Saturday, February 4, 2017

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.

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