Asian Citrus Psyllid Control: A Complete Guide to Protecting Citrus Crops

I come from a citrus background and know the importance of some of the smallest insects can be the largest management issue. Asian citrus psyllid damage (ACP), Diaphorina citri, is a part of them. Microscopic sap feeder which is closely associated with young citrus flush, which is where adults and nymphs feed on developing leaves and shoots. Its feeding can cause the plants to grow in an abnormal shape in young shoots, and successive heavy borings can also place extra stress on young shoots.

The main worry, though, would concern not just the feeding injury. ACP is a significant vector of the bacterium that causes Huanglongbing (HLB) or citrus greening. HLB is regarded as one of the most severe diseases of citrus trees worldwide, with infected trees showing blotchy leaf mottling, fruit growth impairments, fruit dropping and decline. Since there is no cure for HLB, reducing ACP population and early identification of these pests is a key element of disease management.

To the growers, this implies that Asian citrus psyllid control or for Asian citrus psyllid treatment shouldn't rely on one routine spray. Regular scouting with protection of new flush, combined with cultural practices and desired biological control and selective insecticides if needed, comprise a practical program. The goal is to limit psyllid numbers and the few opportunities for the transmission of HLB to take place while still having a viable citrus IPM program.

What do Growers need to look out for when identifying ACP?

The key first step in any Asian citrus psyllid identification management program is to effectively identify the pest. The adults are extremely small (ca. 3 mm long) brown, mottled insects. They have a peculiar feeding posture which is the most helpful field guide. When feeding, the adult will position its head towards the leaf and its body, mostly at a 45° angle. ACP produces a "tilted" position, and is somewhat unique among citrus pests in that regard.

The eggs are tiny for psyllid infestation, yellow-orange and tend to be tucked in the crease of the youngest and new leaves. Early stages (nymphs) are yellow to greenish and are found on the tender flushing. Later (older) nymphs have wing buds and produce distinctive white waxy tubules. The presence of these tubules often helps detection of an infestation especially if the young stems and leaf nodes are investigated under a hand lens.

Scouting I would take particular interest in the new flushes not the mature foliage at random. Eggs and nymphs have a strong relationship to flush development, and it is much easier to make an early determination using a 10× hand lens.

Asian Citrus Psyllid Life Cycle: Why it Matters to Growers

The Asian citrus psyllid control develops through an egg stage, five nymphal instars and the adult stage. The females lay eggs on the young growth and the nymphs are found attached to tender new leaves and green stems. In ideal autumnal flush-growths, it takes about three weeks to reach the adult stage, in nature, and when conditions for rapid flush are always optimal, numbers may also proliferate rapidly.

Citrus flush is crucial for growers due to its relationship with new growth, which is what allows ACP to reproduce. Females are attracted to the appearance of new leaves while nymphs rely on a time of ‘feather' flush and developing green stems. It is therefore never too soon, and always very soon, to use all major flushes as a monitoring period.

From an application point of view, the life cycle is crucial because some events have to take the right place. Growers can be more successful if they monitor before and throughout flush to locate the adults before significant egg laying has taken place and locate the nymphs before they spread everywhere and increase too. If the orchard is not seriously monitored after the leaves have fully expanded then the peak developmental phase of the pest could be missed.

What do Asian Citrus Psyllids do to Citrus Trees?

What are the asian citrus psyllid symptoms? In most cases, initial ACP activity is related to tender growth. Infested young shoots may appear distorted, curled or otherwise develop abnormally and feeding by ACP will cause new leaf tips to turn back. A heavy flush may also be sticky from nymph honeydew and/or waxy secretions, adding also a white flux in the vicinity of the cast skins.

It has to be remembered that while asian citrus psyllid damage is a complication, it is a minor one compared with the much more serious potential of HLB. Infected psyllids can acquire and carry the bacterium that causes citrus greening, but direct feeding results in infection of developing leaves and shoots as well. Leaves may possess an asymmetrical mottling pattern, poorly coloured and misshapen fruit, bitter juice, fruit drop and a progressive tree decline characteristics of HLB.

Regular flushing can result in late flushings and subsequent re-escapes of ACP. In citrus orchards that frequently produce tender growth, these adjustments in scouting frequency and focussed attention on edges of orchard, recently replanted areas and young trees may be necessary as more attractive new growth may be produced.

After detection, what should farmers do about Asian Citrus Psyllid?

