Environmental Concentrations and Aquatic Toxicity Data on Diflubenzuron (Dimilin)

Abstract

The insecticide diflubenzuron (DFB) is commonly used in various mid-Atlantic states for suppression of gypsy moths in hardwood forests. DFB is potentially toxic to nontarget biota because it can enter aquatic systems through aerial application or runoff after precipitation events. Based on this concern, the objectives of this study were to: (1) compile, review, and synthesize literature on the fate, persistence, and environmental concentrations of DFB in both freshwater and saltwater environments; (2) compile, review, and synthesize acute and chronic aquatic toxicity data on DFB effects on freshwater and saltwater organisms; (3) assess possible risk to aquatic biota associated with the use of this insecticide in one specific area (Maryland); and (4) recommend future research based on the data gaps identified from this study.

DFB has low solubility in water and exists as a technical grade (TG) and wettable powder (WP) formulation. The toxicity of both formulations is similar at concentrations < 10 μ.g/1. Organic matter is a major factor influencing the adsorption and degradation of DFB in freshwater, saltwater, and sediment. The half-life of this insecticide in freshwater is =3 days at a pH of 10 and temperature of 36°. At lower pH conditions of 6 and at the same temperature, DFB is more persistent since half-life values of ≈ 9 days have been reported. The half-life of DFB in soil is < 14 days when the particle size was ≈ 2 u, m. The half-life is generally greater in cool, dry soil than in hot, wet soil. Aquatic vegetation acts as a sink for DFB by gradually adsorbing the chemical and releasing it over a period of time.

Freshwater organisms demonstrated a wide range of sensitivity to DFB. Sensitivity was dependent on body composition (i.e., exo- vs. endoskeleton), trophic level, and life stage. During acute exposures, aquatic invertebrates were more than 25,000 times as sensitive to DFB than fishes. The most acutely sensitive species tested was the Amphipod,Hyallela azteca (96-h LC₅₀ = 1.84 μg/l). A mature Plecopteran,Skwala sp., was the most resistant invertebrate species tested in acute tests (96-h LC₅₀ > 100,000 μg/l). In chronic tests, DFB concentrations of 1 μ,g/l or greater were reported to eliminate populations of various Plecopteran (stoneflies) and Ephemeropteran (mayflies) species after 1 month of exposure. A 30-day LC₅₀ of 0.1 fi.g/1 DFB was also reported for the Tricopteran,Clistorinia magnified. Freshwater fish were resistant to acute exposures of DFB as 96-h LC₅₀s were generally > 50,000 μ.g/1. Fish were also reported to accumulate DFB rapidly during acute exposures but were capable of eliminating this insecticide within 7 days. Most of the DFB aquatic toxicity studies with saltwater organisms were conducted with invertebrates. The most acutely sensitive species tested was the premolt stage of grass shrimp (96-h LC₅₀ = 1.11 μ.g/1). The mummichog,Fundulus heteroclitus, the most resistant species tested, had a 96-h LC₅₀ of 32.99 mg/1. The lowest reported chronic effect concentration for saltwater organisms exposed to DFB was 0.075 μ.g/1. This concentration was reported to significantly reduce reproduction in the mysid shrimp,Mysidopsis bahia.

Data from the State of Maryland were used as an example for predicting the potential environmental effects of DFB on aquatic biota in Maryland waters. A case can be made for possible environmental effects given the worst case conditions of the most sensitive species (premolt stage of grass shrimp with a 96-h LC₅₀ of 1.11 μ.g/1) exposed to the highest reported environmental concentration (1.5 μ.g/1 DFB in water). However, in most cases, the present data base would suggest that environmental effects are not likely.

Key Words:

• Dimilin
• diflubenzuron
• aquatic toxicity data
• environmental concentrations
• insecticide