Leaf cutter bees-harmless, useful and often neglected pollinator

August 7, 2011

in Red mason & solitary bees

rose leaf cutter damage

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We have all heard of honey bees, living in large numbers in their hives, bumblebees living in much smaller numbered colonies but many people have not heard of or are aware of another group of important pollinators of our food, particularly fruit and flowers. That group is the harmless solitary bees, which themselves comprise of 3 main groups of which there are around 150 species: miners, leaf cutters and mason bees. Like bumblebees, these important pollinators are under threat from their habitats being destroyed and lack of forage. We were becoming increasingly dependent upon one species of pollinator, the honey bee, possibly because we also gained honey from it as well as some crops! The difficulties that honey bees are currently going through should start to raise alarms that we cannot rely on one species alone and we should start to help the other, less well known wild species in our quest for pollination and hence food supply and food security.

Leaf cutters like mason bees, like to nest in a ready made cavity, unlike miner bees that can construct their own. We can help them by providing cavities to nest in and there are many solitary bees nest boxes available to buy at garden centres. Personally I like watching these fascinating bees and have bought nest boxes, one almost 20 years old,  that allows you to observe them up close, without too much disturbance to the bees.

 

Two different types of solitary bee/insect nesting boxes

The two most common species of leaf cutter bees in the UK are, Megachile centuncularis and M. willughbiella. Besides leaf cutter bees, mason bees and some small predatory wasps will all use these nest boxes. These boxes were positioned onto an east facing wall. They were mainly used by red mason bees, (Osmia rufa ) with a few leaf cutter bees using a few tubes. One was moved to a south facing wall which radically increased the leaf cuter bee usage. The predatory wasps store live but paralised sawflies, spiders and caterpillars inside the cavities, then lay an egg with a hearty feast awaiting the young wasp grub! The different species of solitary bees and small predatory wasps, simply do not bother with each other, all intent on supplying for the own offspring and concentrate on completing their tasks within the observation tubes, each of which is in effect, the nest. So you are able to watch a wasp pile the caterpillars etc., in one tube for its young right next to a mason bee or leaf cutter bee in the next tube! Each separate tube is in fact a separate nest site. If all the tubes are occupied, the females will go off in search of another nesting site.

Mason and leaf cutter bees can easily be observed in the transparent tubes

The white polystyrene bungs at the ends of the tubes are to stop spiders and earwigs entering. If a bee finds a spider inside a vacant tube, she obviously will not use that tube nor will she tolerate an earwig being present inside. Spiders will sit in wait for insects to enter, use it as a shelter and build a web from the tube, earwigs will shelter inside them and eat the pollen and stores of food intended for the bees.

Wooden nesting box for solitary bees- note bottom left cell has been sealed with mud

Another type of commercially available solitary bee nesting box. This one has at least one cell fully occupied and sealed by mud, a trade mark of a mason bee. Leaf cutters in my garden, do not use this type box at all, only the mason bees. Why is this?

After emerging from her cocoon in which she overwintered, possibly March or April, the female leaf cutters bees feed themselves up with nectar for energy and pollen in readiness for egg laying, find a mate and then go in search of a suitably sized cavity. Old tree stumps  or logs with beetle borings or beetle outlets are suitable, as are some plant stems.  Tunnel depth, diameter and material will influence her choice. After selecting a cavity, she will, like bumblebees, undertake a zig-zag orientation flight in which she in effect, takes mental pictures of the entrance hole and its location memorising its exact location. Once found, if it was previously used by another bee, she will clean out the debris before the the cell construction process begins. Leaf cutter bees select a suitable rose leaf, lilac, beech or willow herb leaf, starting from the outer edge and using their jaws, (mandibles) cut a semi circular shape around the outer edge. As the leaf is about to drop she resumes flying and carries the leaf underneath its body, using its legs and jaws, in a aerodynamic position to give the least resistance and drag against the airflow, like an osprey carries a fish it has just caught, with the narrowest edges facing the way it is flying.

Leaf cutter bee cutting a leaf, used as nest lining material

I would imagine that carrying the leaf must be rather tiring for the female leaf cutter as the leaf can weigh a 1/4 of her weight! and recently at Speke Hall in Speke, Liverpool, I saw this female having a breather, rather appropriately on a wooden table with two benches, one of those that we use to have a rest ourselves ! There was another leaf cutter actually inspecting old nail holes in the very same bench!

Female cutter bee with her nest lining material, having a rest at Speke Hall, Liverpool!

