Effects of Field Exposure to Diazinon on Small Mammals Inhabiting a Semienclosed Prairie Grassland Ecosystem. I. Ecological and Reproductive Effects

Abstract

The widespread use of cholinesterase‐inhibiting pesticides in the environment presents increasing concerns about their effects on human, wildlife, and ecosystem health. As a group, these pesticides are generally highly toxic and have great potential for negatively affecting nontarget organisms. Small mammals have proven to be ideal biomonitors of environmental contaminants, and were used here to test for possible effects of a widely used cholinesterase‐inhibiting insecticide, diazinon, in a natural field setting. Using 12 0.1‐ha terrestrial mesocosms, we examined the effects of low‐level diazinon exposure on the small mammal communities inhabiting semienclosed grassland ecosystems. Our primary objective was to test the hypothesis that diazinon, applied at two different recommended label application rates, would not cause any observable adverse ecological or reproductive effects on small mammal populations and communities. Experimental small mammal communities consisting of Sigmodon hispidus, Microtus ochrogaster, Reithrodontomys fulvescens, and Mus musculus were stocked at natural densities and sex ratios inside empty mesocosms. Diazinon 4E was applied at two different maximum recommended label application rates, 0.56 kg a.i./ha (1×) and 4.5 kg a.i./ha (8×), and controls remained unsprayed, with four enclosures (replicates) per treatment. Two 30‐d trials were run during peak rodent breeding seasons and enclosures were sampled on days 2, 16, and 30 of each trial. Recovery of small mammals was not significantly different among treatments, although fewer animals were recovered from the diazinon‐exposed enclosures in both trials. Analysis of trapping data suggested that the normally strong competitive relationship between Sigmodon and Microtus may be altered by the pesticide, favoring Microtus in the diazinon‐exposed enclosures. Incidence of reproductive condition was found to be reduced 20 to 80% and 33 to 100% in diazinon‐exposed males and females, respectively. Reproductive productivity, including percentage of pregnant females and of females giving birth, was significantly reduced in diazinon‐exposed animals. Percentage of pregnant females ranged from 13.6 to 43.5% in diazinon‐exposed animals compared to 40 to 80% for control animals, and percentage of females giving birth ranged from 0 to 17% in diazinon‐exposed animals compared to 22 to 50% for control animals. Generally, the effects found in this study suggest that diazinon was relatively persistent in the sprayed enclosures and that oral routes of exposure (consumption of dead and dying arthropods, grooming) may have been important. Ecological relationships and reproduction in both herbivorous and omnivorous mammals were negatively impacted by diazinon exposure. Overall, ecological relationships in the enclosed prairie grassland ecosystem were disrupted by diazinon, probably through a combination of sublethal effects, particularly reproductive effects, impacting individuals and their populations. This suggests that negative impacts on populations and...

Opinion of the Scientific Panel on Plant protection products and their residues (PPR) related to the choice of endpoints to assess the long term risk to mammals

“…This conclusion was based on the evidence of a number of field studies in which the types of effects detected in laboratory tests were also seen in wild mammal populations. Effects included reduced percentage of females with embryos and reduced embryo number in small mammals exposed to diazinon (Sheffield and Lochmiller, 2001), and delayed reproduction and decreased body weight at birth in rodents exposed to carbaryl (Barrett, 1968;Pomeroy and Barrett, 1975;Barrett, 1988). However, it should be noted that the battery of toxicity tests may not provide appropriate assessments for wild mammal species that have physiologies or life history traits that are markedly different from those of test species.…”

Time and space issues in ecotoxicology: Population models, landscape pattern analysis, and long‐range environmental chemistry

“…Scheffield and Lochmiller [101] have recently reported the results of a microcosm study on the effect of exposing a community of small mammals in a prairie grassland ecosystem to subchronic levels of the pesticide diazinon. They set out a field experiment where they limited the displacement of introduced populations of hispid cotton rats ( Sigmodon hispidus ), prairie voles ( Microtus ochrogaster ), and other native small mammals of the Oklahoma (USA) tall‐grass prairies while exposing them to controlled applications of diazinon in 0.1‐ha exclosures.…”

“… A flow‐compartment model of a population system homologous to the description in Sheffield and Lochmiller [101]. The dynamics of Sigmodon hispidus and Microtus ochrogaster populations are described in terms of birth‐death processes as well as that of a generic arthropod composing their diets.…”

Time and Space Issues in Ecotoxicology: Population Models, Landscape Pattern Analysis, and Long-Range Environmental Chemistry

“…As an example of this latter case, consider a feasible parametric scenario of the sigmodon-microtus-arthropod system under toxic stress described in [101] (Fig. 5).…”