Galls of unknown origin

Those of you familiar with my blog will have noticed a minor fascination with galls lately, in part due to the publication of Redfern (2011). Following a couple of posts about the common Knopper Gall Andricus quercuscalicis, I felt it was time to tackle something distinctly less well known, to be precise, an undescribed species of the gall midge genus Dasineura (Diptera: Cecidomyiidae).

Dasineura ulmaria is a common galler of meadowsweet Filipendula ulmaria leaves, forming reddish-pink pouches with a hairy opening on the underside. However, in recent years (and indeed before, though less commonly), similar galls have been found on dropwort Filipendula vulgaris but with projections and openings on the upper sides of the leaves (see Harris 2010 for details of material seen between 2002 & 2009). As described in Redfern (2011, pp. 147-150), the newly hatched larva of D. ulmaria settles on a young leaf, and as it feeds, nutritive tissue develops beneath and around it, with cells growing and dividing to envelop it. This is typical of much insect-mediated gall formation where larval feeding induces gall production, although the precise mechanisms are porrly understood.With D. ulmaria eggs being laid on the underside of the leaf, the gall also has its opening here, unlike the galls on F. vulgaris as noted above.

Dasineura galls on F. vulgaris

These galls are single-chambered ('unilocular' in gall-speak) and as shown in the photo above, there may be more than one per leaf, and many per plant.

Dasineura gall on a single leaf of F. vulgaris. Note the hole towards the left which may be generalised plant-feeding damage or could be a potential predator of the enclosed larva.

Upper side of a F. vulgaris leaf showing two gall projections and openings.

Close-up of the gall openings showing fringing hairs and elongated cells of the conical projections.

Further investigation of the gall surface shows enlargement of plant cells, especially along the leaf ribs.

Enlarged cells on the outside of the gall, highlighted by reddening/darkening along ribs and veins.

Moving on from surface anatomy of the gall, its dissection reveals the single larva that lives inside.

The tiny (approx. 1mm long) larva in situ showing its yellow colour and segmentation. Also note the somewhat spongy texture of the nutritive cells lining the inside of the gall.

Close-up of the larva (width of photo approx. 0.5mm) showing the small head and tiny (paired) antennae. Also note the segmentation highlighted by dorsal and ventral sclerites, plus a hint of internal structure and gut contents.

In Dasineura ulmaria (and many other galling species), the young larva induces the production of large nutritive cells by the plant which line the gall chamber and provide it with food, especially during later stages/instars. As the larva grows, nutritive cells are consumed and nutrients continue to be supplied from vascular strands and stored starch. As the larva empties successive layers of nutritive cells, replacement nutrients have to pass across several dead layers. In D. ulmaria, nutrients also have to diffuse from vascular strands to the larva via the sclerenchyma - cells which have become somewhat woody but remain alive and connected - and it is possible that this is also the case with the galls on F. vulgaris. The larva pictured above is of the general form expected of Dasineura and although the papillae and structures known as the sternal 'spatula' (underneath the head end) which can be diagnostic (e.g. Smith 1989) were not visible in this young specimen, it is hoped that more mature larvae will be collected during a field visit in June to the same chalk grassland site that produced the specimen photographed here.

As noted by Harris (2010), this gall has been described previously - as D. ulmaria by some authors and D. harrisoni by others, although it may of course turn out to be genuinely undescribed. Therefore, until further work can be undertaken (e.g. on the host range of D. ulmaria and the identity of D. harrisoni which is unclear), this gall is to be treated as distinct but undescribed i.e. Dasineura (undescribed sp. A on F. vulgaris) - evidence that even in over-populated (and entomologist-laden) southern England, new species await discovery!

As ever, watch this space - if there are further developments I hope to be able to post them here.

References

Harris, K. (2010). Notes on gall midge galls recorded on Filipendula ulmaria and F. vulgaris in the United Kingdom. Cecidology 25(1): 6-10.
Redfern, M. (2011). Plant Galls. Collins, London.
Smith, K.G.V. (1989). An introduction to the immature stages of British flies. RES Handbooks for the Identification of British Insects 10(14):1-280.