A Dytiscidae larva (water tiger) spotted in a pond adjacent to the pipeline corridor within the Kenai Wildlife Refuge in June 2020 (USFWS/Matt Bowser)

A Dytiscidae larva (water tiger) spotted in a pond adjacent to the pipeline corridor within the Kenai Wildlife Refuge in June 2020 (USFWS/Matt Bowser)

Refuge Notebook: The little-known predator of the seasonal pond

Not to be confused with the more noticeable surface whirligig beetles that swim in a circle, predaceous diving beetles will most often be under the water tension.

By Sam Artaiz

As the weather warms and summer begins, one thing that always comes to my mind is when should I clean my car? As a kid in Oregon, when school let out and summer began, my dad would ask me to clean the cars. If you grew up like me, drive-thru car washes were a luxury we didn’t use. I would haul the hose over, get the bucket of suds and begin to rinse and wash the car, covering all the little ins and outs of the vehicle.

I usually did two rounds of washing because the dead insects would not come off in the first round. As I would finish rinsing the car, I sometimes noticed, especially on nicer days, live beetles washing off my car. Where are these beetles coming from? Did I miss a spot? I would check the roof, and no, I did not. I found just a few beetles crawling around. I would continue to rinse the car until they were off.

However, I always found another beetle or two just as I had put the hose away and got into the car to move it back to the parking area. Has this ever happened to you? Apparently, this is more common than I realized.

Recently, I stumbled across these beetles while traveling along Mystery Creek road. The beetles are called “predaceous diving beetle” or “true water beetle.” Their scientific family name is Dytiscidae. If you venture into the refuge and follow pipelines, gravel roads or wet areas, you will most likely spot these amazing creatures in seasonal muddy-bottomed puddles.

Adults are small, lopsided, oval and usually jet black. Sometimes they can be mottled or brown on their modified hard forewings and have a small air bubble on their back end. Casting a shadow over them will get them to quickly, yet clumsily, swim away under a leaf using their modified back legs. If you were to hold them in your hands, these back legs would be elongated, flattened and fringed with stiff hairs that look like fine-toothed combs.

Not to be confused with the more noticeable surface whirligig beetles that swim in a circle, predaceous diving beetles will most often be under the water tension. They are uniquely adapted to use an air chamber under their wing casings that captures an air bubble, which they use to breathe with an external gill called a spiracle. The beetle looks a little like the bubblehead charm from Harry Potter.

The air bubble conducts gas exchange within the water gradients to intake oxygen and dissipate carbon dioxide. When oxygen levels in the stagnant puddles become too low for effective gas exchange, the beetle heads to the surface to replenish its air chamber.

As part of ushering in the new generation, eggs are laid on vegetation underwater. Once hatched, the larval stage, nicknamed “water tiger,” lives in the water and breathes using a spiracle or external gills, preventing the need to reach the water’s surface.

Suspended from the water surface-tension, water tigers resemble a centipede from one’s nightmares. With a long slender segmented body, they have three pairs of legs and large articulating mandibles. Their coloration is a slight tan with dark pattern markings. The larva will then leave the water puddle to pupate within mud or wet sand on the puddle’s shore.

Predaceous diving beetles fill a generalist predation role within the ecological food web of seasonal ponds and wetlands. They predate and consume all the living or dead macroinvertebrates that they can handle, from small prey like mosquito and fly larvae to even tadpoles and small fish. Adult predaceous diving beetles hunt by sight through the tannin-stained water and use their sharp mandibles to rip, chew and eat their prey.

On the other hand, larval water tigers pierce then inject digestive enzymes to break down their prey to be liquefied and extracted like a Capri Sun. Although very active in the water, adults are clumsy predators compared to their more nimble larval youth, who hunt by pursuing or sitting and waiting to ambush prey.

Ranging in size from a few millimeters to a whopping 4 cm, the genera Dytiscus, the predaceous diving beetles are a menu item for other predators such as fish, wood frogs, waterfowl, shorebirds and cranes.

Research on these beetles shows a close connection to mosquito control. Whenever I spot predaceous diving beetles in ponds, the site tends to be relatively open and seasonal. Where water bodies are small, predaceous diving beetles fill aquatic roles as top predators. Like a trout in a new pond, consuming mosquitoes, these native beetles and larvae can deliver similar effects in the birdbath or large gravel road puddles.

Their effects on mosquitoes vary from active predation, reducing the actual size of hatched mosquitoes, and overall water avoidance by laying female mosquitoes. Tannin-stained puddles along Mystery Creek road seem to be obvious mosquito breeding grounds. Still, they are devoid of the conspicuous wiggling mosquito larva because a few predaceous diving beetles patrol the shores.

As true beetles, predaceous diving beetles use winged flight to disperse to these isolated safe havens. When early summer hits, many spring puddles dry up as the sun comes out, driving the beetles to find new hunting grounds. They will climb nearby vegetation to dry out and get an elevated base to jump off.

Once in the air, they visually search to find water. Using the sun as their guide, they fly and scan for reflections that signify a new water body. When a water body is spotted, they begin their descent. Because their back legs are modified for swimming, they have no landing gear. Due to their size, a strong landing is needed to pierce the water tension to get underwater. As they pick their target, they begin to make their descent, picking up speed. Ping! Instead of a landing into new hunting grounds, they are sprayed off the roof of a 2008 Subaru Outback. A little dazed, they will climb onto the closest available standing vegetation, dry off, check their bearings and try again.

Samuel Artaiz is a first-time seasonal biological technician at the Kenai National Wildlife Refuge. Find more Refuge Notebook articles (1999–present) at https://www.fws.gov/refuge/Kenai/community/refuge_notebook.html.

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