• Topics: Citrus REC - Lake Alfred | Entomology and Nematology | Rogers, Michael E | Stansly, Phillip A | Citrus Leafminer | Asian Citrus Psyllid (ACP) | SP 043 Florida Citrus Pest Management Guide | Stelinski, Lucasz L

2012 Florida Citrus Pest Management Guide: Asian Citrus Psyllid and Citrus Leafminer¹

Asian Citrus Psyllid

Psyllid Management

The Asian citrus psyllid (Diaphorina citri Kuwayama) has become the most important insect pest of Florida citrus due to the presence of citrus greening disease spread by the psyllid. In other regions of the world where citrus is grown and greening disease is present, use of insecticides to control the psyllid vector has been a major component of greening management strategies. While no scientific data has been collected in these countries to demonstrate that insecticide use has indeed provided a benefit in terms of reducing or slowing the spread of greening disease, anecdotal evidence suggests that reducing psyllid populations via insecticide application does help to slow the rate of spread of the disease. However, it should be noted that elimination of the disease from an area has never been successful.

The products recommended in this chapter for psyllid suppression have been demonstrated in field trials conducted by the University of Florida to be effective for reducing psyllid populations. However, it should be noted that most of these products will have negative effects on natural enemy populations that keep other potential pests below damaging levels. Thus, it is likely that new pest problems may develop as a result of increased insecticide use for psyllid suppression. However, the problems posed by these other potential pests are far less serious than the threat posed by citrus greening disease.

The goal of psyllid management programs is to reduce psyllid populations in commercial citrus groves to as low levels as possible. Management programs should be developed specifically for a given set of growing conditions attempting where possible to reduce the number of pesticide applications used to minimize costs and negative impacts on beneficial insects and mites. The following information is provided to aid in the development of site-specific psyllid management programs.

Factors Affecting Psyllid Populations

The two main factors that affect the abundance of psyllids are 1) the presence of new flush and 2) the temperature.

New flush is required for psyllid females to lay eggs, as well as for subsequent development of the psyllid nymphs. Female psyllids lay their eggs in developing leaf buds and on feather-stage flush that has not yet unfurled. Once young leaves have begun to expand, they are no longer attractive to psyllids for egg laying. When suitable flush is not available for egg laying, psyllids may either remain on a tree feeding on the mature leaves until new flush is available for reproduction or they may leave the tree in search of other host plants on which to lay their eggs. These plants may be citrus trees within the same grove (particularly young resets that flush more often) or trees producing flush in neighboring groves. Therefore, psyllid management practices in one grove may affect future psyllid populations in nearby surrounding citrus groves.

Temperature is also closely linked to the abundance of psyllids in the field. The ideal temperature conditions for psyllids are between 68-86°F. At these temperatures, a single female psyllid can live for as much as 30-50 days and may lay as many as 300 and 750 eggs, according to laboratory studies. When the daily temperatures are above 90°F, the average lifespan of a female psyllid decreases to less than 30 days in the laboratory, with an average of fewer than 70 eggs produced per female at these higher temperatures. Thus, under Florida conditions, psyllid populations will be lower during the mid summer months compared to late spring and even early fall due to both high temperatures and a reduced amount of new flush available for egg laying.

Psyllid Feeding Damage and Disease Transmission

Psyllid feeding damage is limited to new growth resulting in curling and distortion of the young leaves due to injection of toxins present in saliva injected during feeding on plant fluids. Because of the nature of this damage, in the absence of citrus greening disease, past control recommendations were targeted only to young trees on which the new flush comprised a significant portion of the total leaf canopy. Since the citrus greening pathogen is now present in Florida, it is necessary to manage psyllid populations on bearing and nonbearing trees to reduce the risk and spread of greening.

Much of the information on transmission of the citrus greening pathogen by the Asian citrus psyllid is still unclear. Previous studies on psyllid-pathogen transmission suggests that a healthy psyllid feeding on a greening infected plant may be able to pick up the pathogen with as little as 30 minutes of feeding. After picking up the bacteria, there is a latent period that may be as short as 7 days or as long as 25 days before the psyllid can transmit the pathogen. Transmission of the pathogen is thought to occur through salivary secretions, requiring 1-7 hours of feeding for successful transmission to occur. Detailed studies are underway to examine the exact nature of transmission in the Florida environment, including the investigation of whether pathogen transmission might be prevented or reduced by the use of insecticides.

