By Jawwad A. Qureshi and Philip A. Stansly
More than a decade has passed since 2006, when huanglongbing (HLB) or citrus greening disease was identified in Florida. By then, the disease had already spread widely and went unrecognized due to high psyllid populations and a disease incubation period of months or years between infection and symptom expression. Nevertheless, management of the Asian citrus psyllid (ACP) vector is still critical to reduce disease severity in infected trees and spread into new plantings.
Research-based tactics for individual and area-wide ACP control have greatly reduced psyllid populations, helping mature citrus to maintain production and new plantings to survive and produce in spite of HLB. Tolerant or resistant plants and bactericides may reduce, but probably never eliminate, the need for ACP control. Nevertheless, challenges remain due to the rapidity of vector reproduction, dispersal and disease transmission. Therefore, management of ACP in all habitats including organic and conventional citrus, urban areas and abandoned citrus is critical for area-wide reduction of this vector-disease complex and for sustainable citrus production.
ACP requires newly developing buds and young shoots of citrus to develop and reproduce. Young trees flush more frequently compared with mature trees and therefore face a greater risk of infestation by ACP and HLB infection. Consistent and longer-lasting protection from ACP provided by soil-applied, systemic insecticides interspersed with sprays of different chemistry is necessary. The systemic neonicotinoid insecticides, imidacloprid, thiamethoxam and clothianidin, may provide up to six to eight weeks of protection, but even this is not sufficient because of label restrictions that limit the amount of these products that can be used.
Furthermore, these insecticides all have the same 4A mode of action (MoA), so rotation is needed to delay resistance. Cyantraniliprole (Verimark®), a new insecticide with a different MoA (28), would be another systemic option if it were not so expensive. Foliar sprays of different MoAs are needed.
Planting on reflective mulch provides additional protection to young trees by further reducing ACP colonization and therefore HLB pressure. It is an effective tool to augment current control measures for young trees based primarily on insecticides.
Dormant/Winter Sprays: ACP reproduction is rapid. Up to 700 to 800 eggs per female are possible under favorable conditions in spring and summer when temperatures are optimal and flush is abundant. Emigration of infected ACP at the end of spring flush is the main event in the annual movement of HLB. Much of this movement can be preempted by controlling ACP in winter, when populations are down due to lower temperatures and lack of flush needed for reproduction. This weak link in the ACP life cycle is best exploited with sprays of organophosphate and/or pyrethroid insecticides prior to spring flush.
In the interest of resistance management and conservation of beneficials, these insecticides are not recommended during the rest of the year (except for border sprays). Area-wide dormant sprays with these products reduced ACP populations 15-fold in spring compared with an untreated grove. Natural enemies such as ladybeetles and lacewings, which contribute to control of ACP and other pests such as citrus leafminers, aphids and mites, are relatively unaffected by these dormant sprays.
We are testing conventional and organic programs for ACP management to expand options for all growers. Three separate organic programs — rotations of organic insecticides applied alone, with horticultural mineral oil or with insecticidal soap — are being compared with a standard conventional program for ACP control and conservation of beneficials in a block of Valencia and Hamlin oranges. Dormant sprays of the natural pyrethrum PyGanic EC 5.0 alone (17 oz./acre) or with 435 oil (2% v/v) or M-Pede (2% v/v) applied in November, December and January, and Danitol 2.4 EC applied at 16 oz./acre in January all significantly reduced ACP through March. By then, adults started to escalate above a provisional growing season threshold of 0.1 per tap sample. Danitol provided more ACP reduction than PyGanic, especially when applied both in November and in January.
Thus, two dormant sprays were better than one. PyGanic with M-Pede or 435 oil performed better than PyGanic alone. There is no threshold for dormant sprays, which are critical for year-round ACP control.
The last two winters were warm with some rain events, which resulted in unexpected flush and greater than normal populations of ACP. It may be necessary to add another dormant spray under such conditions.
Growing Season Management of ACP: Factors to consider for control of ACP during the growing season include abundance of nymphs or adults, other pests which also need to be controlled, cost, and effects on beneficials. Just seeing one or two adults per 50 taps or single infested flush may not be sufficient to justify spraying an entire block. Thresholds based on annual accumulation of ACP in mature blocks with high HLB incidence have been developed (see Citrus Industry magazine, February 2015). Flush protection is critical because when an infected female has access to a new bud or shoot, it not only lays eggs, but injects the pathogen into those soft tissues, thus spreading the infection to the tree, its progeny and other colonizing adults.
We have been using a provisional 0.1 adults per tap sample threshold for the three organic programs and one conventional program described above. Organic insecticides rotated with M-Pede or 435 oil used 50 percent less insecticide while providing better control than organic insecticides alone. Reduction of ACP was still greater with the conventional program, but not always significantly so, compared with programs using organic insecticides with M-Pede or 435 oil.
We have been testing these spray applications mostly at 100 gallons per acre using a Durand Wayland AF100-32 air blast speed sprayer, and sometimes at 10 gallons per acre using a Proptec rotary atomizer sprayer, and have seen good results with both (see figures 1 and 2). Best yields were obtained using organic insecticides with 435 oil or conventional insecticides and did not differ between the two programs (see Figure 3). Yield did not improve with organic insecticides alone compared to the untreated check, and was significantly less than the check with organic insecticides plus M-Pede, possibly due to a spray on bloom.
The relative effectiveness of conventional insecticides for control of ACP and other pests is listed in Table 1. Smart choices which target multiple pests that may be present in significant numbers with one spray of single MoA will reduce unnecessary sprays and attendant costs.
More research and funding is still warranted to investigate the sustainability of these organic programs, their role in managing other pests and integrated use of organic and conventional insecticides.
Natural enemies including predators, parasitoids, certain fungi and even nematodes play an important role in controlling pests of citrus. Many beneficial insects are rare in citrus groves during winter due to the lack of prey, but increase in spring coincident with new growth and may persist during the growing season.
ACP, citrus leafminers, aphids, weevils and mites colonize flush, as do their natural enemies such as ladybeetles, lacewings, spiders, minute pirate bug, predaceous mites and parasitic wasps, with the potential of keeping these pests under control. Sprays targeted at flush negatively impact beneficial insects, reducing natural control for many pests and often leading to secondary pest outbreaks. Citrus leafminers, aphids and root weevils are associated with canker, tristeza and root rot, respectively. So these diseases increase with increased pest pressure.
We saw that lacewings, spiders and ladybeetles all increased where organic insecticides were used, which may have contributed to ACP reduction. Green lacewings were the most abundant predator and are mobile enough to move in quickly from the surrounding habitats. Ladybeetles, which were formally abundant in our groves, are now rarely seen, presumably due to frequent use of broad-spectrum insecticides. Likewise, the parasitoid Tamarixia radiata was released in all plots in the above experiments, but mostly recovered from ACP nymphs in plots sprayed with organic insecticides. Thus, the best time to release Tamarixia would be in the spring flush after dormant sprays.
Acknowledgments: The authors thank the Citrus Research and Development Foundation for providing funding to some past projects and the California Citrus Research Board for the current organic projects.
Jawwad A. Qureshi (firstname.lastname@example.org) is assistant professor of entomology and Philip A. Stansly is professor of entomology with the University of Florida/Institute of Food and Agricultural Sciences Indian River Research and Education Center in Fort Pierce and Southwest Florida Research and Education Center in Immokalee, respectively.
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