Conservation management of the green carpenter bee Xylocopa aerata (Hymenoptera: Apidae) through provision of artificial nesting substrate

The green carpenter bee, Xylocopa aerata (Smith, 1851), has undergone a severe reduction in its distribution over the previous century, in part because of habitat loss and fragmentation and its dependency on old-growth heathland habitats. In 2007, the last remaining South Australian population, on Kangaroo Island, was negatively affected by a shortage of nesting substrate as a consequence of a series of large, intense wildfires. In an attempt to prevent yet another extirpation of this species, we developed artificial nesting substrates (stalk nests and trunk nests) and placed them at various locations within the current range of the population. Monitoring showed that the bees readily used the alternative nesting substrate in sites where very little natural substrate was left. Reproductive activity was evaluated using X-ray images of winter nests. Based on tunnel lengths and number of individuals, we conclude that conservation action has resulted in successful reproduction and that the number of nests used has increased each year. This is the first time that provision of nesting substrate is shown to substantially improve the breeding capacity of a threatened population of endemic bees in their natural habitat. However, reproduction in artificial substrate was significantly higher in 2016 than in 2017 and 2018. The most obvious reason for the decline in breeding was climatic - low spring and summer rain in the later years limited floral resources and hence reproduction. Global climate change is causing drier conditions and more frequent and intense fires, which are reducing the likelihood of survival of this species. This effect is demonstrated by the latest fire season which left only 5% of the bees' habitat intact on Kangaroo Island. Each large-scale fire locally annihilates bees and destroys old growth nesting substrate which takes at least 35 years to recover, thus increasing the overall risk of extinction.

Automated summary

The green carpenter bee population on Kangaroo Island in South Australia has been threatened by wildfires and climate change, but conservation efforts with artificial nesting substrates have successfully improved their reproductive capacity. However, declining reproduction rates in recent years due to climate change have increased the risk of extinction for the species.