Nesting Biology

Most species of Megachile use existing cavities in which they construct their nests.  Various types of tubular cavities serve as nest sites.  They include dead plant stalks, rolled leaves, spaces among rocks, termite tunnels and burrows in soil.  Old beetle burrows in wood and holes in masonry are popular sites.  The width of the cavity selected depends on the size of the bee, but most species use diameters of 10 mm to 20 mm.  Several species accept artificial nests of bamboo canes, bored pieces of wood and drinking straws.  A few excavate their own burrows in the soil, some use existing holes in the ground, while a number use either location.  In contrast, some species of mason bees build an exposed nest attached to a wall or branch. 

          Possibly the first recorded reference to the nesting biology of Megachile was by Réaumur’s (1742) cited by Kirby (1802) and Jean-Henri Fabre (1915).  Fabre’s accounts of his own detailed observations on the biology of Megachile (1914, 1915) and other insects in southern France have fascinated naturalists for years.  His books awakened my own curiosity in wild bees.  Fabre gave the first detailed account of the use of trapnests to study the nesting biology of these bees.  During the same period Ferton (1896, 1897) studied Megachile species in southern France and on Corsica. 

          Despite the bees’ economic and ecological importance and the relative ease of study, the biology of few species has been studied in detail.  This is unfortunate as the readiness of many Megachile species to utilize trapnests and their fondness for suburban gardens enables the amateur entomologist to study their nesting biology with relative ease.  Apparently the difficulties of identification have discouraged such investigations.  Detailed studies have been conducted on M. brevis (Michener 1953) and M. umatillensis (Bohart & Youssef 1972) in U.S.A. and Krombein (1967) collected information on several species there.  The present author studied the holarctic species, M. centuncularis in southern England (Raw 1988).  The most detailed studies on neotropical species are those on M. concinna, M. zaptlana, M. rufipennis and M. lanata in Jamaica (Jayasingh & Freeman 1980, Raw 1984b, 1985).  With hundreds of publications on its biology, physiology and management, by far the best-known species is M.. rotundata, the principal pollinator of alfalfa. 

          Almost all leafcutter bees construct their nests by a similar method.  A female selects a suitable tubular cavity where she constructs a series of cells end to end.  For each cell she first cuts oval pieces of green leaves which she uses to form a cup which is the base of the cell.  Next she lines the sides of the cavity with a number of layers of oval pieces.  She then provisions the cell with food and lays an egg on its surface.  Finally she cuts several leaf discs to seal the cell.  The food the mother places in each cell is a paste of pollen and honey.  The cell closure is positioned at a short distance from the food mass to allow space for the larva to grow.  M. pugnata omits the cell linings and intercellular partitions.  Rarely M. centuncularis omits the cell lining, the only construction being the leaf discs placed to separate the cells (Markowsky 1933).  Many mason bees use mud and resin and others chewed leaves to build the partitions or even the entire cell. 

          Generally several cells are constructed head to tail in the tunnel.  The females of some species secrete lactones, esters and hydrocarbons in the Dufour's gland which are added to the provisions, but it is not known if they are germicides or nutritional supplements (Williams et al 1986).  Megachile is much more efficient than most herbivorous insects in consuming the stored food.  The growing larva assimilates 54-58% of the energy and 90% of the nitrogen in real growth (Waldauer 1968, Wightman & Rogers 1978, cited in Roubik 1989: 151 & 282). 

          Having consumed the food reserve, the final instar larva defaecates and spins a cocoon.  In warmer climates and in regions with longer summers, development from egg to emergence of the adult can be completed in about a month.  In univoltine species the final larval instar, called the prepupa, hibernates or aestivates and development is completed shortly before emergence from the nest. 

          Females of M. rotundata prefer trap-nests used by the previous generation to new tunnels.  Apparently they detect aromas secreted left by the previous occupants rather than nest residues (Parker et al 1976).  A nesting female of M. centuncularis recognizes her own nest as different from those of her conspecifics (Raw 1992). 

          Among temperate species a female will normally live up to a month and produce up to 30 eggs.  In warm weather she builds and provisions about one cell per day.  Little detailed information is available on the detailed nesting behaviour of tropical species. 

          In many species the female is larger than the male and occupies a larger cell provided with a greater quantity of food.  In a nest containing both sexes the mother bee commonly lays female eggs in the inner cells and males in the outer ones (Raw & O'Toole 1979).  Many species are protandrous, with the adult males emerging a week before the females. 

          As the cells lie head to tail in a narrow tunnel there is a question of how a bee towards the back of the nest can emerge when siblings block the exit.  Presumably all bees nesting in tunnels behave as do M. centuncularis, Osmia rufa L, O. ceorulescens (L) and O. leiana (Kirby) (Raw 1972, 1988).  When a bee towards the back awakens it chews through the partition and gains access to the cell in front.  If that cell is occupied, the bee nips the occupant.  When the bee in front starts to move the bee behind remains still.  Activity of a bee is the signal to the bee behind to stop biting.  When an emerging bee encounters a dead bee in the cell in front it bites the corpse and, as there is no response, it chews its way out pushing the remains behind it. 

