Asian Citrus Psyllid

The Asian citrus psyllid (ACP; Hemiptera: Psyllidae) is a tiny (0.125 inch, 3 mm), mottled-brown insect that is about the size of an aphid. The adult psyllid feeds with its head down, almost touching the leaf, and the rest of its body is raised from the surface at a 45-degree angle with its tail end in the air. No other citrus insect pest positions its body this way while feeding.

Adults typically live 1 to 2 months. Females lay tiny, yellow-orange, almond-shaped eggs in the folds of the newly developing, unfurled, soft leaves, called the feather flush. Each female can lay several hundred eggs during her lifespan.

Nymphs can only grow and feed on the feather flush and young green stems of citrus. Nymphs are typically yellow-orange but can also have a green hue. Young nymphs (instars 1 and 2) lay flat on the surface of new feather flush and on the edges of leaves, and older nymphs (instars 3 to 5) congregate on stems of flush, especially where leaves attach at the nodes. Older instars are easier to identify because they are larger, their wing buds and red eyes are more visible, and they produce visible waxy tubules.

Nymphal development slows at 104°F and continual exposure to 3 or more hours per day of 108°F kills nymphs. Similarly, high temperatures reduce adult Asian citrus psyllid flight and dispersal.

The total generation time for the Asian citrus psyllid is about 3 weeks.

Asian citrus psyllid attacks all varieties of citrus and very closely related ornamental plants in the Rutaceae family (e.g., mock orange, Indian curry leaf, orange jasmine and other Murraya species). The removal of Murraya species in or nearby orchards is encouraged because Asian citrus psyllid can reproduce faster on this genus of plants.

While Asian citrus psyllid adults can feed on mature, hardened citrus leaves, all stages of this pest prefer to feed on, and therefore damage, new leaf growth. The salivary toxin that Asian citrus psyllid injects while feeding can cause the new leaf tips to burn back.

Beyond feeding damage, Asian citrus psyllid vectors the lethal bacterium (Candidatus Liberibacter asiaticus) that causes huanglongbing (HLB or citrus greening) disease. Huanglongbing causes leaves to yellow asymmetrically (blotchy mottle), and results in unmarketable fruit with asymmetrical shape, aborted seeds, and bitter flavor. The disease can kill a citrus tree within 5 to 8 years, and there is no known cure for the disease.

Asian citrus psyllid arrived in Southern California from Mexico in 2008, is now well established in Southern California, and is periodically found in central and northern California. Huanglongbing was first found in residential trees in Los Angeles County in 2012, likely due to illegal importation of infected plant material; it has spread from that area to other counties (Orange and San Diego) in southern California.

In Florida, the psyllid and the disease spread rapidly. It is thought that huanglongbing was present in a small number of Florida backyard citrus trees for a number of years. After the arrival of Asian citrus psyllid, the disease spread into commercial citrus orchards. Florida citrus growers are now applying broad-spectrum pesticides up to fifteen times per year to reduce Asian citrus psyllid numbers and slow the spread of the disease. Pesticides can reduce the number of psyllids, but an adult psyllid can carry the bacteria throughout its adult life and can sometimes transmit the disease before pesticides kill it.

For more information, read UC ANR Publication 8218, Citrus Bacterial Canker Disease and Huanglongbing (Citrus Greening). For more information about the detection of Asian citrus psyllid and huanglongbing, and to view management strategies for various citrus growing regions, see the UC ANR website Asian Citrus Psyllid Distribution and Management. Also see Huanglongbing in this publication.

If you see Asian citrus psyllid in areas of California where it is not yet known to have established (central and northern California), please contact the California Department of Agriculture (CDFA) Exotic Pest Hotline at 1-800-491-1899. Personnel from CDFA will inspect plants for the presence of this psyllid. They send specimens to diagnostic laboratories for identification and determination of the presence of the huanglongbing bacterium in psyllids and plant tissue.

Currently, pesticide applications to California citrus orchards are designed to limit and slow the spread of Asian citrus psyllid and huanglongbing throughout California. Keeping psyllid numbers low helps prevent the spread of the bacterium that causes the disease.

In parts of California where the density of Asian citrus psyllid is extremely low, such as in northern and central California, an eradicative approach is being used. Infested orchards, along with neighboring orchards and residences, are treated quickly in a coordinated manner with ACP effective insecticides. Follow-up monitoring is required to verify the effectiveness of the insecticide application. If psyllids continue to be found after spraying, additional treatments and subsequent monitoring is required.

In the parts of California where Asian citrus psyllid is now well established (southern California), insecticides are applied as part of areawide programs. Applications are made regionally in a coordinated manner with the goal of covering the entire region within a specific timeframe to help control Asian citrus psyllid across the whole region.

