A Trend of Predaceous Diving Beetles

As a naturalist, I get the strangest emails. I try not to check them at home, but when my phone buzzed and the subject said “June bug on steroids?” it was worth interrupting my evening chores. “The past couple nights I’ve heard something hit our window at night when we have lights on and each time I’ve thought ‘that sounded like a June bug… but BIGGER.’” wrote a Museum member.

I opened the video, and sure enough, there was a black, oval-shaped beetle about an inch and a half long walking along the boards of a deck. The message continued, “I’ve never seen a beetle so big here in the Northwoods. Hopefully it’s not a sign that all of the bugs will be mega sized this year after such a weird winter.” Chuckling, I wrote her back, “Looks like a predaceous diving beetle! That’s their normal size!” The next day, I was scrolling through a Facebook group of regional nature observations, when a video of a diving beetle squirming in a bucket popped up with the caption “Biggest June bug ever!!!” Since June bugs are actually beetles, too, I applauded them for getting close.

Predaceous diving beetles regularly fly between bodies of water, and these 1.5 inch-long insects have been observed around homes recently. June bugs look similar, but are only an inch long and don’t have the large, paddle-like back legs of their aquatic cousins.

Have you heard the saying, “two is a coincidence, three’s a trend”? Arriving home from work, a black spot on the shrinking pile of snow near my front door caught my eye. Yep, there was a predaceous diving beetle! Despite being chilled, he was very wiggly once I flipped him over. There was a wide patch on his front leg, which is a character of the males only. I snapped some photos and then scooped him into a bucket, walked him down the hill, and released him near the lake. As luck would have it, a north wind had blown all the remaining ice up against my shore, so there wasn’t access to water. But I put the beetle on bare sand to let him warm up. He must have flown to my driveway; hopefully he can fly the last leg to the lake!

Since dispersing diving beetles have become a trend, it’s clearly my sign to write about them. Have you also encountered one recently?

Usually, you’d expect to see predaceous diving beetles swimming near the shallow edges of ponds and streams where there are plenty of aquatic plants to lay their eggs on, and few insectivorous fish. Their larvae are aquatic, but crawl out of the water to pupate in mud along the shore. Metamorphosis takes about a week, and they crawl right back into the water as adults.

Their continued surf-and-turf flexibility comes in handy if their little pond starts to dry up. Like most beetles, PDBs hide a pair of shimmering hindwings under an armored set of forewings called elytra. When all of their wings are closed, there’s a seam straight down the middle of their back. When open, the elytra look like the doors of the DeLorean in Back to the Future. The two cellophane-like hindwings pop out from underneath to flap away, with their large body and their hairy, paddle-like legs – adapted for swimming – dangling awkwardly.

The elytra don’t just hide more wings, they also trap a bubble of air next to the beetle’s breathing pores, which happen to be located on their rear end. The bubble is precisely sized to sustain their dive while not floating them back to the surface. Once the all of the oxygen has been gleaned from the air pocket (which reportedly takes somewhere between 10 minutes and 36 hours), the beetle swims up to the surface for a resupply. Sometimes they’ll climb fully out of the water and slather a layer of anti-microbial goo around their spiracles to keep their respiratory system healthy. Beetle larvae also have to return to the surface periodically to sip more air, which they store in their tracheal trunk. This handy storage vessel is similar to our windpipe.

Both the larvae and adults are fierce predators. PDB larvae have earned the nickname “water tigers” by ambush hunting with jaws open wide. Once they pounce, digestive enzymes flow through channels in their sharp pincers into the prey, turning the captive’s insides to goo. The larva then sucks out the prey smoothie. Although rarely seen because they tend to hide in the mud, the larvae occasionally bite humans. That their bites are described as “painful, but not medically important” is only mildly comforting. More comforting is the fact that they eat a lot of mosquito larvae.

Adult beetles are often credited with the same digestive enzyme injecting powers, but the most reliable sources I found described them as merely chewers and shredders. They often eat dead stuff, or they might sneak up on or even chase down insects, leeches, snails, tadpoles, small fish, AND mosquito larvae!

So what are these aquatic beetles doing on our decks, cars, and snow piles? Some species, especially those who tend to live in shallow ponds, change locations frequently, and often en masse! Has this dry spring forced them to leave shrinking pools? PDBs are also known to disperse to avoid parasites in their former home, predation, crowding, and competition from their buddies, and even a lack of plants.

Moonlight reflecting off water is a beacon for dispersing beetles, but this means that yard lights, cozy windows, wet roads, puddles, and shiny cars can lure them astray. These fast swimmers are slow and vulnerable on land, which benefits raccoons, skunks, snakes, and other little predators.

As they are flying around, predaceous diving beetles become somewhat waterproof, and may have trouble breaking through the surface tension of their hopeful new home. One way they resolve this is to exude “wetting agents” that breaks the surface tension, similar to dish soap. The more fun option is to dive in at high speed.

If one happens to dive toward your window, the shiny roof of your car, or some other reflective surface, at least now you know that you aren’t seeing June bugs in March.

Emily’s award-winning second book, Natural Connections: Dreaming of an Elfin Skimmer, is available to purchase at www.cablemuseum.org/books and at your local independent bookstore, too.

For more than 50 years, the Cable Natural History Museum has served to connect you to the Northwoods. The Museum is closed until May 1 to construct our new exhibit: “Anaamaagon: Under the Snow.” Our Winter/Spring Calendar of Events is open for registration! Follow us on Facebook, Instagram, YouTube, and cablemuseum.org to see what we are up to.