Reintroduction of the Longhorn Beetle

To breed and reintroduce longhorn beetles in Sweden


The lack of giant oaks is one of the reasons that the longhorn beetle (Plagionotus detritus) has become one of the rarest beetle species in Sweden. Previously the species occurred in several provinces in southern and central Sweden, but in the last decade it has only been found in isolated populations in the Stockholm area. The species is strongly linked to areas with sparse stands of old oak trees and the loss of this habitat has led to the species being listed as Endangered on the Swedish Red List. As part of the Swedish Action Plan for Plagionotus detritus, Nordens Ark was asked by the County Administrative Board of Stockholm to develop a rearing method for the species. The aim of the project was first to develop an effective breeding method in order to create a self-sustaining captive population and when that had been accomplished to annually contribute with animals for reintroduction into suitable habitat within the species' former range.


When the project started in 2006, very little was known about keeping and breeding the species. Plagionotus detritus is a longicorn (Cerambycidae), belonging to a group of beetles commonly known as "wasp beetles". The fully formed beetle is between10 and 19 mm long and has a distinct yellow and black-striped coloration. It also moves and behaves in a way very similar to wasps. The first breeding population was established by collecting oak wood colonised by the beetle's larvae and bringing it to the breeding facility at Nordens Ark. The oak wood was collected from Djurgården in Stockholm, which is the last stronghold for the species; the collections were made without damaging the existing population.


To develop an effective rearing method for the species, the project has tried to imitate the species' natural conditions as much as possible. The enclosures, which are placed outdoors, are covered with a fine-meshed fabric to keep the beetles in, but also to keep other insects out. Since the species loves sun-drenched trees, it is important that the enclosures are standing in a sunlit place. The species develops in recently dead oak wood and emerges as a beetle in late June to early July. As soon as a beetle emerges from the oak in one enclosure, it will be moved to a new one. The new enclosures are prepared with oak wood of the right quality, on which the beetles can lay their eggs. The newly hatched beetles are very active and quickly begin to mate. The female will lay her eggs in the thick bark of the oak and the larva eats its way into the inner bark. Just as quickly as they came, they will be gone again; Plagionotus detritus only lives as the fully formed beetle for a couple of weeks. The larva has a development period of one or two years before it pupates. The exact period is depending on weather conditions and availability of food under the bark. Prior to pupation, the larva will bore into the thick bark of the outermost sapwood and create a pupal chamber. After two weeks as a pupa, it hatches as a fully formed beetle. The beetle starts gnawing its way out of the tree and then the whole cycle will start again.


In 2014, the ex situ population of Plagionotus detritus reached new heights, with a record number of 130 beetles being hatched at the breeding facility and these beetles colonised over 30 oak logs. After years of modifying the rearing technique, the time has come to reintroduce the species. Reintroduction will initially take place at the historical localities in Uppland (Båtfors area) and Småland (Strömsrum). In these areas, the habitat has been restored in accordance with the Action Plan by the County Administrative Board of Uppsala and Kalmar. The reintroduction is done by moving oak wood colonised by the beetle's larvae from the breeding facility at Nordens Ark to the reintroduction sites. The logs are placed near giant oak trees, so that when the beetles hatch they will be close to an oak stem suitable for egg laying. The long-term goal of the project is that there should be viable populations of Plagionotus detritus in at least five Swedish landscapes and that the species is taken off the Swedish Red List.


WAZA Conservation Project 15002 is implemented by Nordens Ark, in collaboration with the County Administrative Boards of Stockholm, Uppsala and Kalmar.




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  • Longhorn Beetle Conservation

    Longhorn Beetle Conservation

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