Solitary Bee Nest

Much can be observed in solitary bee nest tunnels and learned when undertaking annual management and cleaning.

Go to Solitary Bee Hive Management to read advice on annual management and cleaning.

The following are the most commonly seen. The Mason Bee is by far the most common species found so the information here refers to them. The appearance will differ slightly according to the orientation of the Nest chamber in the Solitary Bee Hive.

'Masonry' Mud Cell Walls

Female Mason Bees normally start and finish each cell with a 'masonry' mud wall before finally sealing the entrance with the same.
Some tunnels might be seen that are unfinished and even sealed with no contents.

Tunnel filled with Mason Bee cocoons. The cocoons cannot be seen but the presence of dark larval frass and a distribution of leftover pollen loaf are indications that all of the larvae in this tunnel reached the cocoon stage.
Interesting to note the cell construction. Each cell begins and ends with a mud wall so the walls between cells are double walled. The 2nd cell (from left) seems to be an abandoned attempt at cell building, not uncommon. The last cell has been left empty before a final plug of mud to seal the tunnel.
The orientation of the Nest Chamber means this view is of the bottom of the cells.

Pollen 'Loaf'

Female solitary bees create a 'loaf' of pollen and nectar in each cell as a food supply for each larva.
Often there is some left over but sometimes eggs fail to hatch and larvae can die before reaching maturity, leaving nothing in the cell but the 'loaf' untouched or only partially consumed.

Solitary mason bee cocoons

They are often surrounded with dark tubular pellets of frass (solid excrement of insect larvae), sometimes combined with loose pale silk threads together with any leftover pollen 'loaf'.
Once removed and cleaned, they will seen to be grey/brown, oval in shape and firm to touch.
The size will vary slightly, smaller being males, usually at the front of the tunnel and larger being females, usually at the back of the tunnel.

Compare cell on left with Mason Bee frass with cell on right with Houdini Fly frass.
The orientation of the Nest Chamber means this view is of the bottom of the cells.

Houdini Fly

This kleptoparasitic (parasitise by stealing) fly lays eggs in the cell before it is sealed by the female solitary bee. The hatching fly larvae consume the pollen loaf starving the solitary bee larva.
Affected cells appear filled with a mass of long thin curly threads of orange/brown frass. The fly larvae might also be seen.

Showing some nest tunnel contents after removal.
1: Mass of orange/brown threads of Houdini Fly larval frass. 2: Four Houdini Fly larvae. 3: Healthy Mason Bee cocoon.
Also note yellow masses of leftover pollen loaf.

Pollen Mites

Another kleptoparasite, the mites can quickly multiply within the cell consuming the pollen loaf starving the bee larva. Difficult to see with the naked eye but cells infested with mites will appear filled with granular 'dust'. This can vary in colour, usually yellow to brown. Mites will attach to bees as they emerge in spring to infest new nests.

Showing some nest tunnel contents after removal.
Showing curled Mason Bee larva killed by Chalkbrood (image upper centre). Also see Houdini Fly frass (image left), two Houdini Fly larvae (image lower centre) near a long, dark curled tube of healthy Mason Bee larval frass.