Pesticide use and manufacturing have been one of the most prevalently studied contributions to population decline of M. grisescens. One such study focused on gray bat populations of the Tennessee River area of northern Alabama where scientists and conservators noted a higher than normal gray bat mortality. In this area, since 1947, large amounts of DDTR (DDT (dichlorodiphenyltrichloroethane), DDD, and DDE) flowed through waterways from the DDT manufacturing site located on the Redstone Arsenal near Huntsville, Alabama down to the habitat area of M. grisescens, where heavy contamination of the local biota has occurred. Lethal chemical concentrations of DDT in the brains of adult bats are about 1.5 times higher than in juveniles. Because M. grisescens feed on many types of insects with aquatic larval stages, it is believed that this food source may be the root of the chemical concentrations. Many of the bats tested in different studies were juveniles not able to fly, and thus were likely to have only consumed milk. After concentration through lactation, a few parts per million in prey of the adult gray bat would cause mortality in these juveniles. Under conditions of rapid fat utilization, such as migratory stress or initiation of flight by juveniles, residue mobilization of harmful chemicals may occur, causing mortality. Other pesticides linked with gray bat population decline include dieldrin and dieldrin's parent compound aldrin, which have also increased mortality in other bat species. Even though the manufacture of DDT ceased in 1970 and the manufacture of dieldrin and aldrin in October 1974, heavy contamination of the biota persisted. Recently, however, guano samples from various habitats indicate a decline in certain detrimental chemicals. For example, guano from Cave Springs cave shows a decline of 41% in DDE (a compound related to DDT) between 1976 and 1985 and guano from Key Cave shows a decline of 67% for the same time period. However, it is unknown how long these chemicals will remain in concentrations that will cause harm to wildlife.
