By Jawwad Qureshi
Management of the Asian citrus psyllid (ACP) is needed in multiple environments, such as organic and conventional production systems and urban areas. However, the tools and approach may be different depending on the circumstances.
Chemical control is the primary method used in commercial production to reduce psyllid populations, but the increased use of insecticides negatively affects the environment and biological control agents, which are an important component of citrus pest management. Additionally, repeated use of similar mode-of-action toxins leads to insecticide resistance in ACP, as already seen with neonicotinoids.
The soft tissues of young shoots are needed by ACP females to develop and deposit eggs. Citrus trees produce a major flush in spring followed by sporadic growth in summer and fall. Very few young shoots are produced during winter, which limits ACP reproductive capacity. This is a time when killing most adults has the greatest impact on reducing the probability of significant population spikes when spring growth provides ACP with abundant resources for reproduction.
Foliar sprays of insecticides are the primary tool to target psyllids during the dormant and growing seasons. Therefore, three separate organic spray programs and one conventional spray program integrating releases of the parasitoid Tamarixia radiata were evaluated in a multiple-year study conducted on Valencia orange trees. The objectives of the experiment were to measure the impact of these programs on psyllid suppression and citrus yield and find tools that could be used toward developing integrated and sustainable citrus pest management programs.
Three organic programs, organic insecticide tested alone (program 1) or tank-mixed and rotated with M-Pede (program 2) or horticultural mineral oil (HMO) FL 435-66 (program 3) were evaluated. These were compared with a rotation of conventional insecticides. In the dormant winter season, Pyganic® (natural pyrethrum) was sprayed alone or tank-mixed with M-Pede or HMO FL 435-66.
These sprays were applied monthly in November, December and January. Danitol® (synthetic pyrethroid) was used once in January in the conventional program. The decision to spray during the growing season was made when psyllids reached 0.1 adults per tap sample (10 adults in 100 tap samples). Insecticides used in different programs are listed in Table 1. A total of 81,946, 94,764, 118,386, 69,429 and 31,429 Tamarixia adults were released in 2014, 2015, 2016, 2017 and 2018, respectively.
MONITORING OF PSYLLIDS AND BENEFICIAL ORGANISMS
Psyllids and predators were sampled using the tap sampling method. Samples were collected onto an 8.5 by 11-inch laminated, white paper held on a clipboard. The clipboard was held horizontally under randomly selected branches at 3 to 6 feet above the ground in the outer tree canopy. The branches were struck sharply three times with a short length of PVC pipe.
Psyllid adults and predators observed in the samples were recorded. Shoots containing third to fifth instar ACP nymphs were collected randomly from the trees in organic and conventional programs and in the untreated control. Nymphs were then held in the laboratory for two weeks to allow ACP adults and T. radiata to emerge, or the nymphs were examined under a stereo microscope to check for the presence of Tamarixia eggs for evaluation of parasitism rates.
TAMARIXIA AND PREDATOR ABUNDANCE
Psyllid nymphs parasitized by T. radiata were observed during the growing season from February to October. Mean parasitism averaged between 17 and 27 percent in the untreated control, 15 to 19 percent in the organic programs and 1 to 8 percent in the conventional program. Maximum parasitism in any one month was 40 percent in the organic programs, followed by 35 percent in the untreated control and 10 percent in the conventional program. However, there also were times when nymphs sampled from the untreated control, organic or conventional programs were not parasitized.
There were 0.2, 0.03 and 0.02 spiders, lacewings and ladybeetles counted per tap sample, respectively, in the untreated control. Fewer spiders (0.1 average) and ladybeetles (0.01 average) were found in plots where any type of insecticide program was in place. However, there were slightly more (0.02 average) lacewings found in plots treated with organic programs than in conventional program (0.01 average).
PSYLLID SUPPRESSION AND YIELD
Significant psyllid suppression was observed from dormant-season sprays in the organic programs and conventional programs compared with the untreated control. Pyganic® tank-mixed with M-Pede or HMO FL 435-66 oil sprayed once each in November, December and January provided significantly more reduction in ACP than Pyganic® sprayed alone.
Psyllids exceeded the treatment threshold of 0.1 adults per tap sample by March, which warranted additional sprays during the growing season. On average, 0.3 to 0.6 ACP adults were counted per tap sample in untreated control plots. Compared with the untreated control, psyllid adults were reduced by approximately 80 percent in the conventional program, 60 to 68 percent in the programs using organic insecticides with M-Pede or HMO FL 435-66, respectively, and 36 percent in the program using organic insecticides alone.
Yield was highest in plots where organic insecticides were mixed or rotated with HMO FL 435-66 oil or those treated with the conventional rotation of synthetic insecticides (Figure 1).
Survival and sustainability of citrus production in regions with HLB depends on psyllid management. Significant reduction in ACP using organic insecticides with HMO FL 435-66 or M-Pede indicates potential use of these tools for organic citrus producers and in urban areas where conventional products may not be appropriate. Effective management of ACP in organic citrus and urban environments will help control the spread of the vector and disease to other environments and provide yield benefits.
These findings also provide conventional growers with the option of diversifying their programs by including organic insecticides as part of integrated pest management. Evidence of parasitism in these studies, particularly in organic programs and during the growing season when psyllid nymphs are available despite insecticide use, suggest that releasing T. radiata in commercial production systems will be useful for additional ACP suppression. Rotations of organic and conventional insecticides will be useful to improve biological control and to help reduce the use of or reliance on conventional insecticides and risk of pest resistance already shown in Florida.
Acknowledgements: The author thanks the Citrus Research Board for funding this research and the Division of Plant Industry for providing T. radiata. He also thanks the late Phil Stansly for his valuable suggestions and contributions to these experiments.
Jawwad Qureshi is assistant professor of entomology at the University of Florida Institute of Food and Agricultural Sciences Southwest Florida Research and Education Center in Immokalee.