Potential agricultural insecticide exposure of Indiana bats (Myotis Sodalis) in Missouri
The type and distribution of agrochemical insecticides routinely applied to the major field crops in northern Missouri were described and shown to be coincident with the summer/maternity range of the endangered Indiana bat (Myotis soda/is). The use of insecticides was found to be greatest in the production of corn. In addition, agriculturally significant insect pests causing field crop damage along with those that provide a resource base for the Indiana bat were identified. Two congeneric surrogate bat species (M. lucifugus, the little brown bat, and M. septentrionalis keenii, the northern long-eared bat) were collected from northern Missouri agricultural sites and from winter hibernacula in non-agricultural southern parts of the state to assess the potential impact insecticides have on the Indiana bat. Residues of 8 historically applied organochlorine insecticides were identified in all surrogate specimens , with pp-DOE found in greatest amounts. Two synthetic pyrethroids, permethrin and esfenvalerate , were recovered from 16 individuals and both surrogates. Two biomarker indices suggestive of organophosphate and/or carbamate insecticide exposure, brain acetylcholinesterase (AChE) and muscarinic receptor number (B[subscript max]), were depressed in 10 little brown and 4 northern long-eared bats ([alpha] = 0.05) collected from 6 northern Missouri agricultural sites. The current findings suggest that some individual bats of both surrogate species living in northern Missouri during the summer months are being exposed to sublethal levels of organophosphate and/or carbamate insecticides. The sensitivity, as measured by 24h oral LD[subscript 50] tests in the little brown bat, was determined to an agricultural field grade and an analytical reference grade of the insecticide permethrin. The field grade product had an LD[subscript 50] of 38 mg/kg, while the analytical grade had an LD[subscript 50] of 514 mg/kg. Bats surviving analytical grade LD[subscript 50] tests were examined for their flight ability at 24 hours. An effective dose (ED[subscript 50]) of 79 mg/kg impaired flight in 50% of tested bats, a dose approximately 15% of that found to produce lethality. Flight performance of little brown bats was evaluated in obstacle avoidance tests after exposure to analytical grade permethrin. No relationship was found between performance as measured by the total number of obstacle avoidance passes or the percentage of hits and oral permethrin dosage or brain tissue permethrin concentrations . A brain tissue permethrin concentration of 1-3 ppm appears to be a toxic threshold, below which high performance variability is noted . Above a brain concentration of 1-3 ppm, death prior to completion of the experiment at 96h was common.
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