Feature

Native Bees with New Tricks

New Zealand's "primitive" native bees are cleverer than we thought.

Dr Dave Kelly

Pollination systems worldwide have received much attention because of the often complex co-evolved relationships between plants and their pollinators. Sometimes these can be very specialised, such as the very long-tubed Madagascar flowers visited by equally long-tongued hawkmoths.

The traditional view of New Zealand flowers is that they are rather unspecialised and visited by a range of generalist animal pollinators. However, continuing work on native mistletoes at Canterbury has shown that native bees in three different genera have a very close and complicated relationship with the mistletoes. The bees manage to open flower buds of the red-flowered mistletoe Peraxilla tetrapetala, which were previously thought to require a visit by a bird to open them.

Earlier work on the Peraxilla mistletoes since 1992 at the university has shown that the flowers cannot open themselves. Buds ripen and then wait for a visit from an animal. The most common pollinators are tuis and bellbirds, which seek out ripe buds and twist the top. When twisted, the petals suddenly spring open, allowing the bird to drink the nectar. In the process, the birds pick up pollen and transfer it to the next flower.

This type of "explosive" flower has now been reported from several New Zealand species as well as a number of related mistletoes in the same plant family (Loranthaceae) in Africa and Asia. However, until now it has always been birds, and only birds, which open the flowers. The species of birds vary between regions, from honeyeaters in New Zealand to sunbirds in Africa, and flowerpeckers in India and southeast Asia.

The research at Canterbury is funded by the Public Good Science Fund and involves Canterbury staff members Dave Kelly and Jenny Ladley (who first discovered the explosive flowers), plus Alastair Robertson of Massey University. Field work at Lake Ohau was designed to measure how much pollen might be moved onto flowers by native bees after birds had opened them.

To our astonishment, we found that the bees were not waiting for the buds to be opened by birds, but were opening buds themselves. They do this by biting the end of the bud with their mandibles and pushing with their legs. Since the bees are only about one-fifth as long as the buds, this takes a herculean effort on the part of the bees and, more often than not, the bud does not pop open. Sometimes however the bee bites in just the right place to spring the mechanism; the bud pops open and the bee then harvests the pollen and nectar.

At least three different native bees in two families have shown this behaviour: an undescribed species of Leioproctis (Colletidae), Hylaeus agilis (Colletidae) and Lasioglossum sordidum (Halictidae). New Zealand's native bees are generally thought of as primitive: they are solitary and only active above-ground for the warmer months. There have previously been few reports of close relationships between the native bees and specific native flowers.

However, the bees clearly are much more expert mistletoe pollinators than the "primitive" tag would suggest. No other bee anywhere else in the world has been shown to be able to open large explosive bird-adapted flowers. Exactly how this behaviour could evolve in a bee which never sees its parents and does not have a complex social structure for sharing information is a fascinating puzzle we have yet to resolve.

This discovery, as well as showing another way in which the native flora and fauna were adapted to each other, is important from a conservation viewpoint. The native mistletoes are all found only in New Zealand and in many areas their populations have been declining. One species is already extinct. The main reasons for the declines have been clearance of forests for farming, browsing by introduced Australian possums (which are very partial to mistletoes), and reduced pollination because there are fewer tuis and bellbirds around than in pre-human times.

This is where the native bees come to the rescue. Our research has shown that flowers opened by bees get a good dose of pollen, and are much more likely to be able to produce seeds than if the buds are not opened by any animal. The bees are present at other sites such as Craigieburn near Arthur's Pass as well as near Lake Ohau. Therefore, the bees are helping to keep mistletoe reproduction on track, as well as benefitting by harvesting pollen and nectar to feed their young.

Even if native birds are reduced in densities in some areas, the bees may be able to keep the mistletoe seed production going. However, even our energetic native bees have their limits. Once seeds are produced, the seeds must be dispersed by birds to get onto a new host plant, and this the bees are unable to do.

Overall, this research shows that the co-evolution of native plants and animals is still yielding up some interesting secrets. The flora and fauna here are unique and the bees' behaviour is unmatched elsewhere in the world. Better understanding of the functioning of native ecosystems, including pollination mutualisms, is essential if we are to take effective care of the biodiversity of which we are the guardians.

Dr Dave Kelly is in the Department of Plant and Microbial Sciences at Canterbury University.