Dieldrin and heptachlor residues in dead gray bats, Franklin County, Missouri--1976 versus 1977.

Lethal dieldrin concentrations were found in the brains of dead gray bats (Myotis grisescens) collected during 1976 and 1977 beneath a maternity roost in a Missouri cave. In addition, residues of heptachlor epoxide, oxychlordane, cis-chlordane, and trans-nonachlor increased significantly in both brains and carcasses of bats collected during 1977. These increases appear to reflect a switch by local farmers from aldrin, dieldrin's parent compound, to heptachlor for the control of cutworms. They also constitute an additional threat to this colony of this endangered bat species.