This year, as with the last seven, our coffee breaks in the garden have been enlivened by watching the life and times of leaf-cutter bees (Megachile species).

It all started when a small bonsai dish was filled with a mixture of sandy soil and coarse grit, planted with Sempervivum offsets and set on a table in the garden. Within a year or so the plants were well established and started to ‘heap up’ as the offsets grew through the established plants eventually overtopping them and shading them out. At this stage, in about 2003, it attracted the attention of leaf-cutter bees which exploited small gaps at the edge to burrow under the plants and make their nests. As they dug, the excavated material was tipped over the edge of the dish, in time reducing the level of the soil by about 3 cm. They also tried to throw out particles of grit, but unable to get a foothold on the glazed edge of the dish, gave up after a brief struggle, leaving an accumulation of small stones. Some time during this period a moss established itself where the bees were digging and now it occupies about one fifth of the area, gradually infiltrating the Sempervivum and submerging it.

The bonsai dish in 2006. The activity of the bees has removed a quantity of soil and moss is encroaching. A leaf -cutter bee is approaching its nest hole at the junction of the moss and the Sempervivum.

At the end of May, the first of the latest generation of bees appeared excavating its burrow under the moss. It entered the hole head first and after about two or three minutes backed out dragging the spoil between its front legs and down-bent head. It retreated three or four centimetres before releasing its load and flying off, making a small circuit before re-entering the burrow and repeating the operation. (The excavated material appeared to be largely organic with a few small stones, indicating that it was digging in the decomposed plant material rather than any remaining soil.) This was followed by a period when it left to collect large, rectangular sections of leaf which it held beneath its body, supporting each one with its legs by hooking its claws over the edge of the leaf. As it entered the hole it used only its mandibles to pull the leaf section, some times with a struggle, into the hole. After repeating this operation two or three times the bee next visited the nest with a load of pollen, which it gathers on the hairs on the underside of its body rather than pollen baskets and thus is visible as a rim of yellow around the rear of the abdomen (Fig. c). The final act before it started again on another cell was to fetch a smaller more circular section of leaf to close the cell.

A leaf-cutter bee dragging a large section of leaf into its hole. At this stage it is holding the leaf with its mandibles.

A leaf-cutter bee entering its hole with a load of pollen collected on the hairs under its abdomen and visible around its rear end.

As the leaf-cutter was working, the site was visited by other bees and wasps which lurked around the nest. The most colourful was the ruby tailed wasp, Chrysissp. which was seen on two occasions to nip into the burrow to lay an egg or inspect progress. This is a parasite which lays its egg in the bee’s cell, from which the wasp larva on hatching eats the leaf-cutter bee larva.

Another more sinister looking visitor with its pointed abdomen and dark head and thorax was the bee, Coelioxys sp. (I am grateful to Adrian Knowles for his identification.)

A kleptoparasitic bee (Coelioxys sp.) lurking near the leaf –cutter-bee nest. The red dot is a passing mite.

This, like the ruby tail, kept the nest uNovember 19, 2011 9:14make a hurried exit as the owner was working within. Unlike the ruby tail wasp this bee does not parasitize the larva of the leaf-cutter bee, but is a kleptoparasite which lays its egg in the cell. This hatches before that of the leaf-cutter bee and then destroys its egg before eating its food store (O’Toole & Raw 1991).

The dish was watched closely for about an hour around 11 am on most days. Casual observation throughout the rest of the day suggested that the main period of activity was from about 10 am when the dish was in full sun until mid afternoon. After about six days the bee was not seen again although the hole was left open. Reflecting on the bee’s behaviour, I was amazed at the way in which it performed a complicated series of tasks and the effort involved. Flying with a relatively large piece of leaf acting like a sail clearly took a lot of effort: the bee sometimes landed near to the dish and took a minute or so to summon up energy to finish the task. The source of the pollen and nectar with which the bee provisioned each cell was not identified. To gather pollen on the underside of its abdomen presumably requires a flower with exposed upstanding stamens like a daisy or other Asteraceae, but there were few flowers of this type in the near vicinity.

The regular nest building by leaf-cutter bees over a period of years suggests that each generation returned to the place where it was hatched to make its own nest. The same may be true for the two parasitic species, i.e. the bonsai dish is a small self contained ecosystem ideal for nature study in comfort.

Reference
O’Toole, C & Raw, A (1991). Bees of the World. Blandford Press.