Why spray if you don’t need to? Putting the IPM back into cItrus IPM by ground truthing spray thresholds

Why spray if you don't need to? Putting the IPM back into cItrus IPM by ground truthing spray thresholds

Report Date: 06/07/2021
Project: 19-002   Year: 2021
Category: ACP Vector
Author: Lukasz Stelinski
Sponsor: Citrus Research and Development Foundation

The objective of this experiment is to determine the effect of incorporating action thresholds for ACP on populations of natural enemies as well as secondary pests of citrus. ACP vector control is a basic component of HLB management even in situations where disease incidence is high. The use of action thresholds is a possible strategy to reduce unnecessary insecticide input to improve economic return, and reduce potential for insecticide resistance. The idea is to spray only when one reaches a population level of ACP that cause economic damage rather than spraying for ACP all the time. We suspect that an ancillary benefit of using action thresholds will be to reduce the negative impact of insecticides on populations of natural enemies and/or reduce secondary pest infestations because we spray less for ACP control. This may provide further benefit by improving activity of biological control. While the main goal of this investigation is to establish economic thresholds for ACP control to optimize returns on investment, we investigated this ancillary question of whether reducing insecticide input for ACP control may affect populations of beneficial natural enemies in citrus groves.  A year-long study was initiated on March 5, 2021 in an experimental sweet orange grove under standard cultural practices located in Lake Alfred and with an estimated initial HLB infection level of 100 %. Prior to the start of the experiment, the entire site was sprayed with pyrethroid. Four replicates were established for each threshold treatment. Insecticide sprays for ACP were based on nominal thresholds of 0.2, 0.5, or 1.0 adult per stem tap and ACP treatment sprays were made of different insecticide modes of action in rotations. There were seven applications for the 0.2 adults per tap threshold treatment, five applications for the 0.5 adults per tap threshold treatment, and two applications for the 1.0 adult per tap threshold treatment. Also, a calendar based positive control treatment was established where ACP sprays were applied once per month, irrespective of ACP population density. ACP were monitored by calculating the mean number of adults per tap from ten randomly selected trees across all four plots for each treatment. Tap samples were made by holding a 22 × 28 cm plastic laminated white paper sheet horizontally underneath a randomly chosen branch, which was then struck sharply three times with a 40 cm length of PVC pipe. ACP adults falling on the sheet were quickly counted to obtain the number of ACP adults per stem-tap and tree. If the means reached or exceeded the target economic threshold, all replicate plots assigned to that treatment threshold were sprayed. ACP sampling occurred every 7 to 15 d.  Populations of ACP and natural enemies were assessed every week from March 19 to April 28, 2021 using the stem tap sample described above. Spiders (Araneae), arboreal ants (Hymnoptera: Formicidae), lady beetles (Coleoptera: Coccinellidae) and lacewings (Neuroptera) previously identified as key natural enemies of ACP or other important citrus pests were counted. Counts were made by visually inspecting randomly selected tree branches in earch plot for 2 minutes per sampling period and counting all arthropods found during that period. Inspections were conducted weekly. Arthropods were identified to species level where possible. Ants were collected with pointed round paint brushes and preserved in 70% ethanol and identifications were made subsequently.  Cumulative numbers for Asian citrus psyllid were 156, 129, 143 and 135 for the 0.2, 0.5, and 1.0 psyllid per tap, and calendar treatments, respectively. There were 8, 11, 8 and 4 spiders found for the 0.2, 0.5, and 1.0 psyllid per tap, and calendar treatments, respectively. We counted 25, 50, 41, and 51 arboreal ants for the 0.2, 0.5, and 1.0 psyllid per tap, and calendar treatments, respectively. There were 3, 1, 0. and 0 lacewings for the 0.2, 0.5, and 1.0 psyllid per tap, and calendar treatments, respectively. There were 10, 16, 16, 22 ladybird beetles for the 0.2, 0.5, and 1.0 psyllid per tap, and calendar treatments, respectively. Finally, there were 1, 4, 2 and 1 citrus root weevils for the 0.2, 0.5, and 1.0 psyllid per tap, and calendar treatments, respectively. This experiment will run throughout the remainder of the summer. At this early point in the esperiment, we have not yet observed an effect of reducing sprays by employing treatment thresholds on populations of natural enemies as compared with plots treated with calendar sprays. However, we predict that differences may emerge as the investigation progresses, based on our previous experience. The possible improvement in the effect of biological control as a consequence of using treatment thresholds for timing sprays against ACP may prove to be a means of integrating insecticides with biological control for psyllid management.                                 


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