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Ecological Committee on FIFRA Risk Assessment Methods (ECOFRAM)
Terrestrial Workgroup Report: III. Exposure via Ingestion of Granules

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Introduction

Of the several ways in which birds and mammals may be exposed to granular pesticides, the ingestion of granules is usually considered to be most important (EPA 1992, Best and Fischer 1992). Granules may be ingested accidentally by animals that obtain food from treated soil or vegetation, or they may be ingested intentionally if they are mistakenly accepted as grit or food.

EPA Office of Pesticide Programs currently uses a hazard index approach (LD50s/ft2) to characterize risk of granular products. The exposure component of this index is an estimate of pesticide load available per an arbitrarily chosen unit area. It is not an estimate of pesticide intake for individual birds, and therefore can not be used in conjunction with the dose-response relationship of acute oral toxicity tests to make predictions about the probability of adverse effects.

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Development of New Tool

A new assessment tool is needed that estimates pesticide intake of birds as a result of ingestion of granules. ECOFRAM reviewed two recent attempts to estimate pesticide intake via ingestion of granules using individualbased, probabilistic (Monte Carlo) models (Abt Associates, 1996; Dixon et al. 1997). A Monte Carlo model is under development that incorporates the best features of these models as well as some new ideas.

As a first step, a conceptual model of factors that potentially influence avian ingestion of pesticide granules was developed (Fig 1). The conceptual model has several modular components:

Fig 1. Conceptual Model of Bird Exposure via Ingestion of Granules

flow diagram with 4 sections: 1 has target of granule: grit availability ratio by size class with many inputs including: soil texture data, natural grit availability by size class, granule integrity, rain, spills, hotspots, application rate and efficiency of soil incorporation, and granule size by soil texture size classes; 2 has target of binomial probabilities of ingesting granules vs. natural grit particles by particle size and zone with inputs including: target from section 1, size shape color matrix, granule: grit preference ration, and porportion of grit intentionally ingested; section 3 has target of total grit particle (natural + granules) ingestion rate by particle size and zone with inputs including: gizzard grit counts by grit size class (species & individual variability), conversion of gizzard counts to ingestion rate, and relative bird use of field zones: untreated vs. treated areas (including band vs. spill areas); section 4 has target of pesticide concentration of granules with inputs of applied conc., degradation loss, and washoff loss. Targets of 1 and 3 then go to granule ingestion rate (n/bird/t) by particle size and zone which goes with target of 4 to pesticide ingestion rate from granules (mg/kg bw/t) summed for particle size and zones which goes to intoxication with returns by two route options, one route returns via learned avoidance of granules into section 2 input of granule:grit preference ratio and the other route returns via reduced foraging for grit & food into the target of section 3.

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Levels of Refinement in Analysis of Exposure via Granule Ingestion

Level 1Level 2Level 3
PURPOSE
  • Produce conservative point estimate of exposure level
  • Rapid computation without stochastic modeling
  • Distribution of potential exposure under conservative scenario
  • Distribution of exposure under more realistic scenarios
  • Full range of variation in conditions evaluated

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How does the model work?

The user specifies the granular product (e.g. granular fonofos), crop (e.g. corn), geographic region (e.g. Midwest), focal species (e.g. horned lark) and number of model iterations to be run. In each iteration, the grit ingestion behavior of an individual bird at a crop field with a randomly selected soil type within the region is simulated. The number and size distribution of particles an individual bird is "programmed" to ingest each day is determined by sampling from the avian grit use data base of Best and Gionfriddo (1991). Every instance in which a bird ingests a particle is assumed to be a binomial trial in which the particle ingested could be a granule or a natural grit particle depending on their relative abundance and the bird's preference for one or the other. Granule ingestion is estimated by sampling from a binomial distribution defined by the particle ingestion rate (N) and probability of ingesting a granule (p). The daily dose of pesticide ingested (mg/kg BW) is determined from the number of granules ingested and the pesticide load.

Model Parameter Definition How calculated?
AvlGnlsz Availability of granules of size s in zone z From appl. rate, % surface granules remaining, granule size distribution
AvlGrtsz Availability of natural grit of size s in zone z From soil texture data (% soil mass and mean particle diameter)
ProbGnlsz Probability of ingesting a granule when a particle is ingested of size s in zone z From relative availability of granules and natural grit particles and granule:grit preference factor
ProbGrtsz Probability of ingesting a natural grit particle when a particle is ingested of size s in zone z 1 - ProbGnlsz
GGP Granule to Grit Preference ratio Estimated from literature
PIRsz Particle Ingestion Rate for particles of size s in zone z From studies of gizzard grit number, size and turnover rate, and relative size and bird use of field zones
GIRsz Granule Ingestion Rate for size s in zone z Random outcome drawn from a binomial probability distribution based on PIRsz (n trials) and ProbGnlsz (p)
PIRG Pesticide Ingestion Rate from Granules ∑ GIRsz · GnlWt mg · %AI / BW kg, summed for all particle sizes and zones

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Examples of Inputs and Outputs

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Literature Cited

Best, L.B. and J.P. Gionfriddo. 1991. Characterization of grit use by cornfield birds. Wilson Bull. 103(1):68-82.

Best, L.B. and D. L. Fischer. 1992. Granular insecticides and birds: factors to be considered in understanding exposure and reducing risk. Environ. Toxicol. and Chem. 11:1495-1508.

Abt (Abt Associates Inc.). 1996. Regulatory "cluster analysis" of field corn pesticides. Vols I & II. Report to EPA Office of Policy Analysis. EPA contract nos. 68-W1-0009, 68-D0-0020, 68-W4-0029

Dixon, K., S. Anderson, and M. Hooper. 1997. An individual-based model of chlorpyrifos ingestion and mortality in avian species. Poster PWP058, SETAC Annual Meeting, San Francisco.

Acknowledgment: Special thanks to Pat Havens of Dow AgroSciences for constructing a prototype model.

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