.
- /strong> It hops and flies a short distance when the branch is touched, and may live from weeks to several months in mild weather.
Monitoring and Biological Control First
Regular monitoring detects the psyllid before the new growth fills with eggs and nymphs. The main methods are:
- Inspect new growth weekly: During growth flushes, with a hand lens, looking for eggs, nymphs, and waxy threads.
- Tap method: Hold a white board under a branch and tap it lightly, then count the adult insects that fall.
- Yellow sticky boards: Adult psyllids are attracted to yellow, and these boards are used in official monitoring programs to track their spread. It helps to hang them in the orchard, the nursery, and around orange jasmine hedges.
As for biological control, the most important specialized natural enemy is the tiny parasitic wasp Tamarixia radiata, which lays its eggs under the nymph so that it turns into a hollow brown "mummy" with a round hole from which the parasitoid emerges.
Another parasitoid is Diaphorencyrtus aligarhensis, along with predators such as lady beetles, lacewings, and spiders, and insect-killing fungi. For these enemies to work, you need to control the ants that protect the nymphs for the honeydew, and reduce unnecessary broad-spectrum sprays.
Horticultural oils and agricultural soap are gentler options on natural enemies, but they need to be reapplied every week or two.
Chemical Control With Rotation
The goal is to bring psyllid numbers down to a minimum, especially at the start of each flush, to protect the new growth before it fills with eggs and nymphs. Approaches used in psyllid control programs worldwide include:
- Group 4A (neonicotinoids such as imidacloprid and thiamethoxam): As a spray or a soil treatment. Soil treatment is important for young trees with frequent flushes, and gives protection for weeks.
- Other groups for rotation: Group 28 (such as cyantraniliprole), Group 6 (abamectin), and Group 23 (spirotetramat) on nymphs, and Group 3A (pyrethroids) on adults.
Rotation among these groups is essential, because the psyllid has shown reduced sensitivity to some active ingredients where they were used heavily. Do not spray during flowering, to protect bees and pollinators.
Products That Help in the Program
At Orkida Agri we have prepared the full set of program tools: two pesticides from different groups for rotation, boards for early monitoring, and a sprayer that reaches the new tips:
- Actara 25 WG: systemic thiamethoxam (Group 4A) used on citrus as a spray or soil treatment, which moves in the sap to the new tips where psyllids and their nymphs feed. Thiamethoxam is a core active ingredient in psyllid control programs in countries affected by greening, and we prefer timing it at the start of the flush to protect new growth. Avoid spraying during flowering.
- Exirel 10 SE: cyantraniliprole (Group 28) used on citrus, and registered for the Asian citrus psyllid on citrus in some countries. We use it in rotation with Group 4A in the next flush, because a different mode of action delays the development of resistance.
- Biobest Yellow Sticky Board 40x25 cm: for early monitoring of adult psyllids in the orchard, the nursery, and around orange jasmine hedges, and it also catches aphids and whiteflies.
- Eco Spray 20-Liter Backpack Sprayer, Ultra Model: for covering the new tips from all sides, since psyllids and their eggs are in the shoot tips and inside folded leaves.
Our advice: follow the dose and pre-harvest interval written on the label, because citrus fruit is picked and consumed fresh.
Prevention and Farming Practices
With a disease that has no cure, prevention becomes the real program. These are the most important practices that protect the orchard:
- Healthy seedlings from certified nurseries: Produced under insect protection, and do not move grafting wood or seedlings from infected regions. Many new outbreaks begin with a single seedling.
- Managing growth flushes: The psyllid reproduces only on tender growth. Pruning, nitrogen fertilization, and irrigation after drought at scattered times give it new growth all the time. Unify the timing of these operations so the growth comes out at once and is easier to protect.
- Removing trees infected with greening: After the infection is confirmed, since they do not recover and remain a source of infection. Before removal, spray the tree to kill the psyllids on it so they do not move to neighboring trees.
- Bringing orange jasmine hedges into the program: Or replacing them with ornamentals that are not hosts near citrus orchards.
- Area-wide cooperation: Psyllids fly between orchards and gardens, and the best results came when neighboring farms coordinated their monitoring and spraying dates.
- Balanced nutrition: It helps trees tolerate stress, bearing in mind that correcting nutrient deficiencies does not cure greening.
Frequently Asked Questions
Does the psyllid itself kill a citrus tree?
The psyllid alone causes crinkling and distortion of new growth and brings sooty mold, but its direct damage is limited. The real danger is its transmission of greening disease.
Is there a cure for greening disease?
There is no known cure that heals the tree. Management relies on controlling the psyllid, healthy seedlings, and removing infected trees.
How do I tell greening from nutrient deficiency?
Zinc or iron deficiency gives symmetrical yellowing on both sides of the midrib, while greening gives asymmetrical mottling. Final confirmation requires a laboratory test.
Does the orange jasmine shrub transmit the psyllid to lemon trees?
Yes, orange jasmine is one of the psyllid's favorite hosts and may be a reservoir for it near citrus trees, so it should be inspected and included in the control program.
When do I spray citrus trees against the psyllid?
At the start of each new growth flush, because the psyllid lays its eggs only on it, avoiding the flowering period and rotating among the effective groups.
What are the twisted white threads on the tips of lemon branches?
Usually the waxy secretions of Asian citrus psyllid nymphs, a distinctive sign of it. Inspect the branches with a hand lens and you will find the yellow-orange nymphs beneath them.