Objective: Develop threshold-based models for current use in Florida citrus. The objective of this experiment is to optimize management of Asian citrus psyllid (ACP) by implementing an economic threshold for need-based timing of insecticide applications. The most recent experiment we are establishing as part of this project also involves insecticide resistance management. We will compare implementation of three economic thresholds by also comparing various insecticide rotation schemes that we have developed based on previous research on managing resistance for ACP. These two factors will be investigated simultaneously. The rotations we will test are comprised of various insecticide modes of action that are currently registered and used for ACP control; these are: acetylcholinesterase inhibitor, acetylcholine receptor agonist, an inhibitor of chitin biosynthesis, chloride channel allosteric modulator, sodium channel modulator and a ryanodine receptor modulator. By rotating these modes of action, we should be able to prevent development of resistance in ACP entirely while implementing an economic threshold. Since the thresholds will determine timing and frequency of applications, it will be important for us to determine how this affects the order of rotating modes of action. Our purpose is to determine the economic threshold level that controls ACP populations most effectively and economically, as well as, which insecticide rotation works best with implementation of such a threshold. Previously, we have established a large-scale experiment at two sites: a commercial site in Frostproof with an estimated size of 80 hectares with 15-25 years old Valencias and Hamlins. There are 8 replicate blocks at this site. We have continued monitoring ACP and flush production at this location since January. The second, newly established site is located in Lake Alfred. Trees at this location consist of 2-3 year old Hamlins. This site was chosen because it has historically shown some of the highest levels of insecticide resistance documented statewide. For example, resistance to neonicotinoid insecticides and associated product failures of imidacloprid and thiamethoxam have been demonstrated at this site. Three economic threshold levels (0.2, 0.5, and 1 ACP per tap sample) will be evaluated for two different insecticide rotation schedules at this site. Each threshold will be evaluated in replicated plots with four replicates per treatment. Ten trees will be selected in each replicate plot to monitor psyllid densities following insecticide applications. Also, these trees will be used for collecting psyllid samples that will be used to determine changes in ACP susceptibility to insecticides in the laboratory. We have already begun collecting adult ACP from these plots to determine the baseline insecticide resistance levels compared to the susceptible laboratory population of ACP using a leaf dip assay. Field populations have been collected, and bioassays are currently being conducted. We use commercial formulations of dimethoate, fenpropathrin, imidacloprid, and cyantraniliprole for this testing. Five to six concentrations of each insecticide are tested and replicated five times. We will begin insecticide applications when adult tap numbers reach the experimental threshold. We will continue to collect samples chosen at random from the central rows of each plot. The plots will be sampled weekly, beginning in late March 2020. The tap sample method will be used to determine the treatment threshold. Ten samples will be taken per plot to determine an everage ACP count. For eggs and nymphs, 10 randomly selected flush samples will be collected per plot, and the number of eggs and nymphs per flush samples will be counted. Leaf samples will be collected from each plot as well to determine the HLB infection rate. When counts of ACP adults in any plot reach a previously defined threshold level, a spray will be applied with the next insecticide in the rotation. We will determine toxicity and dynamic insecticide resistant development. Also, we will collect adult ACP from the rotation sites to determine the relative expression of ten CYP4 and six GST genes compared to the laboratory population. Genes will be selected based on our previous research which has identified specific genes associated with insecticide resistance in ACP. These will severe as diagnostic tools for helping us identify the specific mechanisms conferring resistance. Finally, our goal is to investigate determine the most effective threshold ACP population level required to trigger a management spray within the context of effective insecticide resistance management. In this manner, we will develop an economical and sustainable management strategy for ACP with insecticide, which is still currently lacking in Florida. Our newly developed methods will be having a positive impact on the management of Asian citrus psyllid populations by stabilizing or reducing resistance and focusing on economical viability of spraying.