Chemical Control

Nonbearing Trees

Young trees that produce multiple flushes throughout the year are at greater risk of greening infection than mature trees because of the attraction of adult psyllids to the new flush. Even without greening, young trees in the field need to be protected for about 4 years from psyllids and leafminers to grow optimally. Soil-applied systemic insecticides will provide the longest lasting control of psyllids with the least impacts on beneficials. Currently three soil-applied insecticide active ingredients (imidacloprid, thiamethoxam, and clothianidin) are available provide control psyllids on young nonbearing trees.

Imidacloprid, thiamethoxam, and clothianidin are all neonicotinoid insecticides which, depending on the formulation, may be applied as either soil-drench or foliar applications. However, drench applications are by far the most effective way to use these products on young trees. Soil-drenches are best applied using an applicator metered to deliver 8-10 oz of formulated drench solution to each tree. Drench applications should be applied directly at the soil-rootstock interface. Use restrictions limit the number applications that can be made in a growing season. Imidacloprid applications are limited to no more than 0.5 lbs AI/A per growing season, regardless of application method. This equates to 14 fluid ounces per acre for 4.6F formulations or 32 fluid ounces per acre for 2F formulations. Thiamethoxam applications are limited to no more than 0.172 lb AI/A (or 3.67 oz Platinum) per growing season regardless of which thiamethoxam product is used. Clothianidin is currently labeled for non-bearing use only and is limited to 0.4 lbs AI/A (or 12.8 oz Belay 50 WDG) per growing season. Because of the restrictions on the amount of these products that can be used in a growing season, drenches in solid blocks are only feasible in young trees up to about 6 feet in height with the number of applications possible per year dependent on tree size. Soil applications of these neonicotinoids can be applied to resets within blocks of mature trees at greater frequency or higher rates as long as the limits on amount of AI/A applied is not exceeded. Imidacloprid, thiamethoxam, and clothianidin all share the same mode of action and thus these products are not considered alternatives for rotation to prevent pesticide resistance. Foliar sprays of products other than these neonicotinoids should be used between soil-drench applications to provide additional control of psyllid populations and to help minimize the potential for insecticide resistance development.

Bearing Trees

Management options for psyllid control on bearing trees are much more limited than for nonbearing trees. Previously, the only soil-applied product that provided systemic protection of mature trees from psyllids was aldicarb. However, the aldicarb registration in citrus was recently canceled so use of this product is no longer permitted.

At present, the only other chemical control option that has been demonstrated to be effective for reducing psyllid populations on bearing trees is the use of broad-spectrum foliar insecticide applications. Broad-spectrum foliar sprays are most effective when used to control adult psyllids prior to the presence of new flush. Once psyllids begin reproducing on new flush, it becomes increasingly difficult to gain control of rapidly increasing populations. Psyllid management programs should begin by first targeting overwintering adult psyllids during the winter months when the trees are not producing flush. By eliminating these overwintering adults, psyllid populations will be greatly reduced on the following spring flushes. By targeting psyllids early in the year, this should provide enough suppression in psyllid populations to reduce the need for psyllid sprays during bloom when pollinators are present and most pesticide products cannot be applied. Additional sprays for psyllids should be made when observing an increase in adult populations in a grove.

Biological Control

Foliar insecticide applications should only be used when needed to minimize the impact on natural enemies that maintain psyllids at lower levels later in the year. While a single female psyllid can lay as many as 800 eggs, studies in Florida and Puerto Rico have shown that over 90% of psyllids that hatch in the field do not survive to become adults. Many are consumed by predaceous insects such as ladybeetles. The parasitic wasp, Tamarixia radiata, has become established throughout Florida and also contributes some mortality especially in fall. Additionally, there are many potential pests such as scales, mealybugs, whiteflies, etc., that are currently maintained at low levels in Florida citrus because of biological control. Excessive sprays could result in resurgence of these pests.

Other Management Considerations

Management practices used within a grove can also affect psyllid populations, especially those practices that promote new flush such as hedging and topping and fertilization. Trees should always be sprayed with a broad spectrum insecticide prior to or just after hedging, and topping before any flush develops. Management strategies that reduce or limit the duration of flush may help to keep psyllid populations at low levels and reduce the need for additional pesticide applications. Alternate host plants surrounding citrus can serve as a source of psyllids for infestation. Two common host plants, orange jasmine (Murraya paniculata) and box orange (Severinia buxifolia), are host plants for both the psyllid and the greening pathogen. When possible, both of these plant species should be removed from areas surrounding commercial citrus groves.

