Who you gonna gall?

If you are interested in plant galls, you'll know how important it is to correctly identify the host plant as there is often a high degree of specificity between the gall host and the gall causer which makes identification a lot easier. It's not always that straightforward however, as I discovered when I found galls on the leaves of a Mountain (or Alpine) Currant Ribes alpinum.

Galls on a leaf of Ribes alpinum

These are clearly true galls as the red patches are swollen rather than simply being discoloured, and looking Redfern & Shirley (2011) - the standard work on British galls - there are few options on Ribes. In fact, the key quickly moves to an answer, both caused by aphids of the genus Cryptomyzus:

• On redcurrant R. rubrum, galls caused by C. ribis which are yellow-green in colour.
• On R. alpinum, galls caused by C. korschelti which are pink, orange or reddish.

The plant ID is definitely correct (a known specimen confirmed by an experienced botanist), so it looks like C. korschelti, but it's important to check carefully, so let's see the aphids themselves.

Aphids on the underside of an R. alpinum leaf

Close-up of an aphid on the underside of an R. alpinum leaf

By now you should have seen the difficulty - this is a yellow-green aphid, but this implies the aphids are C. ribis which are not, according to Redfern & Shirley (2011), found on R. alpinum. However, rather than having found a new species, I thought it was more likely that this is R. ribis on an unusual (for the UK) host. However, having consulted with an aphid specialist (thanks Fiona!), not only is it unusual to find Cryptomyzus on the underside of the leaf rather than inside the galls, but the galls themselves are too swollen. So, let's look even more closely...

Siphons (or 'cornicles') at the rear of the aphid's abdomen

Cornicles are an important way to separate some aphid species, and these are broadly swollen with a slightly widened rim at the end (this isn't very clear in the photo, but it is there), the whole being approximately bottle-shaped. In Cryptomyzus, the cornicles are narrower - this is a different genus. Without going into too much detail here, it turns out that (thanks again Fiona) it is in the genus Hyperomyzus, specifically H. lactucae. This is rarely recorded in Britain (though this doesn't necessarily mean it is rare, just that not many people look or can identify them), but it has been found here before and is known from Ribes in continental Europe.

Many galls are not well understood and minor discoveries like this can be made quite readily if care is taken to look, especially given that R. alpinum is not especially common in the UK and probably poorly studied. The lack of readily accessible/affordable identification guides (Blackman & Eastop's 2006 2-volume opus was needed for this species, but is not cheap) makes aphid study more difficult (plus some genera are taxonomically confusing and really require genetic analysis and/or research), but as I found, there are specialists who are ready to help and it's always worth asking. Now to let the authors of the gall key know what I've found, then check whether H. lactucae has been recorded in Hampshire before...

References

Blackman, R.L. & Eastop, V.F. (2006). Aphids on the World's Herbaceous Plants and Shrubs. (2 vols.). Wiley, Chichester.
Redfern, M. & Shirley, P. (2011). British Plant Galls (2nd ed.). FSC, Shrewsbury.