The first step in response to ACP is to confirm and assess the situation and not immediately resort to an insecticide. I would first look for adults, eggs and nymphs on young flush, shoot tips, and undersides and edges of tender leaves. Tapping samples and yellow sticky cards may also be used to aid in detecting the adults and direct examination of feather flush yields the best results for locating eggs and nymphs. The following priorities apply to UC IPM management of seedheads: Monitor often through the season and be extra vigilant during periods of active seedheads.

The thresholds for citrus suited to asian citrus psyllid treatment, when to intervene and the purpose of the cropping will differ depending on the citrus production system, ACP and HLB status, and regional recommendations. For orchards where there are established monitoring programs or treatment thresholds, adhere to the treatment schedule recommended by those programs instead of prescribe the same schedule in all orchards.

There is special need for early intervention services as populations of ACP can grow quickly in favorable conditions. Meanwhile, growers should be cognizant of the fact that each stage of the ACP is not as well controlled by any given insecticide. UC IPM acknowledges that eggs and nymphs are hiding in new growth, and that the mobile adults can be distributed by moving via short and long range flights, posing other challenges for the use of contact and residual sprays.

Asian Citrus Psyllid Management: Develop an Integrated Program

Frequent scouting and new-flush monitoring are the first steps to a successful Asian citrus psyllid damage management program. Therefore, cultural methods can be used to control favorable habitats when feasible and biological control or selective insecticides to decrease pest populations if necessary and warranted by pest pressure. The overall objective is to control the psyllid in total eradication rather than to eradicate all of the species.

Growers should also not use the same chemistries over and over again. Persistent selection pressure for resistant populations can be generated with the same mode of action. The rotation of effective insecticides with differing modes of action; only using insecticides where they are needed through monitoring; and use of natural enemies can all contribute to the sustainability of the overall program.

The local regulations are also taken into account. Growers should check if Asian citrus psyllid pesticides are registered and approved for their region, check the approved label and take into account pre-harvest intervals, maximum residue limit and market requirements before selecting a pesticide that will be suitable for use on Asian citrus psyllid. A combination of these factors can determine if a certain treatment can or cannot be implemented in a commercial citrus program.

Asian Citrus Psyllid Pesticides and Insecticides Options – What Growers Should Consider

Traditional insecticides may be important for achieving ACP suppression, especially if populations are boosting and during disease-risk management periods when intervention is needed. But consider the product life stage, application timing, coverage, local registration and resistance-management recommendations when selecting a product. Each year, commercial citrus programs can also involve multiple treatments since each asian citrus psyllid insecticide is only effective to varying degrees against ACP life stages.

Contact products require the target insect to be in contact with the spray deposit. This may be difficult where adults are mobile and eggs/nymphs are inside the developing plant tissue. There are differences in the way they work, systemic products are absorbed by the plant, however use depends on crop, product registration and timeliness of the application as well as the environmental considerations. Thus, it should not be assumed that either type would be universally accepted solutions.

Especially for foliar applications of coverage. Producers should make sure that part of a leaf surface is in contact with the target, and young shoots receive sufficient coverage, and use the amount of water, timing and method specified on the product label. To limit impacts to pollinators, pesticide spraying is not recommended during bloom unless absolutely necessary, so avoid sprays.

Bio and other non-chemical methods can be used as another tier in an IPM program. Studies have reported potential for the use of Isaria fumosorosea as a biological control agent for ACP and compatibility of the fungus with horticultural oils was investigated. But there are some special factors to consider when trying to be compatible; growers shouldn't assume compatibility with all pesticides, fungicides and adjuvants when used with microbial products, as it varies with different formulations and application programs.
 

How can Biological Control be used in the control of Asian Citrus Psyllid?

Biological asian citrus psyllid control can be a valuable alternative or adjunct method of controlling ACP. What is particularly interesting about the entomopathogenic fungi is that their spores can touch the outside of the insect and start to infect the insect through the outside under proper conditions to infix spores deep in the insect's cuticle. The fungus can then invade the host and eventually lead to the death of the host plant. Fungal biocontrol is not as fast acting as traditional insecticides, but it is very sensitive to environmental conditions and the biology of the biocontrol agent.

Isaria fumosorosea has the potential for use against Diaphorina citri, as evidenced by research. The results of a 2012 study showed several strains of I. fumosorosea caused significant mortality of adult ACP in the laboratory; however, greenhouse tests with the Apopka-97 strain significantly lowered established psyllid populations by about 50% during three weeks.

