Who left mud on the windowsill?

December 11th 2021

Who left mud on the windowsill?

On the 26th July 2021, I opened a window in my lounge to set free a moth and noticed dried mud on the windowsill.  Going outside, I saw that there was quite a lot of it occupying the space at the bottom of the window frame (photo 1).  At one end, there were several small greenish moth caterpillars, along with a much larger, legless, pale larva (photo 2).  At the other end, in individual cells, there were cocoons, red-brown in colour and with black bead-like structures attached (photo 3).

I showed the specimens to Ian Evans who took one of the cocoons for examination under a microscope.  He suspected that the greenish caterpillars were prey which had been immobilised by the adult insect to provide food for the pale larva, which would eventually pupate inside a cocoon.  Indeed, under the microscope, the black ‘beads’ appeared to be the head capsules of the prey caterpillars incorporated into the cocoons.

Expert opinion

I sent the photos to Murdo MacDonald who thought a species of potter or mason wasp, Ancistrocerus, was responsible.  He also drew my attention to an egg in the top left hand corner of photo 2 and mentioned a cuckoo wasp, Chrysis vanlithi (photo 4), which lays its eggs in the nests of potter wasps.  When the cuckoo wasp larvae hatch, they destroy all the host eggs and larvae and consume the food store of caterpillars.  It was not possible to be sure which had produced the pink larva and the hatched cocoons. 

The mystery continues..

I looked at potter wasp photos on the internet and remembered an insect which I had photographed in my garden on 16th June 2021 (photo 5).   At the time, I thought it might be a bee, feeding on the geranium pollen, but couldn’t find it in my bee guide.  Murdo identified it as Ancistrocerus scoticus, common along our north-west coast. Could this be the species that had made the window frame its family home? 

Further searching of the internet turned up a photo of a related species, Ancistrocerus nigricornis, which is found further south in the U.K.  It was shown carrying a caterpillar similar to those in the window frame (photo 6).  Also, while having another look into the window frame on 17th August 2021, I saw that there were green legless larvae amongst the prey caterpillars (photo 7). These looked similar in appearance to those of Ancistrocerus nigricornis  in photos on the internet.  I also saw one photo of larger pale larvae attributed to the same species, suggesting that as the green larvae grow, they lose their colour.

There were also fresh white cocoons without any head capsules of prey caterpillars in the window frame on 17th August (photo 8).  I again consulted Murdo about these, but he re-emphasized that we could not be sure which species had produced the larvae and cocoons.  Even if they turned out to be an Ancistrocerus species and I had a photo of Ancistrocerus scoticus in the garden, there was another species, Ancistrocerus oviventris , which has a scattered distribution throughout the U.K. and is also found here. 

So, who did leave mud on the windowsill?  I shall have to look more closely next year.

Gwen Richards


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