In commercial citrus, multiple treatments with insecticides are necessary to control Asian citrus psyllid. No single insecticide fully controls Asian citrus psyllid. Contact insecticides typically do not control all life stages; the eggs and nymphs are tucked inside new foliage while adults are highly mobile and are capable of both short-range and long-range flights. Furthermore, some insecticides show better efficacy against one stage over another. Because systemic neonicotinoid insecticides require root activity for uptake, they are best applied during June through September.

Barriers can provide protection against Asian citrus psyllid by preventing psyllids from reaching trees. Barriers are constructed of mesh and must be at least 12 feet tall. Mesh openings need to be small enough to prevent psyllids from passing through, but still large enough to have high air permeability (50-mesh insect netting is commonly used). If the barrier completely encloses trees, it must be maintained to prevent openings. Any pests contained within the mesh can cause a secondary pest outbreak that must be managed.

Mesh tree covers are useful for new plantings and smaller trees. Replace mesh tree covers as trees grow.

A step beyond mesh covers and fencing for larger trees is Citrus Under Protective Screen (CUPS), where orchards are completely enclosed within a protective mesh house.

Living windbreaks have also been shown to reduce Asian citrus psyllid numbers, but require water and time to establish and reach a height of 12 feet that deters movement of adult psyllids into the orchard. Ideally, living windbreaks should border the entire field with the sides that face any residential areas prioritized.

Barrier and windbreak design should consider the requirements of farm equipment to access and navigate the orchard, and turn at the end of rows. Typically, leaving 30 to 40 feet between the row edge and the barrier is sufficient.

Asian citrus psyllid requires new leaf flushes as egg-laying sites. Timing production activities that encourage new flushes, such as nitrogen fertilizer application and pruning, to occur during the spring and early summer, when psyllids are least active, can reduce the availability of viable egg-laying sites.

Particle film sprays, such as kaolin clay or diatomaceous earth are used in desert areas of California for sun protection, and can also help repel Asian citrus psyllid and reduce successful egg laying. These products wash off with rain and the reapplication rate in California to cover new leaf flushes is unknown. In Florida, particle film sprays are applied every two weeks. These products may not stick well to new growth, especially in lemons. Additionally, particle film sprays may result in secondary outbreaks of pests, such as California red scale, that can be protected from natural enemies by the film.

Reflective mulches have been shown, in Florida, to reduce Asian citrus psyllid damage in young trees by interfering with Asian citrus psyllid's ability to find host trees, while in flight. This technique is most effective when combined with insecticide applications and other management tactics.

Particle films, especially kaolin, and reflective mulches reflect light back into the tree canopy and can increase a tree’s growth rate.

Releases of the parasitoid wasp Tamarixia radiata started in 2012 in southern California and this parasitoid rapidly established in urban areas. Releases of T. radiata have periodically been conducted in the San Joaquin Valley since 2016; however the psyllid population is so low in this region that the parasitoid is not known to have established. Parasitism rates of Asian citrus psyllid nymphs can be as high as 70% in southern California. However, T. radiata parasitism builds slowly during the season and so allows Asian citrus psyllids to spread disease. In addition, insecticides used to control Asian citrus psyllid and other pests in commercial citrus are toxic to T. radiata. Thus, releases are focused on residential citrus where pesticide applications are difficult to achieve.

Conservation of generalist predators, such as syrphid flies, lacewings (adults and larvae), and lady beetles (adults and larvae), is encouraged. Generalist predators are voracious and, like parasitoids, decrease Asian citrus psyllid numbers but do not fully eliminate populations.
Ants can reduce the efficacy of biological control. Ants, especially the Argentine ant in Southern California, feed on honeydew excreted from Asian citrus psyllids and actively protect psyllids from predators, and parasitoids. See Ants in this publication for more information on how to control ants.

The physical, cultural, and biological control methods described above are consistent with organically acceptable methods of pest management.

Using insecticides approved for organic production to reduce psyllid numbers and prevent the spread of the psyllid or huanglongbing is difficult. The persistence of insecticides approved for organic production is only hours to days (not weeks like some conventional insecticides) and they must contact an insect’s body to be effective. Therefore, it is necessary to apply such insecticides frequently (every 7 to 10 days), ensure that coverage is thorough, and it is advised to use at least 250 gallons of water per acre for products approved for organic production, with a ground speed of 2 to 2.3 miles per hour.

Many of the insecticides used in organic production are sensitive to ultraviolet light and should be applied during cloudy days or after dusk. Asian citrus psyllid is less likely to fly at night and are therefore more likely to be sprayed with the insecticide during night applications.

The efficacy and persistence of organic insecticides can be improved by using adjuvants, such as oil. Consider adding adjuvants to your spray mixtures if the label allows it. Additionally, check the pH of spray mixtures, since many organically approved insecticides can degrade more quickly outside of their pH range. See the table below for pH ranges.

