If you’ve ever traveled internationally, you may have noticed that before entering into a country, you often have to fill out a form about whether you are carrying any agricultural products or have been on any foreign farms during your trip. This precaution is because many invasive plants and pests can hitch a ride. The Asian citrus psyllid (Diaphorina citri) is an exotic pest that has been able to rapidly spread across North America in just a few short decades since its initial introduction in the late 1990s.
Psyllids are a group of insects that eat plant sap and specialize in a single host plant or a closely related family of host plants. As the name implies, the Asian citrus psyllid attacks citrus plants. They are also a vector, or carrier, of the bacterial citrus disease, huanglongbing (HLB). HLB is also known as the citrus greening disease. Infected trees will show yellowing of the leaves, stunted fruit production, and eventually die after a few years; there are no cures.
Together, the Asian citrus psyllid and HLB are some of the most economically devastating pests for commercial citrus growers worldwide.
Asian Citrus Psyllid Overview
The Asian citrus psyllid and the HLB disease come from tropical and subtropical regions of southern Asia, where citrus originates. The Asian citrus psyllid was first detected in Florida in the late 1990s. Now, these pests can be found in much of the southern parts of continental U.S., the U.S. Virgin Islands, Hawaii, Guam, and any warm climates where citrus are grown. Many plants in the family Rutaceae and genus Citrus are susceptible including the curry leaf, grapefruit, key lime, lemon, lime, orange, pomelo, tangerine, to name a few.
Psyllids can be misidentified as aphids but they are from different taxonomic families. While psyllids are very small, at 3-4 millimeters long, they look more like miniature cicadas and use jumping as a primary way of getting around but can also fly short distances. Adults have brown mottled bodies and wings. They can appear dusty because their bodies are typically covered in waxy, whitish secretions and they will leave waxy droppings on affected plants. The nymphs of the Asian citrus psyllid are yellow or orange in color with red eyes. The nymphs get progressively larger with every instar or developmental stage. Eggs are around 0.3 millimeters long, almond-shaped, and progress from pale yellow to orange as they prepare to hatch.
Life Cycle of The Citrus Psyllid
Each female of the Asian citrus psyllid species can lay 300-800 eggs in her lifetime and she will lay her eggs on the growing shoots of citrus plants. Nymphs have five instars and mature in 15-47 days depending on environmental factors. Because of their rapid lifecycle, these pests can have 9-10 generations per year – only slowing down during winter or dry months, if these conditions are applicable.
Adult Asian citrus psyllids are typically found in large numbers on the lower side of the leaves of their host plants. They are most active during the growth period of citrus plants. Adults lay eggs on the growing tips of citrus plants and near unfurling leaves. The nymphs can only survive on the new growth where they have recently hatched.
What Do Psyllids Eat?
Adult Asian citrus psyllids feed on the flush or newly developed leaves of citrus plants. They directly affect their host plants by feeding on plant tissue. But more damagingly, they are a vector for the fatal bacterial disease Huanglongbing (HLB). As Asian citrus psyllids feed on an infected citrus plant, they also ingest the bacteria and help spread it to other plants. While there are multiple species of psyllids that can be found on citrus plants, the Asian citrus psyllid is the only species that can transmit HLB. All it takes is just a few psyllids to take down a whole tree.
Nymphs feed on the sap of plant tissues from the growing tips of plants, immature leaves, soft stems, and flowers. As they feed, they also inject a salivary toxin into the plants that causes the new flush tips of their host plants to become deformed or die. The damage to the new flush of citrus leaves will stunt the growth of the plant. A telltale sign of these nymphs is the waxy, curly tube with a bulb coming out of the backend of their bodies which they produce to get rid of waste.
How to Control Asian Citrus Psyllid
Because of the significant threat that Asian citrus psyllids pose to the citrus industry in the U.S., there are many national and state-level efforts to quarantine and monitor these pests. Check your local agricultural commissioner’s website for specific hotlines to call in case you suspect your trees may be infected. Throughout the state of California, for example, HLB has been recorded in residential areas and there are multiple PSAs produced by the state to help home gardeners identify and report the issue. There is no cure for HLB so the only way to manage the disease is by reducing the population and spread of psyllids.
Inspect the growing tips of your citrus trees carefully and frequently. Check the new leave flush and the underside of tender new leaves and twigs. Psyllids and nymphs are very small so you may need to inspect your plants using a magnifying glass. Especially be on the lookout for the waxy tubules produced by Asian citrus psyllid nymphs.
Organic or Chemical Control
Home gardeners can use horticultural oils, insecticidal soaps, and pyrethrins but the spray must make direct contact with psyllids in order to be effective. Because of the rapid life cycle of Asian citrus psyllids, these control methods have to be sprayed every 7-10 days while new leaves are flushing and growing. These methods are typically not as effective as other chemical controls.
Foliar spray like Sevin and Malathion or soil drenching with imidacloprid can be used to combat Asian citrus psyllids. However, these are not organic insecticides. The most effective treatment against Asian citrus psyllids are a combination of soil drenching and foliar sprays applied by professionals.
The Asian citrus psyllids have natural enemies including lacewing larvae, syrphid larvae, spiders, birds, and lady beetles. Creating an environment in your home garden that supports these beneficial natural enemies is a good way to help reduce psyllid populations.
On a larger scale, one of the most promising biological controls is the Tamarixia radiata, a small parasitic wasp. The female wasps lay their eggs under or inside Asian citrus psyllid nymphs and the growing wasp larvae feed on the nymphs from the inside out, leaving behind a hollow “mummy” shell. In California, researchers from the University of California Riverside have been working with the California Department of Food and Agriculture to rear and release these wasps into residential parts of Southern California.
Be aware that ants feed on the honeydew and tubules produced by Asian citrus psyllid nymphs and will defend them to protect this food source. If you know that parasitic wasps have been released in your area by your local agricultural service, you will need to control the population of ants in your garden to maintain the effectiveness of these wasps.
If you think your citrus tree may be showing symptoms of HBL, you need to call your local hotline as soon as possible to get a leaf sample tested. There are also maps available through the agricultural commissioner’s office that document where infections in your local area have already occurred. If there has been a documented case of HBL within 2 miles of your trees, it is likely that your plants have already been infected. Trees may not show symptoms of HLB immediately so you should get your trees tested and removed if they are already infected in order to prevent the spread of HLB to neighboring trees.
Frequently Asked Questions
Q: How do I protect my citrus trees from insects?
A: One of the best ways to protect your citrus trees is by frequent inspections, especially during the growing season. Use a magnifying glass to check the growing tips thoroughly for adults, eggs and nymphs. Apply treatment directly on the pests.
Q: What do Asian citrus psyllid eat?
A: Asian citrus psyllids eat plant tissue from the new growth areas of citrus trees. Their nymphs also eat sap from flowers and stems.