Diving Beetle Facts & Information Guide 2021 (Dytiscidae)
Diving beetles, also commonly known as predaceous diving beetles, are a diverse group of aquatic beetles that make up the family Dysticidae, which literally means “able to dive.” Dysticidae consists of 4,000 or so species, which inhabit every continent except Antarctica.
The diving beetle is named for its adult phase swimming behavior – they will dive underwater for long periods of time, carrying an air bubble in their outer wings and returning to the surface when it runs out. These bubbles are referred to by scientists as “physical gills.” As they swim, the bubble draws oxygen from the water to refill itself. As the beetle consumes the oxygen faster than it is replaced, eventually it runs out and they have to return to the surface to grab a new air bubble. Diving beetles can remain underwater for several hours, with the record being 36 hours!
The greatest threat to diving beetle populations is the disappearance of their wetland habitat and pollution, such as pesticide use and agricultural runoff. They are limited by the presence of large fish populations, and have especially declined in areas where fish species have been introduced for sport fishing and become invasive. The Berne Convention on the Conservation of European Wildlife and Natural Habitats has protected Dytiscus latissimus and Graphoderus bilineatus, which are both endangered in central Europe, as umbrella species for the focus of wetland habitat conservation.
What Do Diving Beetles Look Like?
Diving beetle larvae have crescent-shaped bodies, hairy legs and tail, and the larvae have very long pincers for grabbing prey. Newly hatched larvae breathe atmospheric oxygen though diffusion all over their bodies, and mature larvae breathe through spiracles on their abdomen. Diving beetle larvae look quite different from their adult form, and are typically long, pale, and segmented.
The adults have a football-shaped body and are usually dark brown or green in coloration. Some species have light markings and distinctive patterns decorating their bodies. A few species may have bright coloration to signal that they are toxic. They have short, thin antennae, chewing mouthparts, and fairly large and well-developed compound eyes that help them spot prey. To swim effectively, their two back legs are oar-shaped and covered in long hairs that increase stroke power, while the rest of the legs are tucked in to form a streamlined shape. Their average size is around 2 centimeters long, but others can grow larger or smaller. For example, the great diving beetle grows up to nearly 3 inches, making it one of the largest aquatic predatory insects!
Diving Beetle Habitats – Where Do They Live?
Diving beetles have adapted to live in every kind of inland aquatic environment, and some can even tolerate brackish water. They are most often found in shallow, still, or slow-moving bodies of water. In larger bodies of water, they live amongst the vegetation growing close to the shore to avoid becoming prey to fish.
While most prefer still water, recent studies on certain Dytiscidae species discuss other adaptations diving beetles use to stay underwater for weeks at a time, and to thrive in fast flowing water. Deronectes aubei was found to have tracheal gills under its elytra, which it uses to breathe dissolved oxygen and keep itself from being swept away by the current when submerging by minimizing trips to the surface for air. Additionally, diving beetles have the ability to fly and will colonize new bodies of water if needed.
What Do Diving Beetles Eat? (Diving Beetle Diet)
Adult and larval diving beetles are predators. Larvae have earned the nicknamed “water tigers” due to their insatiable appetite and ambush method of hunting prey. They will float motionless at the surface of the water, and grab passing prey with their large pincers. They will then inject toxic digestive juices into their victim, which instantly kills and partially digests it. The larvae can consume prey much larger than them, and feed mainly on newly hatched tadpoles, fish fry, and other insect larvae. The adults feed on other insects, crustaceans, snails, small fish, and sometimes scavenge for carrion.
Fish are the main predator of diving beetles, but diving beetles are also consumed by birds and mammals, including humans in some parts of the world! If caught, in some species the adults emit an unpleasant chemical odor in an attempt to escape.
Are Diving Beetles Dangerous or Venomous?
Diving beetles can prove to be a nuisance during the warmer months when they fly about in search of mates and new waterbodies, tending to congregate around lights. However, though they can bite, diving beetles are quite harmless to humans and will tend to avoid us. Bites are rare and, while they do hurt a bit, don’t cause any long-lasting effects and their toxic injection is not nearly enough to cause damage to anything large like us, cats, or dogs. Raccoons and other crafty mammals, as well as birds and many fish, regularly eat them, so there is little to worry of toxicity, venom, or a harmful bite about if you see diving beetles swimming or flying about!
The Life Cycle of Diving Beetles
After several molts, mature larvae crawl out of the water and pupate in the mud. Metamorphosis takes a few weeks total, then new adults emerge and return straight to the water. They can live for a few years, and in the northern hemisphere adults emerge in the fall and hibernate in frozen ponds during the winter, then mate the following year. Mating occurs in the water, and males in some species have modified front legs to hold onto the female.
Females lay eggs on vegetation above or on the surface of the water, and the larvae drop into the water upon hatching. Sometimes, diving beetles will lay their eggs close to frog egg clusters, so their larvae can consume tadpoles immediately.
How to Attract Diving Beetles to Ponds – Are They Beneficial?
Due to their predaceous behavior, diving beetles can be a positive addition to a home garden and can help keep your pond water clean, removing dead insects and especially aiding in pest control by eating mosquito larvae and other pest invertebrates. They require well-oxygenated water due to their use of atmospheric oxygen to maintain their diving air bubble, and as such are an indicator of healthy water quality! They are attracted to bright lights at night, which is why they are often found near homes and in manmade pools such as garden ponds, swimming pools, and birdbaths.
Unfortunately, diving beetles won’t coexist well with some other garden critters. Diving beetles won’t last long if you are keeping fish in your pond, because the fish will likely eat them. If you are trying to attract frogs and toads to your garden, diving beetle larvae are likely to keep populations from forming by eliminating tadpoles, their favorite food.
Unfortunately, there is even research that shows that diving beetle predation on tadpoles may significantly affect amphibian conservation plans. A potential solution may be relying on captive breeding to boost amphibian population size until they are stable enough for successful reintroductions. In short, diving beetles are suitable for natural ponds that already have a sort of “balance” in place, but may not be suitable for garden ponds if you wish to have things like frogs and newts in it, as well. If you just have fish, diving beetles can provide a nice protein-rich snack for your fish while helping to eliminate pest insects like mosquitos and midges!