HOW SENSITIVE ARE BATS TO INSECTICIDES

Luckens and Davis (1964) dosed May-captured big brown bats (Eptesicus fuscus) with DDT, and found that oral dosages -40 mg/ kg were always lethal. These results indicated an LD50 (dosage lethal to 50% of animals treated) of <40 mg/kg. Because published LD50's for laboratory rats (Rattus norvegicus), mice (Mus musculus), and rabbits (Oryctolagus cuniculus) were 150, 400, and 300 mg/kg, respectively, the authors concluded that "bats appear to be far more sensitive to DDT than any other mammal yet tested." Subsequently, Luckens and Davis (1965) dosed big brown bats captured in August with dieldrin and endrin, 2 other organochlorine insecticides. They found that ". . . the toxicities of dieldrin and endrin are rather comparable in bats and rats." In a third study, big brown bats captured in late September were dosed with DDT at 301,488 mg/kg (Luckens 1973). These bats were held 1 month and then put into artificial hibernation in early November. During this month, only 2 bats died (dosages 204 and 1,032 mg/kg); all but 1 of 32 bats dosed -458 mg/ kg showed DDT intoxication, as did a few below this level. Bats were aroused briefly in December and again permanently in midor late March. Within 1 month of the March arousal, all bats dosed -458 mg died, but below this level only single bats (from groups of 8, 4, and 4) died at 306, 90, and 60 mg/kg. One month after arousal, the survivors were released. The results of Luckens (1973) appear perplexing in view of those obtained in the first study. However, the researchers theorized that "since these pesticides are fat soluble they probably move right into the fat storage depots when taken in at the time the bat is storing fat for the winter" (Davis 1966), and "... since DDT is a highly lipid-soluble compound, it may have gone directly to a fat depot, taking it out of circulation before lethal amounts could reach the brain" (Luckens 1973:71). The seasonally related increasing proportion of body fat of the bats was probably the factor that resulted in an apparent progression from high sensitivity to great tolerance. The results illustrate that the LD50 does not give consistent estimates of acute toxicities of highly lipophilic organochlorine chemicals when fat levels vary. Even though the initial claim of great sensitivity was countermanded (Davis 1966, Luckens 1973), the idea that bats are unusually sensitive to DDT (or even to insecticides in general) has continued to be repeated and now seems widely accepted (e.g., Wells and Girard 1977, Brown 1978, Esher et al. 1980, Stebbings 1980, Corrao et al. 1985). This idea may have persisted because the first paper in this series (Luckens and Davis 1964) was dramatic and was published in the most widely-read of scientific journals, whereas the second paper (Luckens and Davis 1965) contained no surprising conclusions; the papers that fully explained the results (Davis 1966, Luckens 1973) appeared in specialized publications.