Winter husbandry for solitary bees

By Ron Rock, Bumblebee Conservation Trust Volunteer

And so, we reach the end of another bee year. As I write, there are just a few Common carder bees still in the garden. The Red mason and Leafcutter bees are long gone but mud and leaf filled tubes in the bee nesters are evidence that their work has been done. Another generation is already in place and will emerge next spring and summer. But will it? Are bee nesters a fit and forget option in our gardens? Will the bees emerge and happily go on year after year? Sadly, the answer is probably not.

By attracting these bees to our gardens, we are encouraging them to nest in far higher densities than would occur in nature. As a consequence, parasites find bee nesters just as welcoming as the bees themselves. Left unchecked, pests and disease can completely wipe out your bee nester population in a relatively short period of time. It should be our duty to open, inspect, clean, and replace used nesting tubes annually to ensure the health of our solitary bee guests.

The ‘unwanted guests’ in the bee nesters include mites, flies and parasitic wasps which can all have a highly detrimental effect on the intended occupants. No matter how well maintained, bee nesters will attract a certain amount of unwelcome attention during the course of a season.

When all of the activity has ceased, I move my nesters into a sheltered part of the garden to protect them from rain and to allow the bees to pupate and metamorphosise into adult bees to take place. In nature, the bees would remain quiescent in their cocoons until spring, but at this point I step in and give them a helping hand. I use a combination of routed out trays, cardboard tubes with paper liners and — although some would frown upon it — bamboo tubes drilled out to appropriate sizes. I have never had a problem with bamboo, and I have found that the number of parasitised or failed cells is about equal in all three types of nesting cavity.

The ‘Pests’

(Details of the lifecycles of the following creatures are available on the internet. I will concentrate on illustrating how to keep their numbers in check for the sake of the bees.)

(Chaetodactylus osmiae). Bamboo cane (left) on opening above. One cell is heavily infested with mites. The black specks on the left of the healthy cocoon are larval droppings, this is completely normal. Red mason bee larvae move the droppings out of the way before spinning their cocoon whereas Leafcutter bees incorporate droppings in theirs. The mites are the flesh coloured mass to the right of the yellow powder (mite droppings).

(Cacxoenus indigator) – a cleptoparasitic fly, at least in its larval form. The larvae of Cacxoenus indigator, the assassin fly. These fruit flies enter bee nesters through the tube entrance when the bee is away from her nest. When her eggs hatch, the grubs consume the pollen that was meant for the bee larvae and the bee grub starves to death. If only a few eggs are laid you just end up with a small mason bee, but a larger amount as here means no bee at all.

(Monodontomeros obscurus) – These tiny little wasps can be particularly troublesome, especially in nesters that are populated with thin walled plant stems or unprotected paper straws. Cardboard or bamboo tubes seem to protect against their attacks, at least in my experience. This is a wasp which oviposits up to ten eggs into a cocoon. The resulting larvae then eat the developing bee and overwinter as fully developed larvae within the infested cocoons. Reject any cocoons that don’t feel ‘right’. A healthy cocoon is firm to the touch, though male cocoons sometimes have dimples in them. Keep any you are not sure of in an escape proof container and see what emerges in the spring.

Safe opening of bamboo tubes
It can be a little dangerous opening bamboo tubes as I found out to my cost a few years ago. To do it safely you all you need is a simple device (left). This consists of a length of 75mm x 50 timber screwed to a base of 150 x 25. Two holes about 120mm deep are drilled into the 75 x 50 which are large enough for the bamboo tubes to sit in. Then if you insert a sharp knife 10mm deep across the entry end of the tube and twist it left and right the tube will open cleanly. Keep one hand on the knife handle and the other flat on top of the blade for safety. Never allow children to do this unsupervised.

Cleaning cocoons: initially, the cocoons can be cleaned by sieving them in fine horticultural sand, then you can use a soft artist brush to clean, followed with a damp cotton wool pad if required.

Initial cleaning with a soft artists brush

Before cleaning (right) and after cleaning on the left. Cleaned cocoons, don’t forget to dry them before storage, pat them gently with some kitchen towel then leave them on another piece of kitchen towel to dry for a few minutes. Storing the cocoons when damp can lead to problems with mould which can be simply wiped off should it occur, but why create problems when a little care ensures this doesn’t happen in the first place.

Bamboo tubes should be replaced annually, though ones drilled out to 8mm can have a CJ paper liner fitted and used again. Wooden trays can be cleaned with boiling water, scrubbed with an old toothbrush and left outside to dry before going back in the nester. Tubes and slots should be at least 160mm long. 8-10mm diameter holes are good for red mason bees and 10-12mm for leafcutters. Tubes of 3-7mm may attract other types of bee too. The nesters themselves can be scalded out with boiling water and any maintenance needed doing in the autumn. Store them indoors during the winter, load them with new tubes in the spring and then you can sit back and be richly entertained by these wonderful little bees.

Refrigerate cocoons in plastic trays on a bed of damp kitchen towel. The tub of water keeps up the humidity levels to stop the bees from dehydrating. Add paper towels in the storage trays which should be dry by the way. If stored in a fridge you will need to keep the humidity levels of 60-70% so some sort of humidity indicator will be needed A temperature of around 38 – 40 degrees Fahrenheit is perfect.

Ready for the new season, the box below the nester is a release chamber where the cocoons are placed in the spring. Small nesters like this are easier to look after than the huge ones that seem to be in vogue at the moment. Small is beautiful in my opinion. The flowering currant in the background is a magnet for bees of all types when in bloom, bees at the front door, it doesn’t get any better than that!

Some may say ‘let nature take its course’, but if we put out bee nesters then surely the aim is to increase the population of bees in our gardens. That said, you can reach a bee overload situation in which case I ask friends and neighbours to host a nester in their own garden. By adopting this form of husbandry you can increase the numbers of Red mason bees year after year (given the vagaries of the weather). And if you can get a child or two interested in these lovely little creatures then surely that can only be a good thing!

Look out for another exciting installment in February, when I will give you some advice on putting your nesters outside ready for the new season, and also how to deal with Leafcutter bees. These bees overwinter as pupae so their cocoons should be left well alone until next spring.

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