The whole process of nest making can take some time and as they only live a few weeks, have only one brood a year and never see their offspring, time is very important to them. After mating and feeding up, she has to find a suitable cavity, then she will construct a cell within the cavity, made from the very leaves she has cut and carried back, then she has to find pollen and nectar to provision the cell, tamps down the food with her abdomen, lays an egg on top of the food mound, then seals that cell up with a leaf/saliva mixture and starts again with the next cell. As each particular cell can take nearly 10 hours to build and provision, with an average 21 trips to contruct the cell and 18 trips to provision the cell with pollen, the process that can take a few weeks, particularly if the weather is bad. In essence, her life is based around a complex sequence of stereotyped behaviour: nest searching ➞ nest inspection ➞ leaf gathering ➞ pollen/nectar gathering ➞ egg laying ➞ leaf gathering ➞ leaf sealing and so on until that particular cavity is filled. She may lay 30-50 eggs depending upon her finding enough cavities with 8112 cells per cavity.

I have often seen them inside their cells on cold, wet or windy days, sitting there, with their heads just near the cell entrance, awaiting better weather. As it is the actual provision of food for the cells takes most of the time, it would help if we could provide suitable flowers nearby, ( as unlike honey or bumble bees, solitary bees do not forage over great distances), for them to forage on… (list coming soon!). Whilst awaiting better weather, I have seen them, particularly during the inclement daytime weather and early evenings, prepared to defend their eggs, waiting near the entrance with their jaws open ready to repel, predators. Unfortunately, when they are out foraging for food and provisions the cells are left undefended. This leaves ample time for a host of predators and parasites to enter the cells and lay their own eggs…..  a topic in its own right!

Leaf cutter bee cells with an opportunistic mason bee in the last cell

Above you can see 3-4 leaf cutter “cigars”! each containing provisions, lined with cut leaves and sealed with a leaf pulp. However, this female may have died as she never constructed the very last feel. This was made by an opportunistic mason bee, which you can see is totally different. She has sealed the right od her cell with mud, provisioned it with pollen upon which she has laid her egg and sealed the entrance to the whole tube with mud.

Leaf cutter bee cell ( cigar!) with a young bee inside shortly to emerge leaf cutter bee cell to the right and mason bee cocoon on left

Both of the above cells were lucky. They overwintered in my garage, away from predators. Unfortunately the one below was taken by a bird, possibly a woodpecker that visits the garden looking at the damage to the front wooden cover. This certainly is a case of last in, first out!

Bee larvae predated by a bird whilst still in the cavity

A few little soil mounds in your lawn may well be a female mining bee, excavating a tunnel for her young, a few little chunks out of your prized roses, will likely be the leaf cutter bee, the bee hovering around a few holes in your wall could well be the mason bees. So next time you are enjoying your chilled glass of cider, or crunching into a juice apple, highly likely that the apples used were pollinated by one species of bee or another. Cheers!

See my new Registered Design award winning solitary bee box and bumblebee nest box both of which are radical, practical and educational, offering them a safer nesting environment in which you can observe the bees.

For more information about solitary bees and wasps visit BWARS

For more bumblebee information and to help save bumblebees join the Bumblebee Conservation Trust at Stirling University

Refs: Besides my own experiences and observations…..

Anon, (2009), ” Solitary Bees”, International Bee Research Association”, Cardiff

Mader, E et al (2010)” Managing alternative pollinators”, Natural Resource, Agriculture and Engineering Service, New York

O’Toole, C, (2000), “The red mason bee, taking the sting out of bee keeping”, Osmia publication, Banbury

Raw, A (1998) “Nesting biology of the leaf-cutter bee Megachile centuncularis (L.) (Hymenoptera: Megachilidae) in Britain”.
ENTOMOLOGIST. Vol. 107, no. 1, pp. 52-56. 1988.

 

Thanks for photo of damaged rose from Neil Bromhall an interesting web site….  www.rightplants4me.co.uk

 

 

{ 2 comments… read them below or add one }

marian morrison August 16, 2011 at 10:17 am

Hi George. Read all the above with considerable interest and whilst writing this our leaf cutters are still very busy in the box, amazing!! Please keep me informed regarding the bumble bees. Kind regards Marian.

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nurturingnature August 18, 2011 at 8:06 pm

Hi Marian, I will keep u posted re bumblebees and when i get some time have quite a lot to write re solitary bees! Wish mine were still active ! Cheers George

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