The green carpenter bee (Xylocopa aerata) is one of Australia's most spectacular bees. It's big and bold, up to 20 mm long and bright metallic green with golden hairs. It's also one of Australia's most endangered bees.
Green carpenter bees are usually solitary, like most Australian native bees (although at times they may be semi-social). They harvest nectar and pollen from spring and summer wildflowers to provision a nest tunneled into soft wood for their larvae. Females live for up to 15 months, overwintering as young adults in their maternal nests before mating in spring, building and provisioning their brood nests during spring and summer, then dying before the new generation of adults starts hibernating.
Their nests are branching tunnels in soft wood. Detailed studies of the life cycle and ecology of green carpenter bee by Remko Leijs and colleagues from the South Australian Museum and the University of Adelaide show that they depend on two quite different nesting substrates: dead trunks of Banksia trees that have been infected and softened by white rot fungi, and the dried flower stalks of Xanthorrhoea grass-trees.
These two types of nest sites are both important, and are tied to cycles of fire in the bees' habitat. Grass-tree stalks are relatively short-lived but are abundant for a few years after fire (which stimulates the grass-trees to flower). By contrast, banksias are a long-term resource. Banksia trees that are killed in a fire take around seven or eight years to become soft enough for bees to nest in, then last for up to eight years depending on the age of the tree when it was killed. In long-unburnt areas, some banksias die each year, providing continuous nesting sites for the bees.
Green carpenter bees were once common and widespread in south-eastern Australia, from Kangaroo Island to northern New South Wales. They have declined throughout their range and are considered to be extinct in Victoria and mainland South Australia. They still occur in New South Wales around Sydney, and are hanging on at the western end of Kangaroo Island in South Australia.
And here are two elements of the story of their peril. Both western Kangaroo Island and the Sydney region were devastated by unprecedented bushfires in the Black Summer of 2019-20. On Kangaroo Island, all dead banksia trees in most areas were completely consumed by the intense fires, while all living trees were killed. A new generation of banksia trees will need to regrow from seedlings, and it will be several years before the fire-killed trees become infected by fungi to produce the soft, rotted wood the carpenter bees need.
Grass-trees flower immediately after fire, but it takes at least two years for the flower stalks to dry out and become suitable for bees to nest in. So, there will be a difficult period for Kangaroo Island's green carpenter bees. The few bees that survived the fire will have very few nesting sites until enough dead grass-tree flowering stalks become available. There will then be a longer period when they will be constrained by the lack of suitable banksia wood. Kangaroo Island's green carpenter bees need help.
Fortunately, the study by Leijs and co-workers means that the problem facing the bees after these devastating fires is well-understood. This means that targeted conservation actions can be planned. With funding from the Wheen Bee Foundation, a conservation program has been set up to provide artificial nests—sticks of balsa wood propped into the grass-trees—to see them through. This strategy was working well before the Black Summer bushfires: by the time of the fires, 150 nests had been built in balsa wood sticks placed in nine different sites. But sadly, these were all burned by the fires, at the exact time that the nests were full of brood.
Of course, volunteers for the green carpenter bee conservation program are not giving up, and will replace the balsa stalks to build up populations at the few remaining unburnt sites. These artificial nesting stalks will need to be maintained and replaced until natural nest substrates become available again.
All going well, green carpenter bees will soon again be foraging for nectar and pollen, excavating their nests, and contributing to Kangaroo Island's recovery from the devastating fires.
And maybe, one day, these big, beautiful bees will no longer be in serious trouble.