The diet of water beetles varies greatly due to the fact that there are close to 400,000 beetle species around the world. In North America, nearly 5,000 species of beetles live in water for at least part of their life cycles.
"Water beetle" is a generalized term for any species of beetle that is water-dwelling during at least one stage of its life. Water beetles can be herbivores, scavengers or predators. Sometimes, the same species of water beetle is a predator during its larvae stage and an herbivore during its adult stage. Herbivorous water beetles consume aquatic vegetation, algae, roots and leaves. They can also suck the juices from aquatic plants and roots. Scavenging water beetles feed on decomposing matter of plants and animals. Many species of water beetles have mouth parts, such as large pinching mandibles, to grab and hold prey. Other predatory water beetles inject their prey with enzymes that break down and liquefy tissue. This allows some water beetles to eat tadpoles, snails, worms, dragonflies, other insects and small fish.
Some families of aquatic beetles, such as Gyrinidae and Dytiscidae, spend their entire lives in water. Others, such as Hydrophilidae and Dryopidae, spend their larvae pupa stage in water before becoming terrestrial adults. In order to live in water, many species of water beetles create an air bubble that they carry under their abdomens. Though they are beneficial in ways to their environments, some water beetles that feed on fish eggs can devastate fish populations if they get out of control.