Dung Beetles - important for environment and agriculture

Scarab beetles are one of our largest and most divers beetle families. Most Arizonans are quite familiar with the day active Green Fig Beetle and smaller brown beetles of several  genera that tend to accumulate around porch lights and are often just called June Bugs.

Hercules Beetle larva

 Gardeners among us usually hate the white, c-shaped, fat grubs of scarabs that live in the ground and supposedly feed on the roots of your favorite plants. Some of them might in fact do that.

But most scarabs are decomposers, and therefore very important for gardening and agriculture. Their grubs feed on dead plant material that they digest with the help of bacterial symbionts in their widely extended guts. Hence the impression that they are fat. The strong mandibles of those often very large grubs are able to break down decaying wood and leaf matter, utilize amazing amounts of material,  and thus open it up to smaller decomposers, and finally fungi and bacteria. So eventually the nutrients will again be part of the garden soil and available for uptake by plants..

  Among scarabs, dung beetles have evolved to break down the feces of larger animals. The importance of this 'service' can hardly be overestimated.  Obviously, they are removing waste that would otherwise pose a serious health risk. For example, dung beetles help to remove harmful pathogens like E. coli from soil. But if you consider the amount of dung that  big herds of  grazing animals produce, you'll understand that the mere accumulation of this dung would eventually cover so much surface that the grasses that the herds a feeding on would be displaced by a rather sterile crust of dung.

Dung Beetles of the genus Phanaeus

Beetles in the genus Phanaeus are 'tunnelers'. They excavate tunnels beneath the very fresh dung pile and lower a portion of dung down into the ground below. There, the female will lay an egg in the brood ball and seal the chamber. The larval beetle will feed on the dung as it grows until metamorphosing into an adult and emerging.

Dung Beetles in the genus Canthon are America's typical dung rollers. With their shovel heads, they cut a spherical dung portion from the fresh pile. Most of this work is done by the male. The resource is limited, though fights between rivals happen often. A female joins a successful male, often sits on top of his prize as he rolls it in a rather straight line away from the competition. They bury the dung ball and in the underground chamber she lays an egg on it.

Although there is no longer a source in the US to buy dung beetles of any type, historically, the U.S. government sponsored dung beetle introduction programs. When the local dung beetle population did not seem to be able to handle the waste of Texas' huge Cattle herds, Digitonthophagus gazella (Gazelle Scarab) was brought in in the 1970. Of Indoafrican origin,  it is now perhaps the most widespread dung beetle in tropical and subtropical pastures. (Noriega et al. 2010).  Euoniticellus intermedius was brought to Texas from Africa. Thus dung beetles from traditional feeding grounds of big herds were introduced.  I do not know why dung beetles were not brought in from old buffalo grounds like the midwestern prairies, but instead from Eurasia and Africa. Maybe the introducers thought them more suitable for the Texas climate. The beetles proved invasive. They quickly spread throughout most of the southern U.S. 

Digitonthophagus gazella (Gazelle Scarab) and Euoniticellus intermedius, both introduced

 The introduced species are doing their job. They propagated so successfully that they are found all over Arizona by now. In fact, by now most larger dung beetles we find are of those two species. It is difficult to tell if this is harming the populations of endemic species that they compete with, but it is hard to imagine that they wouldn't. My impression is that the  two introduced species are generalists that can deal with nearly all types of soil, dung types, and exposure. They also find dung sources fast and at great distance and fly well enough to quickly move into new areas.  Where we live, for example, grazing is so poor that cattle may only be brought in every few years. Phanaeus and large Canthon species never show up, only all kinds of small Aphodines plus the two imported medium sized species depicted above.  Our endemic large Canthon imitator, the smaller Canthon indigaceus and the three Phanaeus species seem be more discriminating in their choice of habitat and not so fast at pioneering new spaces.

The Hanging Thiefs (or Murderers, rather) of Patagonia Lake

In the unlikely case that  you've ever tried to photograph the arrival of scores of dung beetles (Canthon imitator in this case) that rain down simultaneously on any fresh pile of cow dung, you will have noticed that the beetles literally dive out of sight right after they have landed. The one in the photo was only slow enough for a shot because the surface of  this cow pie was already getting dry.

 Competition, greed, and the urge to prepare food for their offspring are not the sole motivators for this rush. There is danger lurking at the dung buffet. At a pile of horse manure at Patagonia Lake we watched how the beetles had to pass a gauntlet of 12 huge robber flies to get to the feast. The only reason most beetles made it past the flies was that they all arrived pretty much simultaneously and the lurking robbers were distracted by the need to avoid ending up prey themselves and by courting the ladies among them.  A. Scarborough, our local Asilid specialist, tells me that Diogmites are known to congregate at places with high prey traffic density like bee hive entrances, and also for their high (10% of courting males) rate of cannibalism.

The robber flies seemed to ignore weak but quick prey like bottle flies and only lay in wait for the fat beetles. They were of the genus Diogmites, or Hanging Thieves. We soon got a demonstration of the reason for this name. A fly grabbed one of the landing beetles and carried it off to a mesquite tree.

At first the beetle seemed to have a fighting chance to get away. The effect of the paralyzing bite seemed slow, and as soon as the big beetle felt the tree branch  under his tarsi he started to scramble away, dragging the fly along. He probably would have stripped her off his back by crawling into some tight spot.

But  the fly was able to detach the beetle from his hold by using the enormous reach of her long legs. To keep him  from reaching the branch again,

the fly held the beetle with four legs and used just one pair, and later just one single leg to suspend the weight of the prey and herself. This trick earned the genus the popular name Hanging Thief. The beetle had now lost any chance of fighting back.

Other robber flies tend to crouch tightly above their prey holding it down with their superior body mass and strength. It seems that these slender flies need the suspension tactic to control their strong bulky prey while their paralyzing, tissue-dissolving poison is slowly taking effect.