Bees (members of the superfamily Apoidea) are one of the first wild animals children notice. Common, beautiful and with intriguing behavior, bees are raised to pollinate plants as a multibillion dollar global business, and secondarily to make honey. Yet one sting can kill a person allergic to bees. Many people learn about the complex cooperative behavior of bees, including their dance language first described by Karl von Frisch in 1947, as their first introduction to animal behavior. Yet the familiar honey bee (Apis mellifera) represents just one of nine families and thousands of species of bees that are not as well known.
A few summers ago I noticed a pleasant hum coming from our garden on warm mornings. A fast-growing passion-fruit vine (Passiflora sp.) had taken over part of the garden and large black and yellow bees were hovering around the vines, occasionally visiting the flowers. The flowers from this vine are literally fantastic. They look like alien space stations with elaborate landing pads optimized for carpenter bees. The bees land in the designated landing area, brushing past the stigma as they land.
Pollen transferred from the back of the bee to the stigma is transported down to the ovary where it fertilizes the ovum which eventually develops into a seed. Then, while the bees drink from the nectary, new pollen from the flat anther adheres to their back to pollinate another plant. The anther is attached with a movable joint at the correct height off the corona to force the big bees to rub against the anther to reach the nectar.
I saw honey bees also alight on the passionflowers, but because of their loose fit no pollen stuck to their back as I observed with the larger carpenter bees. The Passiflora flowers are clearly designed for big bees.
The bees with which passion-fruit flowers coevolved are members of the genus Xylocopa. These are large, non-social bees (they have no queen) that excavate holes in wood for their nests leading to their common name - the carpenter bees. Worldwide there are at least 700 species in this genus. Although carpenter bees are larger than most other bees, they are not aggressive. Like all bees, males don't have a stinger. While the females have a stinger, they do not attack people.
According to Prof. Dan Gerling of the Department of Zoology at Tel Aviv University, five species of carpenter bees are native to Israel with Xylocopa pubescens the most common. It was the black and yellow X. pubescens that I watched working my passion fruit flowers.
The female carpenter bee digs tunnels in wood up to 20 cm. long to make nests where she spends most of her life when not foraging or mating. In the spring, after making a nuptial flight during which she mates with a male while flying, the female excavates as many as eight cells in which to deposit her eggs. The brooding cells always start at the end of the tunnel where they are most protected and are built in a row, one after another.
After construction she prepares a mixture of pollen and nectar called beebread which she deposits into the cell. Then she lays an egg on top of it, and seals the cell. Over a period of about a month the eggs develop from larva to pupa to adults, eating only the beebread provided by their mother.
This nest with its developing young is clearly of supreme importance for the success of the mother, so it is not surprising that when suitable wood to make nests is scarce, there is intense competition, including no-holds-barred fights, between bees for the tunnels. If a bee leaves its nest to forage and another bee usurps it, the intruding bee breaks open all the cells, kills the developing larvae, and cleans the tunnel in preparation for its own brood.
X. pubenscens are excellent parents. Even while the females are laying their next brood of eggs, they continue to feed their adult offspring large quantities of pollen and to give them regurgitated nectar by trophallaxis (mouth-to-mouth food transfer). Why do the parents spend energy to feed their adult progeny instead of using that energy to make more progeny?
Gerling watched the behavior of parents and their adult offspring while they were living in their nests by using X rays to see inside the nests. He observed the young competing intensely to be fed and noticed that the bee that the parent met first when entering the nest got the most food. The young bees, therefore, struggle with each other to stay at the entrance of the nest and in so doing they prevent bees from other nests from stealing the tunnel and stealing its contents while the parent is out gathering more nectar and pollen.
It makes sense, therefore, for the mother bee to invest in the young adults who guard the nest and protect their younger siblings from being killed by usurpers. Eventually one of these siblings might take over the nest and become the reproductive and foraging female, while the older female might remain at home and guard. Sisters also switch jobs between guarding and reproducing.
The story can be more complex. Occasionally it isn't a family member that guards the nest. Instead it is an unrelated, defeated female that remains and guards the nest for the usurper. It is not clear why. It could be that if tunnels are very scarce this is the only place for her to live. But why doesn't the defeated female turn around and destroy the brood of the new intruder and then start again with her own brood? As always in science many questions remain. Carpenter bees are relatively solitary, but they show the beginnings of the complex social behavior that allow honey bees to live in a hive almost like a single organism.
Next time you see a large bee foraging on a flower, you might ask yourself who is guarding her nest?
The author has a PhD in behavioral ecology from Boston University.