As bat populations have been reported as decreasing in several countries and pesticides have been suggested as one of the main causes, a survey of the residue levels in bats from an East Anglian area was undertaken together with laboratory tests to evaluate the amounts found. The analytical results showed that bats are more heavily contaminated with residues of DDT-type material than either insectivorous or carnivorous birds. Laboratory experiments confirmed that bats are more sensitive to DDT than other mammals and the rate of metabolism of pp'-DDT to pp'-DDE by Pipistrelles is slower than that of a passerine bird. Comparison of field and laboratory results shows that in the area studied during 1968 and 1969, bats were carrying one third of the lethal level of organochlorine insecticides as “background” residues with just under the lethal level after hibernation. This suggests that organochlorine insecticides could have caused population declines in bats in certain areas.
Recently there have been reports of decreases in bat populations in several countries and pollution by insecticides has been put forward as one of the main causes. The present work was carried out to provide more information with regard to British bats by examining both the amounts of organochlorine insecticides carried in a locality in East Anglia and the lethal levels of the most common material, DDT, after poisoning by this substance. Examination of 30 bats of five species, mainly Pipistrelles from the area around Monks Wood, taken during the period April 1963 to October 1970, showed that 100% of the livers contained pp'-DDE, 82% contained pp'-DDT and 29% contained dieldrin. The mean levels of these materials in the liver was 10·68+4·62+0·29 ppm respectively. The most likely source of these residues is from the food supply of flying insects. It was found that the quantity of DDT material in the bat livers examined during the flying period was at a maximum in March after hibernation and lowest during November.
Laboratory tests with Pipistrelles fed pp'-DDT showed that they were more sensitive to DDT than any other mammals tested other than the American Big brown bat, Eptesicus fuscus. None died at less than 45 mg/kg, half between 45 and 90 mg/kg and mortality was complete above 90 mg/kg. Mortality started to occur when liver and whole body residues of DDT+DDE reached 43 and 45 ppm respectively though these amounts may be higher than normal due to the fatness of the laboratory bats used.
The rate of metabolism of pp'-DDT to pp'-DDE was more slow in the Pipistrelle than in a passerine bird which explains the high occurrence and quantity of the parent compound in field bats compared to field bird specimens. The decrease in the quantity of DDT+DDE in bat livers from one end of the flying period to the other is mainly due to variation in the quantity of depot fat held in the body during these nine months. This is low in March and high in November.
A comparison of field and laboratory results show that bats taken at random in the area studied were carrying a third of the lethal level of DDT+DDE during 1968 and 1969 with just under the lethal level being present after hibernation. Such high “background” levels infer that both the individuals and the population are in danger.