The larvae develop in very old, living, oaks with coarse bark that stand in sunny locations. The species seems to be able to live in these oaks for centuries. The great capricorn beetle can also colonise middle-aged, severely stressed oaks with poor vitality. In most cases, the oaks die after a few years, but there are also cases where the stressed oaks have been revitalised and the beetle larvae have died in the stressed oaks probably due to unfavourable conditions caused by high levels of tannins.
The development of the larvae, which takes place underneath the bark and in the sapwood, is believed to take five years in Sweden. In the late summer of the year before the year when the adult beetle (imagines) is to leave the oaks, pupation takes place in a pupal chamber deep inside the trunk of the oak. But before this occurs, the larva gnaws a tunnel all the way to the bark. Then in June of the following year, when the adult beetles leave the trees, the beetle gnaws through the bark, resulting in traces of red-brown frass at the base of the oak. Mating and egg-laying occur immediately.
Adult great capricorn beetles only rarely been observed drinking sap that flows down the outside of the tree. Most adult great capricorn beetles die quite soon after mating, as they are eaten by birds, but adult individuals have been seen as late as September.
The species has declined in Sweden
The last location in all of Scandinavia where the great capricorn beetle remains is Halltorps nature reserve on the Swedish island of Öland. On the mainland the great capricorn beetle became extinct some 50 years ago. The last occurrences on the mainland were at Alsterån, where elytra (hardened forewings) from the great capricorn beetle were found in a jackdaw nest, and exit holes made by the beetles were found in a fallen top section of a very large and old oak. A flying great capricorn beetle was also observed at Ekhult by Emån in the 1960s. At several locations in Kalmar and Blekinge provinces there are gnawed holes which very probably, were made by great capricorn beetles. There is some uncertainty because the holes resemble those made by the goat moth Cossus cossus.
We do not know for certain why the great capricorn beetle became extinct on the mainland. But the most likely reasons are the removal of hundreds of thousands, perhaps even more than a million, old oaks from 1830 and onwards (see the chapter on the oak in history), the regulation of water- courses (which stopped flooding of the riverine woodlands) and a general overgrowing of grazed oakwoodlands. Before the watercourses were regulated, at for instance Emån and Alsterån in eastern Småland, there were probably plenty of old oaks that survived the annual floods, which other tree species like spruce did not.
These old oaks were sun-exposed, and grew close to the streams and in alluvial meadows. They were good habitats for great capricorn beetles. At Emån and Alsterån it is still possible to find old oaks that remind us of this lost type of habitat. Similar areas still exist in some parts of Europe, for instance near Rogalin in central Poland, by the Warta River. Here there is still a large population of great capricorn beetles in the enormous oaks that grow in meadows subject to heavy annual floods, meadows which are still cut for hay.
Great capricorn beetles – a keystone species
The larvae tunnels in the hard wood create habitats for other species. For instance, a study in Poland shows that bats, Nathusius’ pipistrelle Pipistrellus nathusii, appear to spend the winter in tunnels made by great capricorn beetles. According to another study from central Europe, oaks where great capricorn beetles live are home to more species of saproxylic beetles, including several red-listed species.
Great capricorn beetles are bred at the breeding facility at Nordens Ark. Nordens Ark is a non-profit foundation that works in conservation, breeding, research and education, and sharing knowledge about biodiversity. Here, the beetles reproduce in terrariums containing pieces of oak with coarse bark, where the female can lay her eggs. Mating and egg-laying occur every day, and by feeding the beetles with a nectar solution and fresh fruit, their lives as adult beetles can be extended by several months.
The pieces of oak where the female lays her eggs are searched regularly. All the eggs that are found are collected, and after a few days the eggs hatch and each larva is placed in a separate Petri dish. The dishes are filled with special food mixture consisting of oak woodchips and a nutrient-rich powder. The larvae are checked several times a week, and the food mixture is changed regularly to ensure maximum quality. After every growing season, the larvae hibernate for two months at a temperature of four degrees. After only about two years, the larvae are large enough to develop into pupae. In the wild, great capricorn beetles have a larval stage of four to five years, but with this breeding method, this period can be shortened. After a month as pupa, the metamorphosis is completed, but before it is fully developed, and in order to be fertile, it must remain in its pupal chamber in the Petri dish over the winter.
After the great capricorn beetles are taken out of hibernation, they are fed and have the opportunity to mate. This optimises the chances that they will start laying eggs on the oaks as soon as they are released. The adult great capricorn beetles that are bred are generally about 10-15% smaller than those that have developed in the wild. About 15% of the captive-bred adult beetles have deformities, mainly on their wings and antennae. The cause of this is not known, but handling the pupae is a very sensitive procedure and it is possible that some individuals are damaged.