Mortality

The female of a nesting bee provides the entire food supply for her offspring so she looks for a secure place to locate the nest.  Nevertheless, this rich supply of food and the defenceless offspring suffer the attacks from numerous organisms.  Predators and other organisms which kill the developmental stages of solitary bees are of three sorts.  Cuckoo bees and some wasps and beetles kill the egg and eat the stored food.  The parasitoid wasps, Leucospis, Melittobia, Monodontomerus and Tetrasticus and bee-flies attack later, killing the growing larva, the pupa or even the adult before the host can leave the cell.  A third group, which includes dermestid and tenebrioid beetles and flies consume the stored food and apparently incidentally kill the bee larva. 

          Many organisms have been recorded attacking Megachile.  The bees are Coelioxys, Dioxys and Stelis.  Wasps include Leucospidae (Leucospis), Eulophidae (Melittobia and Tetrasticus), Torymidae (Monodontomerus), Pteromalidae (Pteromalus, Dibrachys and Phaeacra), Sapygidae (Huarpea and Sapyga), Mutillidae (Dasymutilla), Chrysididae (Chrysis) and Ichneumonidae (Aritranis and Sphaeropthalma).  Beetles are Dermestidae (Anthrenus, Trogoderma), Cleridae (Trichodes), Meloidae (Lytta and Nemognatha, Ptinidae (Ptinus) and Tenebrionidae (Tribolium, Aprostocetus, Phyllobeanus, Cryptolestes and Tenebrioides).  Flies are Bombyliidae (Anthrax) and Tachinidae (Aritranis, Dibrachys and Phaenaera).  Two moths have been found consuming the food in the cell (Plodia interpunctella (Huebner) - the Indian meal moth and Vitula edmandsae Rogonot - the dried fruit moth).  Mites include Chaetodactylidae (Chaetodactylus), Saproglyphidae (Vidia) and Suidasiidae (Tortonia). 

          The most common cuckoo bees attacking Megachile are members of the genus tip of the Coelioxys The abdomen of the female of Coelioxys is pointed with which, as first reported by Ferton, she pierces the wall or the cap of a cell of the bee to deposit an egg (illustrated in Roubik 1989: 167).  The egg hatches quickly and generally before the host's egg.  The first larval instar is agile and bears long sickle-like mandibles (Graenicher 1905, 1927, Baker 1971).  The larva searches for the host bee’s egg which it punctures with the mandibles and sucks dry.  The second instar larva has normal mandibles and consumes the stored food of pollen mixed with honey which was provided for the host.  The wasp, Sapyga pumila behaves like Coelioxys (Torchio 1972).  The adult female pierces the cap of the cell with her ovipositor to lay her egg.  Likewise, with its long sharp mandibles, the first instar larva destroys the bee’s egg (and any other wasp’s egg which may be present) before consuming the stored food.  The eggs of clerid and meloid beetles are laid on flowers.  The first instar larva of the beetle attaches itself to a foraging bee and is carried back to the nest (Eves & Johansen 1974). 

          Several management techniques have been developed to diminish the attacks of predators on species of Megachile of economic importance as pollinators (Eves & Johansen 1974, Torchio 1972).  The emergence trap utilizes the tendency for predators to emerge from the cells before their hosts.  There are two types, both employing ultra-violet light which attracts the insects.  The lamp is placed above a tray containing insecticide or oil or is located inside a box bearing small tubular openings with glue.  In both, the predatory insect is trapped and killed before the host bee emerges.  The method is said to be efficient in controlling predators in trap-nests that are brought indoors to overwinter. 

          As the insects which attack the offspring of nesting bees do not construct their own nests they spend the night in safe cavities.  A second control technique is the use of night station traps which are similar to trapnests provided for the host bees, but of a smaller diameter to prevent the bees' entry.  However, this technique must adversely affect the populations of small species of cavity nesting bees. 

          Chalkbrood fungus (Ascosphaera species) attack the young stages of Megachile.  In particular A. aggregata specifically attacks Megachilidae and is a serious cause of death of M. rotundata (Vandenberg & Stephen 1982).  Several studies on Megachile and related taxa which construct nests comprising linear series of cells have revealed high rates of unidentified mortality of the developing brood (Danks 1971, Jayasingh & Freeman 1980, 1984b, 1985, 1988).  When an adult female emerges from her natal nest, she must pass through all the cells in front of her and thus come into contact with the contents of all the cells including those in which her siblings died.  This is an ideal situation for a pathogen to pass to the next generation. 

          Various organisms attack adult Megachile, particularly when they are on flowers.  These include conopid flies (Physocephala and Megaselia), crab-spiders (Thomisidae) and Strepsiptera.