Imidacloprid Soil-Drench Application in Young Citrus Trees of Florida: Sorption, Degradation, and Systemic Effects On Asian Citrus Psyllid.

Imidacloprid (IM) is a systemic neonicotinoid insecticide applied to young citrus trees as a soil drench to control Asian citrus psyllid Diaphorina citri (Kuwayama), vector of the bacteria believed to cause citrus greening disease. Processes to model fate and transport of IM in soil and plant were studied under laboratory and field conditions. The sorption study used Immokalee Fine Sand soils of Florida from three depths (0-15, 15-30, 30-45 cm) analyzing both A and E horizons. The 24 hours batch-slurry equilibrium showed low values for sorption partition coefficient or Kd, between 0.20 for E horizons, and 1.68 for A horizons (adjusted to organic carbon contents: log Koc of 2.19 and 2.38). The pesticide degradation showed zero-order trends for all depths, i.e., non-affected by the initial IM concentration of 10 mg per kg of soil. This resulted in half-lives (t_1/2) between 0.9 to 3.1 years, the highest and the lowest t_1/2 in E and A horizons, respectively, possibly due to differences in organic carbon and microbial activity. Also, column experiments using saturated soils and steady-state water flow showed IM breakthrough curves fitting the two-site model. This data suggested that IM was prone to leaching, but persistent in these soils. Following this lead, field experiments during summer 2011 on young unbearing citrus trees (<2 years old) evaluated IM soil-drench applications around the trunk with initial concentrations (0-15 cm depth) between 22.1 to 36.9 mg kg^-1. IM showed almost complete leaching from all depths (0-75 cm) about four weeks after application with final concentrations between 0.1 to 1.1 mg per kg of soil. Nonetheless, young citrus trees with soil-drench treatments compared to control trees with no application showed effective systemic action of the active ingredient to control D. citri even six to eight weeks after application.