Radio telemetry reveals extensive dispersal capabilities of reintroduced Great Capricorn beetles (Cerambyx cerdo) in oak habitats at their Northern range limit

Amid accelerating global biodiversity loss, reintroducing and translocating endangered species have become pivotal conservation strategies. This study used radio telemetry to investigate the dispersal and movement patterns of the reintroduced Great Capricorn Beetle (Cerambyx cerdo) within Tromtö Nature Reserve, Sweden.We tracked 50 beetles (29 females, 21 males) from 17 June 2022 to 17 July 2022, quantified their movements within the landscape, and recorded the distances moved across oak-rich areas at the northern edge of the species’ range.

Female beetles were significantly larger and heavier than males, with longer antennae. The beetles were relocated an average of 9.5 times. Total movement distances ranged from 2.8 m to 822.2 m, with no significant association of movement distance or movement speed with sex, body size or mating status.

We conclude that C. cerdo can reach oaks several hundred meters away and that dispersal distances were not associated with sex, body size or mating status. We recommend future studies employing more advanced telemetry techniques to refine our understanding of the species movement ecology and ensure successful reintroduction and survival.

Methods
Rearing and conservation programme at Nordens Ark

To address the urgent conservation need of the critically endangered C. cerdo in Sweden {Eide, 2020 #3407}, Nordens Ark initiated a pioneering breeding program in 2012, in collaboration with the County Administrative Boards of Kalmar and Blekinge, as part of Sweden's national species conservation action plan. The breeding stock was sourced from Halltorps Hage (coordinates 56.81N, 16.59E), the last natural stronghold of C. cerdo in Sweden, to minimise the impact on the wild population. Larvae were reared in controlled conditions within Petri dishes containing a custom-formulated substrate made from oak (Quercus spp.) shavings, providing optimal nutritional and environmental conditions. The larval stage lasted approximately two years, resembling the species' natural development cycle. During this time, larvae were monitored regularly for growth and development. Upon reaching pupation, individuals were closely observed during their transition to adulthood, typically in late summer. After emergence, adult beetles are overwintered in specialised refrigeration units set to temperatures that mimic natural winter conditions to ensure reproductive maturity. This overwintering process is critical for synchronising the reproductive cycle with natural environmental cues. Since 2017, Nordens Ark has released between 150 and 300 beetles annually into carefully selected protected areas in Sweden, including Björnö (coordinates 56.77N, 16.36E) and Tromtö (coordinates 56.16N, 15.49E), which provide habitats conducive to C. cerdo survival and reproduction. The selection of release sites was based on detailed habitat assessments, focusing on the availability of veteran oak trees and suitable microhabitats. Post-release monitoring is ongoing to assess the emergence of reintroduced C. cerdo from pupae and investigation of habitat utilisation. In captivity one-two days before release, 19 of the female beetles were mated, while 10 remained unmated. 

Study area

This study was conducted in Tromtö Nature Reserve, in Karlskrona municipality, Sweden (Fig. 1a,b). The reserve is characterised by a rich diversity of habitats, most notably featuring ancient and hollow oak (Quercus spp.) (Figure 1c), which are critical to the lifecycle of C. cerdo (Fig. 1c,d). The area is managed through a traditional grazing regime involving sheep and cattle, which helps maintain the open landscape and the health of veteran trees. A telemetry field study was conducted over five weeks, from June 16 to July 17, 2022. 

Radio telemetry methods

The C. cerdo beetles used in this study originated from the Nordens Ark breeding program. We equipped 50 individuals (29 females, 21 males) with NanoPin transmitters (model NTP-1, Lotek Wireless Inc., Newmarket, Ontario, Canada). Each transmitter weighed 0.13 g with 11 mm length and 5 mm diameter dimensions, featuring a 12 cm flexible antenna (Fig. 1d). Transmitters were activated using a NanoTag activator (model NANOTAG IR ACT) and had an operational battery life of up to 30 days. Prior to release, we measured each beetle's body weight to the nearest 0.01 g using a Kern EMB series precision balance, and recorded body length, antenna length (to nearest mm), and pronotum width (to nearest 0.1 mm) using digital calipers. Beetles were equipped with radio transmitters on their pronotum using cyanoacrylate glue (Super Loctite Super Glue Original), with the transmittor antenna extending along the abdomen to minimise interference with movement. Beetles were released in five batches between 17 and 27 June 2022 (18 on June 17, 16 on June 20, 13 on June 22, 4 on June 26, and 2 on June 27) onto 22 suitable ancient oak trees (Quercus spp.) within Tromtö Nature Reserve. Release trees were selected based on trunk size, sun exposure, and overall tree health.

We conducted daily tracking using a Biotracker receiver (model BIOTRACKER8 146-154 MHz) with a whip antenna (model SLA/FT-2). One person systematically searched Tromtö Nature Reserve and adjacent areas up to 3 km away. Each beetle was when searched for once per day during these searches. Beetle locations and activity were documented using Field Maps (Esri), which automatically recorded time, user, and coordinates for each observation.