Bats benefit man by consuming immense numbers of flying insects. However, when they roost in large numbers in man- made structures, their accumulated droppings, odors, mites and lice cause problems. There is also the potential threat of rabies, but this is rare.
The best way to exclude bats permanently from a building is to prevent their gaining entrance. To do this, brightly illuminate the inside of the roosting area at night and from the outside, observe where the light can be seen.
As an alternative for detecting the openings, observe the darkened roosting area from the inside during a bright day. Close these openings with wood, caulking, hardware cloth or masonry. Close all but one or two openings wherever convenient and close the remaining openings just after sunset when the bats are outside.
Bats are sensitive to high frequency sounds and can be repelled by commercial high-frequency sound devices. An inexpensive device can be made by attaching two or three silent dog whistles to an air pump, such as a large aquarium pump. Place the device in the roosting area and run continually until the bats leave.
Bats may also be repelled by brightly illuminating their roosting area at all times. The attic of an average size house may require four or more 100 watt bulbs to drive out the bats. Place the light bulbs to illuminate all potential roosting sites. The bats should leave after several days of illumination. Reapply if necessary.
Bats may be repelled by the odor of napthalene. Spread 100% granules or flakes on floors or between walls. Use at least 5 lb., per 2,000 cu., ft., of air space in the roosting area. Large openings, such as a garage door or large ventilators, must be closed if this technique is to be effective.
Because bats may be attracted by the odor of bat urine and droppings, all droppings should be cleaned up and the roost area washed with a disinfectant, if possible, after the bats are repelled.