Mystery gall time

January was quiet on the blogging front - a combination of grotty weather and mucho other work. However, I'm back with a mystery gall sent to me by Phil Budd from the Southampton Natural History Society (although he found it in Enfield, Greater London). He found it (with about 50 others) in a sallow tree, Salix cinerea and had noted the presence of what looked like moth larvae living in/on it. Beyond this, identification remained elusive and so it was sent to me for further scrutiny.

Gall (approx 20mm diameter) on a Salix cinerea twig - it has clearly developed on one side of the twig and has a swollen and irregular surface.

One side of the gall was softer with fibrous material, possibly the remnants of a galled bud.

My first stop was the excellent Redfern & Shirley (2011), a standard (and affordable) work with excellent coverage of British galls, though the rapidity of change in cecidology (gall study) means here are always new species being added and new host-galler association being found. It quickly became clear that this is either a species not found in that book, or an unfamiliar form of a gall which is included. Either way, it was time to open the gall and look at the the larvae.

Larva (approx 12-14 mm long) from the Salix cinerea gall.

Opening the gall showed that there were several larvae inside and that they were still alive. I removed one which, from the arrangement of three pairs of true legs towards the front, a gap in the middle, then prolegs towards the rear, was indeed a moth larva. It has a darkened head capsule and first segment, bristles and a pinkish-brown colour, with paired dots on the segments in dorsal view.

Larva in dorsal view, noting the paired dots clearest on the front few segments.

This looks (to me) very much like the larva of Cydia servillana, a moth in the family Tortricidae which, although uncommon, is known to cause galls on this tree species. However, the gall is an elongate spindle-shaped swelling nothing like the knobbly and irregular gall seen here. It also contains a single larva, unlike the case here where there were several larvae together. So, what are the possibilities?

• It could be C. servillana creating an unusual form of gall, or utilising an existing growth of the type sometimes caused by the tree's response to a wound of infection.
• It could be a different species of moth which I can't find reference to or which hasn't been recorded before, at least not as a galler of S. cinerea.
• An unknown gall causer such as that noted on catkins in Redfern & Shirley (2011).I have my money on this option...
• Something I haven't thought of. Also distinctly possible!

Although I couldn't identify this for certain, there were some things I could do...

• As the larvae are alive, try to raise them as adults and identify the moths that emerge.
• Ask a gall-specialist - in this case I forwarded this post to the British Plant Gall Society.

As it happens the BPGS responded very quickly and confirmed one of my maybes/suspicions - it is probably the unknown call-causer, maybe a virus or phytoplasma (specialised bacteria that parasitise the phloem and are transmitted by sap-feeding insects much as malaria is transmitted by mosquitos) that distorts catkins, and the larvae are simply using the structure for shelter. So, the gall itself remains a mystery, but the moth may be identifiable if it develops to adulthood - if so, I'll post an update. Until then, you can see that the larva I removed is alive and well...

The gall is held above the larva which then climbs onto it and begins to investigate the various holes and crevices.

The larva continues to explore.

After a few minutes, the larva entered the gall - hopefully it will pupate and emerge as an adult.

Reference

Redfern, M. & Shirley, P. (2011). British Plant Galls (2nd ed.). FSC, Shrewsbury.