Mating and Nesting Habits

Many bees produce specific pheromones, and these are often important in bringing the two sexes together. In some species such as Centris adani the males deposit the marking pheromones on the female during copulation and this makes them unattractive to other males. In some cleptoparasite genera such as Nomada the males produce a pheromone in their mandibular glands which mimics the substances produced by the hosts Dufour's gland, this is sprayed over the female during mating, presumably making it easier for the female to gain entry into the hosts nest.

Orchids of the genus Ophrys emit mimic pheromones which attract male bees of which ever species is being mimicked, males of Andrena, Anthophora, Colletes, Eucera and Tetralonia amongst others fall for these lures. The males attempt to mate with the flowers and end up pollinating them. Females of most species only mate once. However in a few other species the females are receptive all season, e.g. Panurgus spp. In some species of Nomadopsis and Perdita the male remains coupled with the female while she forages, thus preventing other males from mating with her. Males look for females at flowers, at the nest site where they emerged, and at other non-resource sites which are called 'encounter sites', In any one area there may be one lone male patrolling from one encounter site to another or there may be hundreds. When the number of males is very large they may form swarms and patrol synchronously from site to site. In some species such as Osmia rufa the males may have home ranges which represent only a part of the total encounter sites available.

In some species males mark the flowers that mark the boundaries of their range with pheromones. In some non-aggressive bees such as Andrena, Nomada and Triepeolus males may sent mark non-flowering plants to mark the edges of the patrol routes, these markings may be attractive to other males which then join in the patrol. When a receptive female is encountered by one male he will attempt to mate with her, if more than one male is present the others will probably also attempt to mate with her and a large mass of bees, all but one being male, will tumble to the ground. Otherwise non-aggressive males may well fight when a receptive female is present.

In some more aggressive species e.g. Anthidium manicatum and Hoplitis anthocopoides the males are territorial. Territorial males then either patrol their territory , e.g. Megachile and Callanthidium, or they can wait at a single encounter site which they defend from other males, e.g. species of Xylocopa. As always in nature there are variants and some Anthidium males patrol along a route when few other males are around but switch to defending a single flower clump when other males are numerous. The size of male territories ranges from about 0.5m² to 6m². Males probably never hold a territory for an entire breeding season and in some cases territories change owners after relatively short times. This is partly controlled by the density of resources and the amount of competition. In Anthidium manicatum it has been shown that the best territories are held by the largest males. and that territory holding males forage more and copulate more than do non-territory holding males. In circumstances like this you get selection for large males and males tend to be larger than females.

Copulation is normally brief and often involves the mating pair getting mobbed by all the nearby males, the exception to this is the Panurgines. Nomadopsis males may either find the female at a flower and fly with her 'in copula' to the nest or vice versa, depending on the species.

In some gregarious nesters such as Anthophorines and Panurgines, mating takes place over the nesting site, or else it can take place inside the burrow, as in some Andrena and Paracolletes. However, mating normally takes place at, or in a flower being visited by the female for pollen, or as in the case of Nomia melanderi it occurs both at flowers and/or at the nest. Males of some species of Andrena have been observed patrolling along hedgerows, presumably in order to maximise their chances of meeting and mating with a female.

Many solitary bees nest near where they emerged, especially soil, wood and wall nesting species. There may sometimes be hundreds of thousands nesting in one area. Most nest in the ground, in well drained soils of varying degrees of slope, however Megalopta ipomoeae is a nocturnal bee which nests in damp soil in tropical forests. Most of the Bees which nest in plants use plants with pithy stems, i.e. bramble, raspberry, elder, maple or bamboo, whilst only a few, like Xylocopa sp. gnaw out holes in solid wood, and Megachile sp. and Osmia sp. use many different cavities, from holes made in wood or soil by other insects to snail shells and man-made objects.

Many ground nesting species pile the soil from the burrows up in a little volcano hillock around the nest (e.g. Andrena haemorrhoidalis), whilst others build entrance tubes or turrets. The depth of the burrow depends on the species and soil type, those that nest in very dry soil lend to dig deeper holes. The Xylocopines, most Anthophorines, Megachilids and some Colletids construct their nest cells end on end.

In most of the solitary bees the cells are lined with wax or varnish-like waterproofing material and sometimes the tunnels of the Colletids are lined with a thin, cellophane-like material. These substances are secreted by the salivary glands in Andrenids, Colletids and Hallictids, but are abdominally secreted in the Anthophorines. However Megachilids, the leaf cutters, use bits of leaf or petals, while some use mud and/or pebbles.

The cells are provisioned with pollen and usually some nectar in species of Andrena. The pollen is generally made into a dough-like ball with the nectar but in Lithurge sp. it is left quite dry, whilst in many Colletids and Anthophorines the food may be a semi-liquid gruel. In most Anthophorines about half the food is liquid and this portion is eaten first.

A few species lay their egg part way through provisioning the nest (e.g. Lithurge fuscipennis) but by far the most common procedure is to fully provision the nest then lay the egg and close the cell. Most lay the egg on the food but the Emphorines insert the egg under the food. Species of Colletes, Melecta and Nomada stick the egg onto the cell wall.

Development is usually quick, taking only a few weeks to pupation. The bee may then remain a pupa until a suitable emergence time. In species with more than one generation per year this may be quite soon after pupation is finished, but in the many univoltine (having one generation per year) species it will mean over-wintering as a pupa. over-wintering may be extended to several years when the weather is poor. Alternatively species like Andrena and Osmia emerge from the pupa and overwinter as inactive adults in the cell, or in some species of Xylocopa and Ceratina may emerge in the fall and then overwinter gregariously in old burrows or other suitable places. Most do not form a cocoon, the exception to this is the Megachilids.

Nesting females normally nest/sleep in their burrows while many males and not yet nesting females may gather into groups, often containing a mixture of species consisting of up to several hundreds of bees for the night. Some bees, like some Sphecid wasps, grasp the stem of a plant with their mandibles and let the rest of the body relax, either head up, or as in Coelioxys sp. with the head facing down. Others hold the stem with their legs as well, either head down (Melissodes obliqua and M. perplexa), or head up as in M. bimaculata. Nocturnal bees sleep in flowers during the day.