Earth-borer Beetles

Article | Updated 4 years ago

Earth-borer beetles are members of the scarabgroup and have become of special interest in recent years because of their association with underground fungi, particularly those that form symbiotic associations with plant roots. Such fungi (termed ‘mycorrhizal’) assist plants to obtain essential minerals from the soil and many (if not most) of our native plants are dependent on the fungi for their health. The beetles feed on the underground fruiting bodies of the fungi and help spread their spores. Studies at the WA Museum in collaboration with the WA Herbarium have begun documenting which kinds of fungi are eaten by each species of earth-borer. WA Museum studies have focused on the life-histories of the beetles which are extremely poorly documented.

A beetle crawling across the sandy ground

A common south-western Australian earth-borer beetle, Blackbolbus frontalis, male
Image copyright WA Museum 

How to recognize earth-borers

Similar to other scarab-group beetles in general form (short, domed body, spiny legs and short antennae with terminal clubs) but distinguished by the following combination of characters:

1. Antennae terminating in relatively large, subspherical or egg-shaped clubs, each  formed of three segments (arrowed below).

The bulbous antennae of the Earth-borer beetles

Earth-borer beetle Antennae
Image copyright WA Museum 

2. Pronotum strongly developed and, viewed from the side, extending lower than lateral margin of wing case or ‘elytron’ (see image below).

Illustration of where Pronotum, Antennae and elytron are located on the beetle

Earth-borer beetle
Image copyright WA Museum 

3. Usually brown (ranging from light tan to dark tan), rarely black (most dung beetles are black).

4. Spines or projections often present on head and/or prothorax of males (absent in some species; similar spines and projections can occur in dung-beetles and other scarab-like groups).

5. Many (but not all) species squeak when handled.

6. Size small to medium (body length 5-30 mm).

The earth-borer subfamily (Bolboceratinae) occurs almost world-wide but is best represented in Australia where 166 species grouped into ten genera are recognized. Western Australia has 100 of the species and eight of the genera. The beetles occur virtually over the whole State inhabiting a wide variety of habitats but are most common in areas with sandy or sandy loam soils and are less common or absent from areas with predominantly hard or stony soils.

Habits of earth-borers

Earth-borers, though common inhabitants of our bushlands, are seldom seen. With few exceptions, they emerge to fly only after dusk and then only after heavy rain when the soil is damp. They are strong and noisy fliers and tend to be attracted to lights. This is when they are most likely to be seen.

Most of the time, they remain hidden in burrows in the ground.

Usually, each burrow is marked on the surface of the ground by a characteristic pile of excavated soil. Termed a ‘push-up’, the pile is formed as the beetle pushes loads of loosened damp sand up from below. Periodically, a column of damp sand is forced out of the burrow entrance like tooth-paste from a tube. One by one, the columns dry, topple over and accumulate to form a pile of cylindrical lumps looking much like a cluster of mammal droppings (see image below).

Sand castings from a beetle burrow

A ‘push-up’ or tumulus of an earth-borer beetle above the entrance to its burrow.
Image copyright WA Museum 

The shafts excavated are quite round in crosssection, just slightly wider than the beetles excavating them, and usually descend vertically from a few centimetres up to 2 metres or more. Typically, the beetles plug the shafts above them with compacted soil and are usually found at the lower ends.

The adult beetles are known to feed on the fruiting bodies of soil-dwelling fungi and these include what can loosely be called ‘truffles’. These fruiting bodies form within the soil and remain closed (unlike mushrooms, puffballs and earth-stars which dehisce to release their spores into the air). Truffles rely on animals eating them to disperse their spores. Other kinds of fungi are consumed as well (for more information see Houston & Bougher 2010).

Immature stages are known for very few earthborers. Females of at least some Australian species produce just one relatively enormous egg at a time in underground chambers. Where larvae are known, they are completely white, soft-bodied and have poorly developed legs. It appears that they feed only minimally on humus or not at all (Houston 2011).

Given the low rate of egg-production, it is clear that the beetles must be long-lived in order to lay a sufficient number of eggs to sustain the population.

A series of beetle portraits showing facial features of three speices

Males of three Bolborhachium species (B. recticorne, B. tricavicolle and B. coronatum, respectively). Their characteristic horns and other projections are fully developed only in the largest individuals and may be reduced or absent in smaller males. Not all kinds of earth-borers exhibit such adornments.
Image copyright WA Museum 

References:

Houston, T.F. & Bougher, N.L. (2010). Records of hypogeous mycorrhizal fungi in the diet of some Western Australian bolboceratine beetles (Coleoptera: Geotrupidae, Bolboceratinae). Australian Journal of Entomology 49(1): 49-55.

Houston, T.F. (2011). Egg gigantism in some Australian earth-borer beetles (Coleoptera: Geotrupidae: Bolboceratinae) and its apparent association with reduction or elimination of larval feeding. Australian Journal of Entomology 50:164– 173.

Howden, H., Howden, A. & Holloway, G. (2007). Digging down under: Australian Bolboceratini, their habits and a list of species (Coleoptera: Scarabaeoidea: Geotrupidae). Zootaxa 1499: 47- 59.