Bee Caution

Citrus growers should be aware that most of the broad-spectrum insecticides recommended for psyllid control cannot be applied when citrus is blooming because of the impact these products may have on pollinators. Planning ahead to control psyllids prior to the presence of bloom will help reduce the need to apply pesticides during the bloom period. Check the pesticide label for restrictions on application of a product when trees are in bloom.

Citrus Leafminer

Adults of the citrus leafminer (Phyllocnistis citrella) are tiny moths that hide within the canopy during the day and emerge at night to lay eggs individually on young, expanding leaf flushes. The egg first appears as a tiny dew drop, usually alongside the midvein on the underside of an unexpanded leaf. The larva emerges directly into the leaf tissue, mining first along the midvein, then back and forth as it makes its way to the leaf margin where pupation occurs.

Leafminer populations decline to their lowest levels during the winter because of cool temperatures and the lack of flush for larval development. Populations of leafminer build rapidly on the spring flush, although their presence is not apparent until late spring as populations increase while the amount of new foliage decreases. Throughout the ensuing warm season, leafminer populations vary with the flushing cycles and subsequent flushes are often severely damaged. The summer period of high leafminer damage coincides with the rainy season when canker spread is most likely.

Citrus leafminer greatly exacerbates the severity of citrus canker caused by Xanthomonas axonopodis pv. citri. This insect is not a vector of the disease. Nevertheless, leafminer tunnels are susceptible to infection much longer than mechanical wounds. Tunnels infected by canker produce many times the amount of inoculum than in the absence of leafminer. Control of leafminer should be optimized in areas where infection by canker is high. Natural enemies already present in Florida have responded to leafminer infestations, causing up to 90% mortality of larvae and pupae. These natural enemies include the introduced parasitoid Ageniaspis citricola that has established throughout most of Florida and is responsible for up to 30% of this mortality mostly later in the year.

Leafminer Management

Nonbearing Trees

On young trees, use of the soil-applied neonicotinoid insecticides is the most effective means of preventing mining damage on the new flush and has little direct effect on natural enemies. Soil drenches directly to the base of the tree with neonicotinoids have been shown to provide at least 8 weeks control of leafminer. Injection through the irrigation system is less effective because a large portion of the material falls beyond the root zone. Compared to soil-applications, foliar-applied insecticides provide a shorter duration of protection lasting only about 2 weeks depending on weather conditions and the uniformity of flush pattern.

Soil applications of neonicotinoids should be made about 2 weeks prior to leaf expansion to allow time for the pesticide to move from the roots to the canopy. Avoid applications 24 hr prior to significant rainfall events, which will result in movement of the product out of the root zone before it can be taken up by the plant. When the residual effects of the spring application have worn off, typically during the mid-summer rainy season, foliar sprays of other materials can be used on small trees to reduce leafminer damage if necessary. Reapplication of imidacloprid is not recommended during this part of the season because of the likelihood of the material being washed away by frequent summer rains.

Bearing Trees

If canker is present in a grove (or in a nearby grove), healthy trees with leafminer damaged leaves are more likely to become sites for new canker infection. The only products currently available for leafminer control on large trees are foliar insecticide sprays. While a number of products are effective against this pest, achieving control of leafminer using foliar sprays on large trees is difficult because of the unsynchronized flush typically encountered during summer and fall. However, since leafminers affect only developing leaves, coverage of peripheral leaves in the canopy should be adequate to exert suppression when applying foliar pesticides. Foliar sprays should be timed to coincide with the appearance of the first visible leaf mines that occur immediately following the feather leaf stage or about 13 days after budbreak. At this time, insecticide applications will provide protection for most of the leaves in the new flush.

Recommended Chemical Controls

READ THE LABEL.

Some product labels specify rates per acre, while others specify rates per volume delivered (e.g., per 100 gallons). Refer to label for details on how product should be mixed for desired targets.

Rates for pesticides are given as the maximum amount required to treat mature citrus trees unless otherwise noted. When treating smaller trees with commercial application equipment including handguns, mix the per acre rate for mature trees in 100 gallons of water. Calibrate and arrange nozzles to deliver thorough distribution and treat as many acres as this volume of spray allows.

Tables

The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other extension publications, contact your county Cooperative Extension service.

U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Nick T. Place, Dean.

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