While mixing of insecticide active ingredients is not generally recommended for citrus pest management, experience from growers indicates that organic control of Asian citrus psyllid may be an exception to those recommendations. In addition to adding adjuvants, mixing two active ingredients can increase efficacy, the residual effect, or both.

Some insecticides approved for organic production are approved for use in eradicative treatment programs, requiring two applications 7 to 10 days apart. Follow-up monitoring is required to verify that the insecticide application was effective. If more psyllids are found, additional applications must be made.

Organic insecticides are also permitted in areawide, suppressive, treatment programs in regions where Asian citrus psyllid has established (southern California). For each conventional treatment in the fall and winter, two organic treatments need to be applied 7-10 days apart.

In areas where Asian citrus psyllid isn’t established (northern and central California), if it is found, contact your local county agriculture commissioner’s office, and report it.

Use a combination of methods to monitor for Asian citrus psyllid every 2 weeks throughout the season. Young orchards and replanted trees have more new growth and flush more often, therefore are attractive to psyllids. Monitor more frequently in these circumstances.

Detect adult psyllids by visually surveying the new flush, and by using sweep net surveying, tap sampling, or yellow sticky cards. Eggs and nymphs are limited to new growth, so direct visual monitoring efforts on feather flush. Focus particularly on scouting the feather flush at the edges of an orchard.

The methods described below can be effective at certain times of the year and under certain conditions. Sample 10 trees each on the north, east, south, and west borders of the orchard; borders are breaks in citrus plants, generally the size of a road. Also sample 10 trees in the orchard center for a total of 50 trees sampled per orchard (standardized sampling is necessary to compare infestations among orchards). Psyllids tend to be found along edges, and thus it is acceptable to focus sampling efforts on the edges of orchards.

Visual Survey

The best way to find psyllids is to examine tiny new leaves of the feather flush as they are developing and look for adult psyllids laying eggs, nymphs with their waxy tubules, and eggs. Use a 10X or higher hand lens or a head loupe to look for small, hard-to-see nymphs.

Examine the newest flush on two branches per tree on each of the 50 trees surveyed for all psyllid stages (egg, nymph, and adult). Record whether psyllids are found. Describe the leaf flush stage as feather flush, growing flush, or fully expanded leaves to record the suitability of the flush for nymphs.

Asian citrus psyllid produces honeydew, which in turn can provide food for ants or nutrition for sooty mold. The presence of honeydew, ants, or sooty mold may indicate the presence of Asian citrus psyllid, but may also indicate the presence of other honeydew-producing insects.

Sweep Net Survey

A sweep net survey is effective only for adults and is more effective than tap sampling.

1. Stuff a citrus branch into a large sweep net and shake vigorously.
2. Repeat this method on all four sides of the tree.
3. Examine the contents of the net for adult psyllids.

Tap Sampling

Tap conditions, this sampling method returns only small numbers of psyllids unless psyllid numbers are very high in the orchard.

YOU WILL NEED:

• A white clipboard or plastic covered sheet of paper with a grid to help you count psyllids quickly.
• Squirt of detergent mixed with 500 ml of water in a spray bottle.
• One-foot section of PVC pipe or other device to beat branches such as a mallet.

INSTRUCTIONS

1. Mist the detergent solution onto the clipboard to hold the psyllids in place.
2. Select a branch from about halfway up the tree that is 0.5 to 2 inches in diameter. Hold the clipboard 1 foot below the branch and strike the branch 3 times with the pipe.
3. Count and record the number of winged psyllids (adults) collected on the clipboard.
4. Scrape the psyllids off the clipboard and reapply detergent solution if needed.

Conduct visual monitoring along with sweep or tap sampling every 2 weeks, especially during periods of flush. Psyllid sampling detects new infestations and shows if a pesticide application was effective.

Yellow Sticky Cards

Yellow sticky cards are used by the CDFA to trap and detect adult psyllids at a rate of one trap per 1/4 mile (16 per square mile). Traps are more attractive when there is no young leaf flush, and especially attractive after a flush has hardened off. Psyllids are more attracted to leaf flush than sticky cards because there are volatile cues in addition to the color cues in leaves. This method is only effective for detecting adults and generally is not effective for estimating in-field populations.

For maps of the distribution of Asian citrus psyllid and huanglongbing, and more information on Asian citrus psyllid management, see the UC ANR Asian Citrus Psyllid Distribution and Management website.

Eradication in New Areas of Infestation

When Asian citrus psyllid first appears in a region, numbers are low, and the population can potentially be eradicated locally if treated aggressively over an 800-meter (875-yard) radius of detection. Two insecticides should be used, preferably a foliar for knockdown and a systemic for a more persistent effect. Dual treatment is recommended because the foliar application may not be 100% effective against the highly mobile adults and nymphs tucked inside folds of leaves.

