Little brown bats eat flying insects, primarily moths and flies, consuming roughly half their body weight every single night. Their diet spans a remarkable variety: researchers analyzing little brown bat diets across Canada identified nearly 600 distinct prey species. Despite that diversity, the bulk of what they eat falls into a few major insect groups that shift with the seasons.
Moths and Flies Make Up Most of the Diet
Moths are the single largest component of the little brown bat’s diet. Early in summer, moths account for about 35% of what these bats eat, and that share climbs to roughly 55% by late summer as moth populations peak. Flies, including mosquitoes, gnats, midges, and crane flies, are the second most consumed group, representing about 45% of the diet in early summer before tapering off as moths become more available.
Beyond those two mainstays, little brown bats also eat caddisflies, small beetles, wasps, various hoppers, and even spiders. They’re generalists, meaning they don’t specialize in one prey type. Instead, they eat whatever soft-bodied flying insects are most abundant on a given night. Most of their prey ranges from 3 to 10 millimeters long, roughly the size of a mosquito up to a small moth.
Why Water Sources Matter
Little brown bats are strongly tied to rivers, ponds, and streams, and that connection is largely about food. Many of the insects they eat, including midges, caddisflies, and mayflies, spend their larval stage underwater before emerging as flying adults. These emergence events create dense clouds of insects right above the water’s surface, essentially a buffet line for bats.
Research in riparian (streamside) forests has shown that bat foraging activity tracks insect abundance closely. When scientists experimentally blocked aquatic insects from emerging along a 1.2-kilometer stretch of stream, bat activity in the surrounding forest dropped in response. The flux of insects emerging from water is one of the most important factors driving where little brown bats choose to hunt. If you’ve ever watched bats swooping low over a pond at dusk, they’re almost certainly picking off these freshly emerged aquatic insects.
How They Catch Their Prey
Little brown bats were long considered strict “aerial hawkers,” meaning they catch insects in midair while flying. They use echolocation, emitting rapid pulses of high-frequency sound and listening for the echoes that bounce back, to detect and track small insects in complete darkness. This lets them zero in on a mosquito-sized target while flying at speed.
But these bats are more flexible than scientists originally thought. Research has confirmed that little brown bats also glean prey directly from surfaces like leaves, bark, and rocks. This means they can snatch a resting moth off a tree trunk, not just intercept flying insects. That behavioral flexibility likely helps them exploit a wider range of prey, especially on cold or windy nights when fewer insects are airborne.
How Much They Eat in a Night
A typical little brown bat weighs between 7 and 14 grams, roughly the weight of two or three pennies. Despite their tiny size, they eat about half their body weight in insects every night they’re active. For a bat weighing 8 grams, that’s around 4 grams of insects, which translates to hundreds of individual mosquitoes or dozens of larger moths.
Nursing females eat far more. Lactating mothers need to produce milk while also maintaining their own body condition, so their nightly intake jumps to 110 to 150 percent of their body weight. A nursing bat weighing 10 grams may consume 15 grams of insects in a single night, an extraordinary metabolic feat for such a small animal.
Seasonal Shifts in Feeding
The little brown bat’s diet and feeding intensity change dramatically across the year. In spring, when bats first emerge from hibernation, they’re depleted and hungry. Flies and early-emerging aquatic insects dominate the menu simply because moths haven’t yet reached peak numbers. As summer progresses, moths become increasingly available and take over as the primary food source.
Late summer and early fall bring the most intense feeding period. Bats of all ages and sexes enter a phase of rapid fat accumulation to prepare for hibernation. Adults begin packing on weight around mid-August to mid-September, gaining as much as 0.1 grams per day. That may sound trivial, but for a bat that weighs under half an ounce, it represents body mass increases of up to 38%. During this pre-hibernation binge, bats feed as aggressively as possible on whatever insects remain abundant, building the fat reserves they’ll slowly burn through over five to six months of winter dormancy.
Once temperatures drop and insects disappear, little brown bats stop eating entirely. They hibernate in caves or mines, surviving solely on stored body fat until spring brings insects back.
Their Role in Controlling Pest Insects
Because little brown bats consume such large quantities of insects nightly, they provide meaningful pest control. Mosquitoes, gnats, and midges are frequent prey items, and these bats also eat moths whose caterpillar stages damage crops and forests. Agricultural pests like various hoppers and small beetles round out the list of economically relevant insects they help suppress.
A single maternity colony of little brown bats can number in the hundreds or thousands. Multiply hundreds of bats by half their body weight in insects per night across an entire summer, and the collective impact on local insect populations is substantial. This ecosystem service has become a growing concern as white-nose syndrome, a fungal disease, has killed millions of little brown bats across North America, dramatically reducing their numbers and the pest control they once provided.