Leaf cutter bees, rarely seen by many people but ever so useful!
I designed my award winning solitary bee nest box for numerous reasons. I found the other commercial ones turned out to be’ death traps’ for the leaf cutters and red mason bees and I and my children had difficulty viewing what was going on. Now everybody can view what is going on inside the cavities and even the cells with red mason bees! See 3 different species including leaf cutters in the video with interesting facts below.
Pollinators of our food under threat
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 250 species: miners, leaf cutters and mason bees. Like bumblebees, these important pollinators are under threat from their habitats being destroyed, pesticides and lack of forage. We were becoming increasingly dependent upon one species of pollinator, the honey bee, possibly because we also gained honey and other hive products from it as well as some crops from its pollination services. The difficulties that honey bees are currently going through should start to raise alarms. We simply cannot rely upon one species alone to pollinate our food. We should start to help the other, less well known wild species in our quest for pollination and hence food supply and food security.
Where will they nest?
This photograph was taken from one of my solitary bee nest boxes and is a larva inside its leaf cell
A large diversity of leaf-cutters (Megachile) are strictly ground-nesters, as are a good number of Osmia bees. For both, the nests are typically short, shallow, and sometimes branched. (pers. comm. JH). Miner bees can construct their own cavities in sandy/loamy soil. Usually, leaf cutters like mason bees, like to nest in a ready made cavity. This could be hollow stems, holes in walls, fence post holes and other similar cavities, providing the width was to their liking. Leaf cutter bees will also nest inside containers, such a hanging baskets and plant pots, esp. M. willughbiella. I suspect that the compost is something they can easily manage as they force their way through to make cavities. The leaf cells will contain a mixture of nectar and pollen and are very fluid as you can see above. They have to be waterproof or the liquid food would simply ooze out of the cell.
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. However, at the time they were probably the best available for bee observation. They do have major drawbacks.
Moisture cannot wick away causing fungal infection of the pollen and the bee larva dies. Raw states ” Glass tubes have been found undesirable, because condensation on the inside of the tube killed the occupants”
Being completely circular and acrylic any moisture that enters on the bee or pollen wicks into the surrounding material. i.e. pollen. This always carries a risk of fungal infection of the larval food, and thus its death, especially with red mason bees. The acrylic tube restricts the air flow and retains moisture. I have lost many red mason bees because of this. Yes they are fine for observations and have been used for years. But later in the year I did find that many bees were killed by the dampness and mould that grew in the tubes as you can clearly see above. I want to observe bees and their parasites. I want to increase bees numbers, not kill them! I would not use them now. The smaller the tube the greater the infection risk and I found the same thing happened to solitary predatory wasps. The tubes area real pain to clean for adults and children. They made management of the bees difficult. Hence my design of nest box which I now sell! They have been a tremendous success and more importantly a success for solitary bees!
Easily observe what is going on inside the cavities
The two most common species of leaf cutter bees in the UK probably are, Megachile centuncularis (Patchwork leaf cutter) and M. willughbiella. Besides leaf cutter bees, mason bees and some small predatory solitary wasps (with the optional extra nesting blocks) have all used the nest boxes. The predatory wasps store living but paralised sawflies, aphids 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 their own offspring and concentrate on completing their tasks within their cavity, each of which is in effect, the nest. So you are able to watch a wasp pile the caterpillars etc., in one cavity for its young with a mason bee or leaf cutter bee in the nest box! Each separate cavity is in fact a separate nest site for the particular bee os wasp species. If all the cavities are occupied, the females will go off in search of another nesting site.
The white polystyrene at the ends of the tubes are to stop spiders, earwigs and other undesirable 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 occupy cavities as a shelter, probably before deciding to make a web from the cavity. Earwigs will shelter inside them and eat the pollen and stores of food intended for the bees.
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? My research has found that without proper management, (details come with my instructions) these type of nest boxes, can also be a death trap for bees, over time when disease can spread from the cavity or mites take over each cavity leading to the death of the bee larvae. Drilled wooden blocks, a simple to make DIY attempt, is now old technology and again allows for the increase of pests and diseases inside beach hole. Management of these is impossible. Bamboo tubes are cheap and cheerful, but make management difficult. Management of bees can greatly enhance the bee population.
Their life begins.
After emerging from her cocoon in which she overwintered, possibly late May or June in my garden, 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. Cavity 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 cell construction process begins.
The whole process of nest making can take some time. They only live a few weeks so time is of the essence. Inclement weather stops them. They 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. Leaf cutter bees select a suitably soft 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 her body, using her front legs and jaws, to make it as aerodynamic as possible to give the least resistance and drag against the airflow. Ospreys carries a fish it has just caught, with the narrowest edges facing the way it is flying. She uses these pieces to line the outer walls of the cavity, making a hollow cigar shape. She uses different disc shaped pieces of leaf to cap an individual cell within the cigar shaped tube.
Then she stocks the cell with pollen and nectar using much more nectar than the red mason bee, tamps down the food with her abdomen, lays an egg on top of the food mound, then closes that cell using the disc shaped pieces of leaf sealing it 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/nectar, the process 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 if she is lucky, depending upon her finding enough cavities to fill.
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! 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!
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 in my instructions!) 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!
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. Would the leaf cutter bees be able to cut their way out through the mud wall with their leaf cutting mandibles ?
Yes you can view what is going on, BUT there is no oxygen exchange and no escape for moisture. I have found scores of dead bees over they years inside the tubes over the autumn and winter months. They are a devil to clean and remove viable cocoons. I needed a much easier option and one which did not kill the bees.
Both of the above cells were lucky. They were overwintered, 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!
A few little soil mounds in your lawn may well be a female mining bee or even a solitary wasp 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
For a MS word doc info sheet about the three most commonly seen in gardens leaf cutters bees from the Natural History Museum, download here
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
Pers. comm JH James H. Cane , USDA-ARS Pollinating Insect Research Unit Utah State University, Logan.
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.
Raw, A (1972) “The biology of the solitary bee Osmia rufa”, Trans. R. ent. Soc. Lond. 124 (3), pp. 213-229, I fig. 1972
Thanks for photo of damaged rose from Neil Bromhall an interesting web site…. www.rightplants4me.co.uk