Scat Scavengers: a water beetle takes to land, only to prosper in piles of poo.

By Edwin Alvarez & Alec Roseland

Dung beetle or “dunk” beetle? Though it doesn’t belong to the dung beetle family, Sphaeridium scarabaeoides plays with poop just as much as its famed cousins. But there’s a catch: this beetle belongs to a family called the water scavenger beetles, which, as you’d expect, primarily inhabits aquatic environments. With such unusual behavior compared to its brethren, one would expect this beetle to be a star in the world of entomology. Yet, much of its life history remains shrouded in mystery. Even though they are common, they live in such obscurity. They don’t even get their own common name! To address this oversight, we will give them, by the authority vested in… ourselves… the honorary title of “dunk” beetles.

While the native range of dunk beetles extends across most of Europe, these caca-craving critters have proliferated throughout North America including right here in the Lehigh Valley (Bethlehem) after arriving accidentally around 1883. Female “dunk” beetles oviposit their eggs into cow pats, upon which the larvae will eventually feast when they reach maturity. This dung-eating behavior is known as coprophagy (one of those words you thought was only suitable for the SAT), but poop is not the only menu item for these water scavengers. Only after they mature into adults do the beetles begin to adopt their dung diet.

Dung beetles play a vital role in nutrient cycling by transporting dung and dispersing it into the soil. The strategies that adult dung beetles use to fit this ecological niche vary immensely between species, but many fall under three distinct categories within the scientific literature. First are dweller dung beetles that lay their eggs inside the dung, and when they hatch, the larva will feed on the dung. The dwellers will not transport the dung, so they are in a race against time with other organisms that might find that pile of dung to be a tasty treat. Next are tunneler dung beetles that dig tunnels underground and then transport balls of dung into those tunnels. This is important from a human perspective because if the dung were just left out on the ground, many parasites and bacteria could grow. The third common type of dung beetle is roller beetles, which roll their dung into a ball to transport it to another place, bury it, and then lay eggs in the dung under the soil. The rollers take some aspects of both dwellers and tunnelers in that they will lay their eggs in dung like the dwellers and also roll and bury the dung like tunnelers.

Dunk beetles are remarkable because they don’t neatly fit into any of these categories. Who can blame them for not wanting to be a typecast! They’ve got their own thing going, and that’s great! They are considered water scavengers that also take advantage of dung as a food source. They thrive in cow pastures since there is an excess of dung to go around for larvae to feed on and flies that also make their homes on the dung.
One essential nutrient that is often limiting for plants is nitrogen. Dung is rich in nitrogen, so when the dung beetles bury the dung, nitrogen can be released into the ground. From there, bacteria turn the organic nitrogen found in the dung into a form that is useful for plants. Surprisingly, dung by itself is not an excellent fertilizer as it has organic nitrogen, and most plants cannot use nitrogen in that form. Burying the dung also prevents other parasitic insects from laying their eggs on the dung). Predation, in turn, suppresses the fly populations, meaning our dunk beetle plays a valuable role in integrated pest management by minimizing unnecessary insecticide application. It’s also super common for a seed to find itself inside the dung. When the dung beetle buries dung, they’ll also spread these seeds, which help our plants thrive and reproduce!

Horn flies are a menacing pest to cattle – making the dunk beetles a highly beneficial insect. Horn flies grow and reproduce in “cowpats.” After the larvae mature into adult horn flies, they feast on the blood of cattle. Horn flies don’t immediately kill cows, but they cannot grow as much due to constant blood-sucking, and they sometimes injure themselves trying to swat away the horn flies. I mean, imagine if these horn fly populations were allowed to thrive, the cows would surely get annoyed. However, our dunk beetle larvae are predators of horn fly larvae and eat any other occupants in their dung. That means if no other insects inhabit the dung, the water scavenger beetles will start to eat each other! Larvae can get fatter with additional larvae nutrition, but cannibalism is the next best thing if that isn’t available! That way, the larvae reduce competition and get nutrition from their brothers and sisters.

Hopefully, you see why “dunk” beetles are so bewildering! Of all of its intriguing quirks, a question emerges: how did a primarily aquatic insect decide it wanted to thrive by playing in cowpats? “Real” dung beetles form a group of species that can be traced back to the same common ancestor. Since our little friend falls outside of that group, it must have learned its behaviors after splitting away from them evolutionarily many years ago. It is likely that the beetles’ progenitors became isolated from their closest relatives and had to adapt to a landlubbing lifestyle. With any luck, further scrutiny of the fossil record will reveal the origin of these cowpat connoisseurs.
In the meantime, consider expressing your gratitude to the water scavenger beetles if you happen to spot a few in your nearest pasture. These members of the water scavenger family made land – to end up, quite literally, cutting the bullcrap! Their thankless job helps ensure that there’s a patty on your burger and cream in your coffee.