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Stag beetle underground predators

Stag beetles spend at least two years in the larval stage and this hasn't gone unnoticed by some predators. For instance: foxes, badgers, rats and wild boars are known to raid their nests. But they have more elusive predators: the ones that also live underground.
Below are two examples.

First, cardinal beetle larvae. These larvae are predacious; they feed on the larvae of other insects and even though they have not been caught on the act, they are often found together with stag beetle larvae.

Cardinal beetle larva (Pyrochroa sp.), top right, and a mature stag beetle larva (Lucanus cervus).
Photo courtesy of Alison and Peter Jarrett; July 2010, Rainham, Kent.

Cardinal beetle larvae are rather flattened; this allows them to move under the bark and other shallow places. These larvae were found under a chipboard; note the shallow channel leading up to the stag beetle larva.

Second, carnivorous shelled slugs. Yes, there are slugs which do not eat plants! In my own garden I have found, over the years, a relatively rare or under-recorded shelled slug: Testacella haliotidea. It was always associated with places where there were, or could have been, stag beetle larvae.

Shelled slug, left, and young stag beetle larva.

Shelled slug, left, and first instar stag beetle larva (Lucanus cervus).
Photo by Maria Fremlin; 3 November 2011, Colchester, Essex.

These slugs hunt at night and are known to feed on earthworms. Several have been recorded in allotments in my area where I have also found one once.
In any case, there is no proof that they actually feed on stag beetle larvae, just that they share their habitat. However, stag beetle larvae have a high mortality rate. This is not surprising, the place where they live is teeming with life and they might have more enemies. Centipedes come to mind. No wonder that when stag beetle larvae are ready to pupate, make sure to do it inside a cocoon; the pupae, unlike the larvae, are unable to defend themselves.

Further reading:
Bowdrey J. (2008) Allotments are the stronghold of the shelled slug (Testacella haliotidea Draparnaund) (Mollusca:Testacellidae) in Colchester. Nature in North-East Essex, 2008: 51-52.
Taylor S. (2015) The carnivorous shelled slug family Testacellidae in Essex. Essex Naturalist (New Series) 32: 235-238. [PDF]

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Last modified: Tues Feb 16 2016

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