Leafcutter Bees of the Tribe MEGACHILINI

By Geoff Tribe (Images by Jenny Cullinan and Karin Sternberg)

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Megachile frontalis (Identified by Dr Connal Eardley, National Collection of Insects, Pretoria)

This large and abundant tribe known generally as leafcutter bees, even though nests may be lined with various materials besides leaves, is found in every continent where the majority of the species belonging to this tribe are solitary. Unlike other solitary or sub-social bee groups, secreted cell linings are absent. Material for cell walls and linings is instead brought in from outside and, depending on the species, includes pieces of leaves, chewed leaf material, resin, mud, or pebbles.

The distinguishing feature of the family Megachilidae is that the females of non-parasitic species carry pollen by means of a scopa on the venter of the abdomen instead of on the hind legs. The following genera are included in the Megachilini: Chalicodoma, Chelostoma, Heriades, Hoplitis, Megachile and Osmia.

Leafcutter bees in southern Africa are stoutly built, brown or black bees up to the size of a honeybee, hairy and with a conspicuous yellow tuft of hairs on the underside of the abdomen where the pollen grains are collected on this special apparatus. The female leafcutter bee seeks out a tube or tunnel that is of the correct dimensions which is dry and sheltered. Such cavities include hollow stems, holes in a wall, deserted cavities constructed by wood-boring beetles and even man-made holes in buildings.

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The female Megachile venusta cuts a fairly thin oval piece of leaf from a smooth-leaved plant and carries it between her forelegs to the selected nesting site. The leaf segment is pressed into position with her head at the base of the tube. This process is repeated until both the base and walls are lined with overlapping leaf segments. The first small cup about 10mm deep is then filled to three-quarters with a yellow paste of pollen and honey. On this she lays a single egg before sealing the cup with leaf segments and then repeats the process with one cell on top of the other until the tube is filled with such cup cells containing brood. Finally the entrance is stuffed with assorted pieces of leaf of different shapes and sizes before being sealed with a plug of chewed leaf cemented with saliva. There appears to be just one generation per year.

The fully grown larva spins a cocoon which lines the inside of the cell and after excreting, coats the inside with the excrement which dries to form a water-proof varnish. Here it remains for several months until the following season, after which it pupates and emerges two or three weeks later.

A number of the Megachilidae have become ‘cuckoo’ bees where they lay their eggs in the nests of related bees, their larvae feeding on the honey and pollen paste stored by the host which results in the starvation of the host larva.

The related tribe of Anthidiini is much smaller and less abundant than the Megachilini and use the same materials as above for lining their nests except for mud. Genera include: Anthidium, Trachusa, and those with Anthidium as the root. The communal carder bees (Immanthidium) are included here. As their name implies, various cottony fibres which they strip from hairy plants are used to construct the nest. One such carder bee nest made out of a cottony material and the size of a fist was discovered in the hills above Pietermaritzburg in 1972 built within a huge discarded concrete water pipe. Here it was protected from rainfall and the entrance hole to this mass of ‘cotton’ was streaked with yellow pollen. Inside were several individual cells provisioned with a honey and pollen paste.

References:

Michener, C.D. 1974. The Social Behavior of the Bees. Belknap Press, Massachusetts, 404pp.

Skaife, S.H. 1979. African Insect Life. Struik, Cape Town, 279pp

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 The Author…

Geoff Tribe

Geoff Tribe

Getting the shots…

at the beach

Jenny Cullinan

bee paradise

Karin Sternberg

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