The most effective foliar insecticides are: pyrethroids, thiamethoxam alone, or mixtures of neonicotinoids with other products because they are broad-spectrum, have the longest residual activity, and are toxic to all stages they contact. Whenever possible, apply insecticides to the perimeter of an orchard first to prevent psyllids from escaping the application area.

The most effective systemic insecticide is imidacloprid, which persists for several months (depending on tree size and irrigation system) and moves into the new leaves to kill the hard-to-reach immature stages.

• Apply systemic insecticides when root growth is occurring (June through September) for best root uptake.
• Apply to soil; it remains effective for 2 to 3 months.
• Imidacloprid requires 3 to 4 weeks for uptake into mature citrus to begin to kill pests.
• Pre-wet soil before the insecticide is applied. For optimum uptake, apply to newly planted trees or trees irrigated by drip, microsprinkler, or low-pressure irrigation systems. Emitters must provide even, uniform distribution of water. Lightly pre-wet soil for several hours before application to break soil surface tension. Once the irrigation system reaches operating pressure, inject the insecticide into the system over a calculated time interval (generally 2 hours) to allow uniform distribution throughout the system. The use of a dye marker in the treatment solution is recommended to determine when lines are clear of the treatment. Once the solution has cleared all irrigation lines and emitters, continue irrigation to move the insecticide into the active root zone but do not overirrigate or cause runoff. Wait 24 hours before subsequent irrigation.
• Imidacloprid is toxic to bees; do not apply during bloom because bees may be drawn to irrigation water.
• Imidacloprid added to irrigation water works best when roots have been trained to a specific irrigation pattern for a period of time; uptake will likely not be as good if drag line sprinklers are used or sprinklers are moved or modified.
• In some situations, such as heavy clay soils, soils high in organic matter or where there is limited post-treatment irrigation, imidacloprid uptake may not be ideal. In these situations, systemic thiamethoxam (Platinum) may provide better Asian citrus psyllid control, although likely not provide the persistence of imidacloprid.

If either of these treatments (a foliar or systemic application as listed above) cannot be used, then use any combination of two effective foliar insecticides. Always start with a foliar broad-spectrum insecticide (pyrethroid, organophosphate, neonicotinoid, or carbamate) for good knockdown. Make the two applications within one psyllid generation (4 weeks) over as wide an area as possible to ensure control. The size of the area is defined by the geography and continuity of citrus trees, but at a minimum, all orchards intersecting 800 meters of the find site should be treated. If part of an orchard is within the 800-meter radius, the entire orchard should be treated.

Suppressing Established Populations

Once Asian citrus psyllid has established itself in a region and can no longer be locally eradicated, apply insecticides routinely throughout the season and especially during the fall and winter months to suppress psyllid numbers. CDFA liaisons will coordinate fall and winter treatments in these regions to achieve control over a wide area. The treatment threshold is any life stage of psyllid present on 2 or more flushes out of 10 flushes in the monitoring block. Insecticides should be applied whenever any border of the orchard is infested at this level, and border applications are sufficient for suppression if the central area of the orchard is found to be free of psyllids. Even if Asian citrus psyllid numbers are below the treatment thresholds, low numbers still present a risk of transmission of huanglongbing.

• Fall sprays (August through November): A very important application period is in the late summer through fall just prior to and during the fall flush. In areas where psyllids are well-established, two fall applications are needed to control the fall population.
• Winter sprays (December through February): The second most important application period is winter-early spring before the feather flush forms and female psyllids begin to lay eggs. Apply a pyrethroid; the most effective insecticide during the winter because it works well in cold weather and has less of an effect on dormant/reduced natural enemy populations.
• Growing season: Make insecticide applications at the beginning of new growth flushes. Use any combination of broad-spectrum or selective foliar insecticides that are part of the routine management of other pests and effective against Asian citrus psyllid. In addition, during June through September, put on one application of a systemic neonicotinoid (imidacloprid or thiamethoxam) to provide extended nymphal control and to discourage psyllid feeding.

Psyllids first infest the border trees in the orchard when populations are low. During the fall, if the orchard is at least 5 acres in size and the psyllid numbers have been kept very low, then one of the foliar treatments could be applied as a border-only spray instead of a whole-orchard spray.

For resistance management purposes, rotate between insecticides from different classes and avoid using any individual chemical class more than once a season. It is critical to not lose the effective classes of insecticides (especially neonicotinoids and pyrethroids that are quite persistent) to Asian citrus psyllid resistance.

Ground spray applications are preferred because they achieve better coverage than aerial applications.