Mosquito spraying, overheating theories in bat deaths

Velvety free-tailed bats are one of nine bat species found in the Cayman Islands. - Photo: File

Environmental officers are investigating a mysterious spate of bat deaths between Prospect and Rum Point this summer.

Deaths of velvety free-tailed bats, one of nine bat species in the Cayman Islands, were first reported in July by Ocean Club residents in Prospect, said Fred Burton, manager of the Terrestrial Resources Unit for the Department of Environment.

Residents noticed an unusual number of dead bats around bat boxes that were constructed through a former conservation programme.

“We followed up and began monitoring the bat boxes islandwide, and found short-term mortality events happening at Prospect Point, Ocean Club, Willow Point, Pedro, Bodden Town, and in the Rum Point area of North Side,” Burton said.

“Timing in these places varied. Occasional dead bats were found at other locations but those I’ve listed were the places where there was significant mortality.”

The Department of Environment is exploring a number of theories, including mosquito spraying and thermal stress.

Burton said the department is working with the Mosquito Research and Control Unit to investigate the possibility of chemical spraying as a culprit. The MRCU did not respond to CAyman Compass questions about the topic before press time on Monday.

“MRCU has been helpful in sharing their spray data with us and we should be able to rule this in or out,” Burton said.

“Another hypothesis is thermal stress. The mortalities occurred in an extremely dry and hot period. These bats like to live ‘hot’ and may often be quite close to their thermal maximum. So heat stroke is a possibility.”

Another possibility is disease. Burton ruled out white-nose syndrome, a disease that has killed millions of bats in North America but does not occur in the tropics.

Rabies is another disease Burton ruled out, as the symptoms do not match. But to send tissue samples to a lab in the United States, testing must be done for rabies first.

The US Centers for Disease Control and Prevention will test the bats’ brain tissue for the DoE and once deemed rabies-free, the rest of the tissue will be sent to a recommended lab for toxicity testing.

The DoE does not have an overall population count of bats in the Cayman Islands. Monitored bat boxes, however, have been observed to house between 100 and 300 velvety free-tailed bats each.

Population estimates have not been made for other species. Many Cayman Islands bat colonies have been lost. Before the Crystal Caves were developed for tourism, for example, Burton said another disturbance there killed off a huge population of Brazilian free-tailed bats.

“Bats living in caves are extremely sensitive to disturbance by humans, especially when flashlights and camera flashes are involved,” Burton said.

“This is why the ‘Bat Cave’ on Cayman Brac, for example, has no bats any more.”

Velvety free-tailed bats do not depend on caves for habitat. They are known to inhabit tree hollows, rock crevices and attics. The bat boxes were established through a National Trust for the Cayman Islands programme to provide alternative habitat and discourage bats from roosting in buildings.

There are about 40 bat houses erected on CUC utility poles across Grand Cayman. In Cayman Brac, there is little need for bat boxes because the Bluff provides so many crevices, a National Trust fact page explains.

Velvety free-tailed bats feed at dusk, hunting mosquitoes, flies, beetles and moths.

Bats are the only native mammal of the Cayman Islands.

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