What it is
Bats are the only mammals that fly. There are more than 1,400 species worldwide, roughly a quarter of all mammal species on earth.
Most are insectivores. The rest split between fruit and nectar feeders, fish hunters, and a tiny handful of vampire species. The ones in your garden eat insects. The ones in tropical orchards pollinate them.
A little brown bat (Myotis lucifugus) can take 600 to 1,000 mosquito-sized insects per hour. Over a summer that scales to hundreds of thousands of insects per individual.
Why it works
Insect bats hunt by echolocation. They scream at 20 to 200 kHz and read the echoes off prey wings. In a single pass they can identify, intercept, and eat a moth in less than half a second.
The pest control bill they save North American agriculture sits between 3.7 and 53 billion USD a year, by a 2011 Science estimate. Corn earworm moth, codling moth, cucumber beetle, and mosquitoes all show up in bat guano DNA studies. Lose the bats and someone has to pay the spray bill.
Fruit and nectar bats run a different shift. Agave (every bottle of mezcal), durian, banana, mango, and guava all rely on bat pollination or seed dispersal. A single Mexican long-nosed bat will visit hundreds of agave flowers a night. Frugivorous bats drop seeds far from the parent tree, which is why bat-dispersed pioneers colonise deforested patches faster than bird-dispersed ones.
Build for them
You need three things: roost, water, and dark sky.
Roost. A standard four-chamber bat box holds 50 to 200 individuals. Mount it 4 to 5 m up on a pole or wall facing east or south-east. Morning sun, afternoon shade. Black or dark brown paint in cool climates, light grey in hot ones. Internal chamber gap 19 to 25 mm. Rough surfaces or mesh ladders inside so they can grip.
A single box rarely works. Put up three at once, different aspects and heights. Bats audition them over a season and pick the best.
Veteran trees with cavities outperform any box you can build. Leave the snags up. Loose bark hides roosting Pipistrellus. A 50 cm hollow trunk can house 30 bats through winter.
Water. They drink on the wing, skimming open water. A pond longer than 3 m on its open axis gives most species room to scoop. See ponds and dams.
Dark sky. White security lights kill bat foraging. Switch to amber or warm LED below 3000 K. Aim downward. Bats avoid lit corridors and starve out of well-lit gardens.
Plant for them
Bats hunt where insects gather. Insects gather where flowers and water meet.
Night-flowering plants pull in moths. Evening primrose, jasmine, honeysuckle, datura, and night-scented stock all peak at dusk. Plant a strip of them along a hedge line and bats will work it from twilight to midnight.
Old pollinator habitat advice still applies. Long flowering season, diverse species, no broad-spectrum spray. Pesticides do not just kill the target pests. They strip the bat menu and the bats leave.
For nectar-feeding bats in the tropics, plant agave, ceiba, banana, and night-blooming columnar cacti. These trees evolved with bat pollinators and produce poor fruit without them.
When it goes wrong
White-nose syndrome. A fungal disease (Pseudogymnoascus destructans) has killed over 6 million North American bats since 2006. If you find dead bats in a roost in winter, do not touch them. Report to your state wildlife agency. Clean boots and gear between caves.
Bat in the house. Open every external door and window, close internal ones, dim the lights, and walk out. The bat will leave within 20 minutes. Never swat. If it does not leave by morning, a gloved hand and a small box solve it.
Empty box. Wrong aspect, wrong height, or no nearby water. Move it, or add a second box on the opposite face. Three years is a reasonable wait before you give up on a location.
Rabies fear. Less than 0.5 percent of wild bats carry rabies, and a healthy bat never approaches a human. Any bat acting tame or grounded should be left alone and reported. Vaccinate dogs and cats. Do not handle bats with bare hands.