Rapid uptake and retention of neonicotinoids in nursery citrus trees as a safeguard against Asian citrus psyllid (Diaphorina citri) infestation

Abstract

There are specific regulations in the USA for the inter-state shipment of containerized citrus from production nurseries located within Diaphorina citri Kuwayama (Asian citrus psyllid; ACP) quarantine zones. To ensure trees are protected from the insect, nurseries must treat trees with an approved systemic neonicotinoid at least 30 days, and not more than 90 days, before trees can be shipped. The objective of this study was to reevaluate the necessity for a 30-day pre-shipment restriction by providing regulators with further data on the uptake of imidacloprid, thiamethoxam and dinotefuran, during the days immediately following treatments. In previous studies, ACP-effective thresholds were determined for 1-year old containerized citrus trees by correlating residue concentrations in leaf tissue with ACP efficacy. In this study, we used target concentrations to compare the uptake and retention of these neonicotinoids in 4 cultivars of 1-year old containerized citrus trees. ACP-effective thresholds were achieved within 3 days of treatment with the current lowest label rate (6.43 g AI/m3 soil) of generic formulations of imidacloprid, and within 1 day of treatment with the maximum label rate (97 g AI/m3 soil) of the name brand formulation Admire Pro®. The establishment of dinotefuran at ACP-effective thresholds was erratic in citrus cultivars, and residues of both dinotefuran and thiamethoxam had largely dissipated before the 30-day pre-shipment period had expired. The uptake and retention of all three neonicotinoids were significantly compromised under excessive watering (400% ET), although ACP-effective thresholds were still achieved with all imidacloprid treatment rates and thiamethoxam within 3 days. Overall, our results strongly support shortening the 30-day pre-shipment restriction to at most 3 days for trees treated with either imidacloprid or thiamethoxam. And, dinotefuran should be removed from the list of systemic insecticides approved for quarantine treatments.