Last July, Ravalli County staffers heard shrieks coming from the third floor of the administration building. The shrieks emanated from county employees who had just discovered dying bats crawling on the floor. Commissioner Ray Hawk says two dead bats were sent to state authorities to be tested for rabies, but came up clean. After that, Hawk says, the specimens were sent on to a laboratory in Michigan for an extensive toxicology test. In mid-January, Ravalli County officials finally received the results: The bats had died of DDT poisoning, and their corpses showed extraordinarily high levels of the poison—more than 4,000 parts per million.
"Not only had they ingested it in some way, but they had the powder on the bats themselves," Hawk says.
Dichloro-diphenyl-trichloroethane has been outlawed since 1972 due to its toxicity. Hawk says that since bats don't typically range very far from their roosts, he suspects that the source of the DDT is near the county administration building, which is in the middle of downtown Hamilton.
"Somewhere within a mile of here there's some heavy DDT, which could be anywhere. There's lots of old barns around here," Hawk says.
The state Agriculture Department has taken swabs of the county building to test for DDT residue, and will determine its next steps once results come back, according to Communications Officer Andy Fjeseth. Fjeseth says state officials have not yet searched the area around the building for signs of the DDT's source.
"Without knowing the location of the source, it's difficult to determine the threat level," Fjeseth says. "If anyone should find or come across any DDT, they should call us at the Department of Agriculture, and we can work with them to properly dispose of it."
Bryce Maxell, program coordinator for the Montana Natural Heritage Program, has studied bats for 12 years. He says he has no doubt that someone in Hamilton sprayed DDT directly onto the bats. In his experience, it's common for people to be overly alarmed when they encounter bats roosting in their roof.
"I've heard horror stories where there were big buildings where janitors were instructed to go up and spray pesticides directly on the bats, or bag them up, put them in garbage sacks and put them in the trash," Maxell says.
Bats roost in buildings only in the summertime. They spend winters hibernating in caves and other remote habitats. Maxell recommends that people take steps in early spring to prevent bats from roosting indoors. Non-lethal methods include sealing crevices and installing bat roost boxes by late April. He notes that residences with bat roosts report fewer mosquitoes in the summer, since bats are insectivores.