If you build it, they will come !

Nature isn’t always predictable. When we placed a ‘bee hotel’ in what we noticed was a popular spot for Battleby’s red mason bees we couldn’t be sure the insects would take to the potential new home.  We needn’t have worried. They have loved it … so much so that we have had to add an extension ! 

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Athayde Tonhasca of SNH Installing a bee hotel for Red Mason Bees at Upper Battleby. ©Lorne Gill/SNH

We’re delighted to report that our red mason bee ‘hotel’ at Battleby is being well used. These busy bees deserve all the help they can get. They can make up to 75 visits to flowers daily, no wonder they’re exceptional pollinators.

When you hear about pollination, probably ‘honey bees’ come to mind, or perhaps ‘bumblebees’. These bees are indeed important pollinating agents, but many other insects contribute to this vital ecological service – particularly solitary bees and hoverflies. Among these mostly overlooked pollinators, mason bees certainly deserve to be better known for their ecological importance.

The ‘busy-bee’ bee

Some people may be surprised to know that honey bees are not that efficient as pollination goes: the pollen they collect is moistened and carried tightly packed on their corbicula (pollen baskets: the hairy cavities in the bee’s legs in which it stores the pollen) so pollen grains are not easily dislodged. Moreover, honey bees learn quickly how to collect nectar with minimal contact with the flower’s anthers (thus reducing the chances of pollen transfer) and have high flower constancy (the trait of visiting the same type of flower over and over), which is bad news for plants that need cross-pollination between different varieties, such as apples.

Thus, paradoxically, honey bees’ efficiency as food collectors reduces their efficiency as pollinators. These shortcomings are offset somewhat by the huge numbers of bee workers and the fact that they are so amenable to management.

Mason bees (genus Osmia) on the other hand carry dry pollen loosely attached to their scopa (a dense mass of hairs with similar function of the corbicula). This means that more pollen grains have a greater chance of contacting the flower’s stigma. In addition, these bees collect only pollen, and have low flower constancy.

A male Red mason-bee (Osmia bicornis). Battleby. ©Lorne Gill/SNH

All of these traits make mason bees very efficient pollinators, so much so that over 80% of the orchard area in the main apple-producing region in Japan is pollinated by one mason bee species, Osmia cornifrons. Other related species have shown to be efficient pollinators of apples, pears, cherries, blueberries and other crops in the US, Canada and Europe, and methods to rear and manage large populations of these bees have been developed and are improving.

A welcome arrival

Here in Scotland, one species has been rapidly spreading from the south since 2006 – the red mason bee (Osmia bicornis). This is potentially good news for wild flowers and crop production, since this bee is an effective pollinator of rapeseed oil and a number of crops grown under polytunnels and glasshouses, such as strawberries and raspberries. Osmia are cavity-nesting bees; they make themselves at home in existing cracks and crevices in walls – hence the common name ‘mason bee’.

The red mason is particularly interesting for pollination purposes because it readily occupies man-made structures set up within agriculture fields for their nesting. Even better, populations can increase six-fold in one year under management if conditions are adequate, that is, the habitat is flower-rich.

Considering the significant decline of honey bee colonies in the UK and the rest of Europe, mason bees and other native species may represent an ‘insurance’ against the heavy dependency on the honey bee for pollination services.

Small but perfectly formed

The red mason bee is about the size of a honey bee, and is covered with orange-red hairs. The female is black-haired and sports two facial ‘horns’ below the antennae, which are used to pack down wet mud in the nest. The male is smaller and has a tuft of white hair on the front of his head. The underside of the female’s abdomen has a dense mass of orange hairs (called scopa), which are used to transport pollen.

Once a female occupies one of these cardboard tubes, she will construct a series of compartments (brood cells) and stock them with pollen as food for her offspring. She will then close the nest entrance with a mud plug. The larvae will eat the pollen and emerge as adults the following year to start the cycle again.

Cross section of Mason Bee nest

A mason bee nest is simple but effective. An empty cell at the nest entrance is a barrier against predators and parasites. The front cells contain male eggs with smaller balls of pollen. The cells in the back contain larger balls of pollen and female eggs – females need more food

Red mason bees are seen typically from April until to June, peaking in May, when most fruit trees are in flower. They visit a wide variety of plants and are excellent pollinators of raspberry, plum, pear and apple.