Dynamics of diflubenzuron (dimilin®) concentrations in water and sediment of a supratidal saltmarsh site following repetitive aerial applications for mosquito control

A field study was conducted to monitor changes in diflubenzuron (DFB) and a degradation product, 4-chlorophenylurea (CPU) in water and sediment collected from a supratidal mosquito breeding lagoon. Three applications of a 0·4% sand granule followed by three applications of a 25% wettable powder formulation were made to the site. Substantial differences in the dynamics of both DFB and CPU concentrations in water were noted among applications. Non-linearity of the logarithm of DFB concentrations in water as a function of time was also evident in some applications. In such cases, the half-life parameter does not characterise the dynamics of the process and is of questionable value.

For four of the six applications, there was strong evidence of a decrease in DFB concentration in water. However, there was not significant evidence of a decrease in DFB concentrations in sediment, which appeared to be a major site for DFB adsorption. Water concentrations for each analyte (DFB, CPU) were negatively correlated with sediment concentrations for the same analyte, suggesting that an exchange of both analytes occurs across the water/sedimetn interface. Also, positive correlations were noted between DFB and CPU in both water and sediment. These correlation findings are counter-intuitive, if it is assumed that changes in DFB and CPU concentrations are mainly due to degradation of the former to the latter.

While DFB concentrations in the supratidal lagoon water tended to remain above 0·4 μg litre⁻¹ for 7 days post application, it seems unlikely that this toxic concentration would affect planktonic larval crustaceans in adjacent estuaries if DFB entered these waters via runoff or by flooding of supratidal areas. The persistent DFB concentrations in the water and sediment (∼100 μgkg⁻¹), however, could be detrimental to detrialfeeding populations of marsh crustaceans (e.g. Uca).