How Many Insects Does a Bat Eat in a Night? Real Numbers

Why the numbers vary so much

There is no single answer, because the total depends on several factors:

• Prey size: a bat feeding on midges that weigh a fraction of a milligram needs thousands of them, while a bat eating large moths or beetles may need only a few dozen.
• Bat size: a 5 g (0.18 oz) pipistrelle needs far less food than a 20 g (0.7 oz) big brown bat, though small bats eat more for their size.
• Season and weather: bats eat most in late summer and autumn, when they fatten up for hibernation, and little on cold, wet or windy nights, when insects are scarce.
• Breeding: pregnant and especially nursing females need much more energy than males.

So “1,000 insects” may be accurate for one bat on one night and far off for another.

Common pipistrelle: thousands of midges

The common pipistrelle of Europe weighs only about 3.5–8.5 g (0.12–0.30 oz) and feeds mainly on small flies, such as midges, gnats and mosquitoes. The Bat Conservation Trust in the UK estimates that one pipistrelle can eat up to about 3,000 tiny insects a night.

That large number makes sense because midges are so light. Even 3,000 of them add up to only a gram or two, around a quarter to a third of the bat’s weight. On poor nights, the true count is far lower.

Little brown bat: up to its own weight

The little brown bat of North America is one of the best-studied bats. Researchers who measured the energy use of wild females found that, in late pregnancy, a female of about 9 g (0.32 oz) eats around 5.5 g (0.19 oz) of insects a night, or roughly 60% of her body weight.

During nursing, the demand rises. Lactating females, weighing about 8 g (0.28 oz), were estimated to eat around 6.7 g (0.24 oz) of insects a night, close to 85% of their weight. At the peak of milk production, when pups are about two and a half weeks old, the calculated need is close to 10 g (0.35 oz) a night, more than the mother’s own weight. These figures come from energy measurements, not from counting individual insects.

The “1,000 mosquitoes an hour” myth

A widely shared claim says a bat eats 1,000 mosquitoes an hour. It traces back to experiments around 1960 in which little brown bats were released into rooms filled with either mosquitoes or fruit flies. Over short bursts, the bats caught as many as about 10 insects a minute, which some writers multiplied up to 600 or 1,000 an hour.

Wild conditions are different. Insects are rarely that dense, bats do not hunt nonstop, and they take a wide range of prey. Mosquitoes are part of the menu for many bats; a DNA study of little brown bats in Wisconsin found mosquito DNA in about 72% of the droppings sampled. But midges, moths, beetles, mayflies and caddisflies usually make up much of the bulk. Bats help keep insects in check, yet they are not a complete mosquito control.

How scientists measure what bats eat

Researchers use several methods:

1. Droppings: bat guano contains pieces of wings, legs and shells that can be identified under a microscope.
2. DNA metabarcoding: modern labs read short DNA sequences from droppings to identify dozens of prey species at once, even soft insects that leave no visible parts.
3. Weighing: bats are weighed as they leave and return to the roost; the gain shows how much food they caught.
4. Energy budgets: measuring how much energy a bat uses, for example with doubly labeled water, lets scientists calculate how many grams of insects it must eat.
5. Capture rates: high-speed video and acoustic recordings of feeding buzzes show how often a bat attacks prey in the wild.

How bats catch so many insects

Most small bats hunt by echolocation. They send out high-pitched calls and listen for the echoes, which reveal the size, distance and direction of flying insects. When a bat closes in, its calls speed up into a “feeding buzz” of up to around 200 calls per second. Learn more in How do bats use echolocation?.

Bats often scoop insects with the wing or tail membrane before bringing them to the mouth, which lets them catch prey that would slip away from the jaws alone. They usually eat in flight, so a single bat may make several captures a minute when insects are swarming.

Why bats matter to farmers

Many insect-eating bats feed on moths, beetles and bugs whose larvae damage crops, such as corn earworm and cotton bollworm moths. In Texas, huge colonies of Mexican free-tailed bats are known to feed on these moths high in the sky.

A 2011 study in the journal Science estimated that insect-eating bats save farmers in the United States roughly $3.7 billion to $53 billion a year by reducing crop damage and pesticide use, with a central estimate of about $23 billion. The wide range reflects how hard such values are to measure. The same study warned that losses from white-nose syndrome and wind turbines could have real costs for agriculture.

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