Spiders, ants, ground beetles, frogs, lizards and snakes all eat fleas. Nematodes, also called roundworms, eat flea larvae, which are fleas before they become adults.
All of these animals are natural predators of fleas. A home surrounded by these animals – with heavily wooded yards, for example – is likely to have fewer fleas than a home that does not. If chemicals are used to kill these flea predators, especially spiders and ants, then the flea population can dramatically increase. Many of these predators eat flea eggs as well, which decreases the flea population before the fleas are even born.