For the fungus more additional research has been tested under semi-field conditions. Studies reported in 2018 have shown the mortality of adult ACP at laboratory and semi field experiments which indicates that the use of entomopathogenic fungi as parts of integrated ACP management is possible. In fact, global field tests have assessed foliar sprays and autodissemination strategies with I. fumosorosea in home orchards of citrus plants.

An important IPM consideration, however, is that there may be interactions between biological control agents. A parasitoid of nymphs, Tamarixia radiata is a significant natural enemy in some citrus systems. Laboratory studies have shown that there is potential for I. fumosorosea to cause some detrimental effects in T. radiata when exposed simultaneously. The researchers proposed that reducing antagonism would be alleviated if the timing of fungal application and when parasitoids work was staggered.

It is not meant to be an indictment on fungal biocontrol. Rather, the intent of biological tools should be to plan them within the broader context of the orchard like everything else with natural enemies, timing, and product compatibility.

Understanding the role of Isaria fumosorosea in IPM for Asian Citrus Psyllid

What Is Fumokar?

Fumokar is an innovative biological insecticide made from the fungus Isaria fumosorosea strain CJ901. It is used for Asian citrus psyllids as well as other piercing and sucking pests such as whiteflies, aphids, thrips and broad mites. The product used is wettable powder,which has been formulated with I. fumosorosea.

The key to growingers using Fumokar is that it is meant to be a biologically based insecticide in an Integrated ACP Management Program, not a substitute for all other ACP management technologies and practices. It is a microbial in-service to complement the other approach taken to control psyllid numbers.

What effect does Isaria fumosorosea have on Psyllids?

Infection of the insect occurs once the fungus comes in contact with a susceptible insect. In favourable conditions, spores adhere to the insect's cuticle and start to grow and infect the surface of the body. It is now able to grow in the host causing its physiological upset and eventually its death. According to Novobac the feeding cessation, inactivity and mortality are a part of this infection process.

Environmental conditions can affect performance due to the requirement for successful spore germination and penetration in order to enable a successful infection. Humidity, temperature, UV exposure and spray coverage are therefore factors that need to be taken into account. Novobac specifically states that humid conditions favour the action of fungi and it is important to cover all the relevant leaf surfaces and published research has demonstrated variability between different environment conditions for the activity of I. fumosorosea.

What Growers Need To Think About Fumokar Application?

Novobac suggests starting Asian citrus psyllid treatment with Fumokar once the initial early signs of management are observed and notes that Fumokar is a primary method to control Asian citrus psyllid infestation in a foliar application. The manufacturer also has recommended application rates and environmental information; growers should always rely on the most recent label and manufacturer's directions for the application rate, crop use, and application technique that are specific to his/her growing environment.

Fumokar should really be regarded as part of an ACP suppression programme and not as a cure in itself. Its greatest practical use seems to be within a program solely based on monitoring, rather than relying on antiquated support of beneficial organisms or implementation of management strategies that are not compatible with it.

What are the options for farmers to use Fumokar in conjunction with other asian citrus psyllid control?

The key to a practical seasonal program starts BEFORE the citrus flush is mature. I would make the first report of adult psyllids, eggs and nymphs are located in trees as their new shoots appear. If ACP is suspected its diagnosis should be confirmed prior to initiating treatment. Regular monitoring of pest activity can then help establish if there are increases, decreases or recolonization after treating pests.

The next step is to defend the new flush. Asian citrus psyllid treatment reproduction is linked closely to the tender growth so that monitoring should be done more often during active flushing. Monitoring is recommended at regular intervals during the season and direct examination of feather flush should be performed for the purpose of identifying, assessing, and managing ants.

Reference:

• Hoy, Marjorie A., Raghuwinder Singh, and Michael E. Rogers. "Evaluations of a novel isolate of Isaria fumosorosea for control of the Asian citrus psyllid, Diaphorina citri (Hemiptera: Psyllidae)." Florida Entomologist (2010): 24-32.
• Avery, Pasco B., et al. "Compatibility of Isaria fumosorosea (Hypocreales: Cordycipitaceae) blastospores with agricultural chemicals used for management of the Asian citrus psyllid, Diaphorina citri (Hemiptera: Liviidae)." Insects 4.4 (2013): 694-711.