By Jawwad A. Qureshi and Salman Al-Shami
The citrus under protective screen (CUPS, Figure 1) system is designed so citrus can grow inside screened enclosures to protect it from huanglongbing (HLB) and the Asian citrus psyllid (ACP, Diaphorina citri). ACP is the primary vector of the pathogen responsible for causing HLB, known as citrus greening disease. This vector-pathogen combination is currently the most serious threat to the citrus industry.
It is becoming challenging to grow citrus in the traditional open-air production system due to the high incidence and spread of HLB, mainly by ACP. It is also difficult to bring young trees into production once they become infected with HLB and face colonization by ACP. HLB impacts both the quality and quantity of the produce, affecting processed and fresh fruit markets.
CUPS is gaining acceptance among citrus producers as a way to grow citrus free of ACP and HLB, particularly for fresh fruit growers. CUPS provides an environment of high temperature and humidity unlike traditional open-air production systems. It also excludes large predators, so understanding the occurrence of the arthropods (both harmful and beneficial in this system), is essential to developing effective monitoring and management plans.
MONITORING METHODS
Monitoring CUPS regularly is necessary to identify pest problems so they can be managed. Researchers have been monitoring four established CUPS structures with 40-mesh screen at the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) Indian River Research and Education Center in Fort Pierce, Florida.
CUPS and open-air production system controls were planted with Ray Ruby grapefruit on two rootstocks (sour orange and US-897) in two planting systems (in-ground and potted) at a density of 1,957 trees per hectare. Each CUPS and control were planted with 128 trees and 96 trees, respectively.
Yellow sticky traps, visual inspection, pheromone lures, tap sampling and suction sampling devices were employed to detect pests, predators and parasitoids. Each CUPS and control were divided into four plots. Four yellow sticky traps were installed in each CUPS and control, and 40 randomly selected trees were sampled using tap, suction or visual methods every two weeks. Pheromone traps were used to monitor the activity of citrus leafminer (CLM) males at two traps per CUPS and control.
ACP DETECTION
Since its establishment in 2013, no psyllids were detected in the CUPS until 2017 when the first adult was observed on a sticky card. Hurricane Irma in 2017 and another storm in 2019 caused significant damage to CUPS and exposed the structures to pest incursions while being repaired. This allowed the psyllids to enter and establish in the CUPS. Shoots infested with ACP immatures were then observed. However, those infestations were eliminated by manual removal of the infested shoots followed by foliar sprays of insecticides. Otherwise, ACP was either rarely or not observed in the CUPS.
Detection of ACP adults on the yellow sticky cards was more common than with the tap or suction sampling methods. In 2019–2020, 1.3 to 1.5 ACP adults were observed per card compared with 0.2 to 0.3 adults per tap or suction sample in the traditional open-air control. In CUPS, 0.05 adults were observed per card between two years. No adults were observed in the CUPS with any of the three methods in 2021, while 2.4 were detected per card just outside the CUPS walls in the traditional open-air control (Figure 2). The rare incidences of ACP occurrence in the CUPS never translated into shoot infestations except when CUPS were damaged and exposed.
PESTS AND PARASITOIDS
Pests (mites, CLM, thrips, scales and mealybugs) and beneficial organisms (parasitoids and small predators that attack several pests) were able to selectively enter CUPS through the permeable screen or doors. Some of the larger pests (weevils and grasshoppers) and predators (ladybeetles and lacewings) were excluded.
The rust mite (Phyllocoptruta oleivora) and red spider mite (Panonychus citri) were common in the CUPS and caused damage to foliage and fruit. Six species of predatory mites were also observed in the CUPS with two at levels comparable to the open-air control. This article mainly describes the occurrence of insect pests and their natural enemies in CUPS. Pest and predatory mites were described in another article in the March 2021 issue of Citrus Industry.
CLM was another abundant pest in the CUPS. Male CLM were frequently detected in the pheromone traps. Although females were not monitored, their abundance in the CUPS was reflected through significant reproduction and larval occurrence and damage in the foliage, but at a much lower level compared to the traditional control. Its populations were reduced by more than 80% in the CUPS.
Parasitoids, which attack CLM larvae such as Pnigalio spp. and Ageniaspis citricola, were able to follow CLM in CUPS and contributed to its mortality.
Localized infestations of Caribbean black scale (Saissetia neglecta), snow scale (Unaspis citri) and Florida red scale (Chrysomphalus aonidum) were also observed. U. citri infestations were common on the trunks and limbs. C. aonidum and S. neglecta were more common in the canopies colonizing shoots, leaves and fruit. Interestingly, C. aonidum was found only in CUPS and not in the traditional open-air control. Its parasitoid, Aphytis holoxanthus, was more abundant in the CUPS compared to the control based on captures on yellow sticky traps.
Parasitism rates of up to 25% were also observed in the CUPS. Citrus mealybug (Planococcus citri) occurrence was sporadic and observed at low levels on foliage and fruit. Florida flower thrips (Frankliniella bispinosa) populations were notably high in the CUPS compared to the open air. They were detectable with visual observations, yellow sticky traps and the stem tapping method. They induced significantly more scarring on the fruit in the CUPS than in the open-air control (Figure 3).
Maintenance sprays conducted in the CUPS were effective in suppressing pests such as mites and CLM for a short time. Repeated applications were needed.
Jawwad Qureshi (jawwadq@ufl.edu) is an associate professor at the UF/IFAS Southwest Florida Research and Education Center in Immokalee. Salman Al-Shami is a postdoctoral research associate in Qureshi’s lab at the UF/IFAS Indian River Research and Education Center in Fort Pierce.
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