WILD IDEAS
The hard-working dung beetle
In a recent column I mentioned spotting a dung beetle while I was hiking a trail in Shenandoah National Park. It was valiantly trying to extract some dung from a pile a passing mammal had left. I’ve always been fascinated with beetles since I saw my first Hirschkaefer (German for “stag beetle”) on the military base in Germany where my family was stationed a year after I was born. We kids played with the three-inch, hissing black monsters with huge hornlike pinchers—one of my earliest and fondest memories, although I’m sure the beetles didn’t enjoy it.
The German stag beetles ( Lucanus cervus) feed on decaying wood and are in a different beetle family than dung beetles, but they still look more like the bug on the trail than one of Virginia’s smaller, mostly iridescent-green species, so I wondered if it was a nonnative. Dung beetles live on all continents except Antarctica and in a variety of climates. The United States, along with Australia, has been importing exotic species of dung beetles for years to solve an agricultural problem – infestations of livestock by parasitical fly larvae. The flies lay their eggs in livestock manure. If the livestock are grouped together for extended periods, manure (and parasites) accumulate and contaminate forage plants, which are then ingested by the livestock.
Dung beetles help solve this problem by burying and consuming the dung, including any fly larvae on it, or using it as a nutrient ball for their eggs, which they lay in it. Some small species of dung beetle have even been known to attach themselves to livestock to await the food delivery. Some species also eat mushrooms and decayed plant leaves and fruit. Their acute sense of smell leads them to their food.
Since our native dung beetles did not co-evolve with livestock species, which are not native, and therefore are less enthusiastic about using livestock waste; species that are better adapted to livestock dung have been imported into the United States in great numbers. While such biological solutions to human-created problems have been known to go terribly awry, leading to infestations of exotic species that push out our native species and cause crop damage, among other problems, so far the importation of dung beetles seems only beneficial.
On my way back up the park trail 20 minutes later, the beetle I’d spied earlier had made progress in its efforts, working over, under, and around a chunk of dung until it was well on its way to becoming a ball. Unfortunately, I didn’t have time to wait until the bug completed its task so I could determine if it was a “tunneler” – a species that buries its food in a tunnel under the dung heap – or a “roller,” which rolls a dung ball to a suitably soft spot of earth, then creates a tunnel. With the former, the males often do most of the rolling, with the female hitching a ride or helping. Dung beetles are also known to steal dung balls, a behavior that early researchers mistook for communal helping, so tunnelers and rollers work quickly to hide their score.
Mating and egg-laying occurs in the tunnel. In some species, the female blocks off nesting chambers to guard against predators or to protect their young. Another group of dung beetles neither tunnel nor roll dung balls. Nor do they protect their young. Instead, the female lays her eggs in the dung pile and leaves. Males of all species also appear not to be involved with raising or protecting the young.
From checking photos and descriptions at BugGuide.net, I’m guessing the beetle in the park was most likely Onthophagus taurus, a tunneler species in the scarab ( Scarabaeidae) subfamily of dung beetles. If it was, it had to be a female, since it lacked the beautiful curved pinchers of the male, which resemble the horns of a water buffalo. ( taurus is Latin for “bull.”) The males use the horns in combat with other males over mating rights.
Because of their ability to bury large amounts of manure, Onthophagus gazelle and Onthophagus taurus are the dung beetle most often imported into the United States. O. gazella was released in Texas by U.S. Department of Agriculture scientists in the 1970s and is now common throughout the state and in other areas. According to the Texas A&M Extension website at insects.tamu.edu/fieldguide O. gazelle removes 80 percent of the cattle droppings in the state. O. taurus, accidentally introduced into Florida in the early 1970s, was later intentionally released in several other areas of the country and has spread throughout the eastern United States.
Dung beetles offer ecological services other than controlling parasitical flies in livestock, including distributing nutrients and making them more accessible to plants, aerating the soil through burrowing, increasing infiltration of water into the soil and reducing pollution from manure-contaminated runoff. The American Institute of Biological Sciences reports that dung beetles save the U.S. cattle industry alone an estimated $380 million annually.