Risks of Methidathion Use to Federally Threatened California Red-legged Frog (Rana aurora draytonii) Pesticide Effects Determination Environmental Fate and Effects Division Office of Pesticide Programs Washington, D.C. 20460 February 18, 2009 Primary Authors: Colleen Flaherty, Biologist Keara Moore, Chemist Pamela Hurley, Toxicologist Secondary Review: Brian Anderson, Biologist James Hetrick, Senior Scientist Branch Chief, Environmental Risk Assessment Branch 3: Dana Spatz 2 Table of Contents 1. Executive Summary...............................................................................................8 2. Problem Formulation ..........................................................................................16 2.1 Purpose..........................................................................................................................16 2.2 Scope..............................................................................................................................18 2.3 Previous Assessments...................................................................................................18 2.3.1 Methidathion Interim Registration Eligibility Decision, 2002................................18 2.3.2. Organophosphate Cumulative Assessment, and Methidathion Reregistration Eligibility Decision, 2006.........................................................20 2.4 Stressor Source and Distribution ...............................................................................20 2.4.1 Environmental Fate and Transport ...................................................................20 2.4.2 Mechanism of Action ...........................................................................................25 2.4.3 Use Characterization ...........................................................................................25 2.5 Assessed Species ................................................................................................................29 2.5.1 Distribution...........................................................................................................29 2.5.2 Reproduction........................................................................................................32 2.5.3 Diet ........................................................................................................................32 2.5.4 Habitat ..................................................................................................................33 2.6 Designated Critical Habitat ........................................................................................34 2.7 Action Area...................................................................................................................36 2.8 Assessment Endpoints and Measures of Ecological Effect ......................................39 2.8.1. Assessment Endpoints for the CRLF .................................................................39 2.8.2 Assessment Endpoints for Designated Critical Habitat ...................................40 2.9 Conceptual Model ........................................................................................................43 2.9.1 Risk Hypotheses ...................................................................................................43 2.9.2 Diagram ................................................................................................................43 2.10 Analysis Plan ................................................................................................................45 2.10.1 Measures to Evaluate the Risk Hypothesis and Conceptual Model................46 2.10.2 Data Gaps .............................................................................................................49 3.1 Label Application Rates and Intervals ......................................................................50 3.2 Aquatic Exposure Assessment ....................................................................................51 3.2.1 Modeling Approach .............................................................................................51 3.2.2 Model Inputs.........................................................................................................51 3.2.3 Results ...................................................................................................................54 3.2.4 Existing Monitoring Data....................................................................................55 3.3 Terrestrial Animal Exposure Assessment .................................................................59 3.3.1 Modeling Approach .............................................................................................59 3.4 Terrestrial Plant Exposure Assessment.....................................................................62 4. Effects Assessment ...............................................................................................62 4.1 Toxicity of Methidathion to Aquatic Organisms ......................................................63 4.1.1 Toxicity to Freshwater Fish ................................................................................64 4.1.2 Toxicity to Freshwater Invertebrates.................................................................67 4.1.3 Toxicity to Aquatic Plants...................................................................................68 4.2 Toxicity of Methidathion to Terrestrial Organisms .................................................68 4.2.1 Toxicity to Birds...................................................................................................69 3 4.2.2 Toxicity to Mammals ...........................................................................................71 4.2.3 Toxicity to Terrestrial Invertebrates .................................................................72 4.2.4 Toxicity to Terrestrial Plants..............................................................................72 4.3 Use of Probit Slope Response Relationship to Provide Information on the Endangered Species Levels of Concern .................................................................................73 4.4 Incident Database Review ...........................................................................................73 4.4.1 Terrestrial Incidents ............................................................................................74 4.4.2 Aquatic Incidents .................................................................................................75 5. Risk Characterization..........................................................................................75 5.1 Risk Estimation ............................................................................................................75 5.1.1 Exposures in the Aquatic Habitat ......................................................................76 5.1.2 Exposures in the Terrestrial Habitat .................................................................80 5.1.3 Primary Constituent Elements of Designated Critical Habitat.......................87 5.1.4 Spatial Extent of Potential Effects......................................................................89 5.2 Risk Description...........................................................................................................93 5.2.1 Direct Effects ........................................................................................................97 5.2.2 Indirect Effects (via Reductions in Prey Base)................................................105 5.2.3 Indirect Effects (via Habitat Effects) ...............................................................109 5.2.4 Modification to Designated Critical Habitat ...................................................110 6. Uncertainties.......................................................................................................112 6.1 Exposure Assessment Uncertainties.........................................................................112 6.1.1 Maximum Use Scenario.....................................................................................112 6.1.2 Aquatic Exposure Modeling of Methidathion.................................................112 6.1.3 Potential Groundwater Contributions to Surface Water Chemical Concentrations ................................................................................................114 6.1.4 Action Area Uncertainties.................................................................................114 6.1.5 Usage Uncertainties ...........................................................................................115 6.1.6 Terrestrial Exposure Modeling of Methidathion............................................116 6.1.7 Spray Drift Modeling.........................................................................................117 6.2 Effects Assessment Uncertainties .............................................................................117 6.2.1 Age Class and Sensitivity of Effects Thresholds .............................................117 6.2.2 Use of Surrogate Species Effects Data .............................................................118 6.2.3 Sublethal Effects.................................................................................................118 6.2.4 Location of Wildlife Species..............................................................................119 7. Risk Conclusions ................................................................................................119