Insecticides and bridge-roosting colonies of Mexican free-tailed bats (Tadarida brasiliensis) in Texas

Abstract

Approximately 100 million Mexican free-tailed bats spend the summer in Texas, many roosting in highway bridges located near intensive agriculture. Bats may acquire insecticide residues indirectly through the food chain or directly by exposure to sprays. Many insecticides are lethal to non-target wildlife and sublethal exposures may disrupt thermoregulation, food consumption, and reproduction. To determine exposure, specimens and guano were collected from 24 bridge colonies throughout Texas. Samples were analyzed for organophosphate (OP), carbonate (CB), organochlorine (OC), and pyrethroid residues. Brain cholinesterase (ChE) levels were also measured to determine potential OP and CB exposure. Developing embryos were removed from pre-partum females, cleared and stained, then examined for gross physical deformities that might be linked to chemical exposure. Organochlorine residues were found in all guano samples with dichlorodiphenylethylene (DDE) exceeding 3 ppm (dry weight), a level previously associated with migratory mortality in newly-volant free-tails from Carlsbad Cavern, NM, at five sites. Dieldrin and endrin residues were found in guano from three colonies. Analyses of carcasses (n=17) and milk (n=3) from six colonies revealed dieldrin levels up to 1.39 ppm (wet weight) in adult carcasses and 8.71 (wet weight) ppm in milk. DDE was found in all carcass and milk samples with levels up to 48.3 ppm (weight wet) in carcasses and 3.45 ppm (wet weight) in milk. Pyrethroid residues ranged from 0.02 to 0.10 ppm in guano samples from seven colonies; however, no residues were detected in milk or carcasses. Cholinesterase levels of four individuals from three counties appeared to be depressed, but gastrointestinal tracts from fifteen individuals revealed no OP or CB residues. Mean colony ChE level decreased significantly with both increased crop-acreage and acreage treated with insecticides by county. No embryos were found with deformities. Although no longer used, OC compounds are present at levels that may affect Mexican free-tail populations in Texas, specifically migrating young of the year. Widespread exposure to anti-cholinesterase insecticides may also put populations at risk.