Different Midges

Doubtless most folks reading this will have heard about the fearsome Scottish Midge, and trust me the rumours are entirely true. Here on Skye they are a minor annoyance most days, occasionally they will have me running back indoors before I am indeed bled dry, but usually they are entirely tolerable. Happily there are many other species of midge that do not descend in swarms to drive warm-blooded creatures into a panic. Yesterday, whilst in a Portree cemetery, I noticed several plants of Slender St John's-wort exhibiting deeply fuschia-coloured leaflets at the top of the stem. I popped three of these flushed tips into a pot and today I had a closer look at them beneath the microscope. 

Hypericum pulchrum with vividly pink-flushed uppermost leaves

Slender St John's-wort very often exhibits a pink flushing to the leaves at this time of year, but these plants really stood out as being just a bit too pink! A short distance away was a stand of unaffected Slender St John's-worts, clearly this warranted further investigation.

My initial thought was that they were infected by a fungus, but investigations quickly drew a blank. So then I figured it had to be invertebrate-induced and my mind turned towards thoughts of mites and gall-midges. I carefully peeled back one of the leaves and discovered several layers of very small green leaves within plus a sprinkling of suspicious-looking fly grubs. A-ha, a gall-midge it is then! But which?

I jumped online and soon realised that my midge grubs were either Dasineura hyperici or Dasineura serotina. Various features can be used to separate the pair, but the different websites I visited seemed to contradict each other. I even tried using the AI chat thingie on my laptop and it assured me I definitely had one, before going on to totally assure me that I had the other... Eventually, I have decided that I have Dasineura serotina which appears to be massively under-recorded in Britain and, as a bonus, is a lifer for me.

The two leaves have formed a tightly fitting 'purse' or 'pocket' and not a cabbage-looking mass

The fact that the discoloured leaves are tightly fitting is a strong pro-serotina feature. Dasineura hyperici forms a more cabbage-like gall where the terminal leaves are all bunched up together, a growth form also sometimes described as a loose ball. So far so good, serotina is the clear favourite.

Outer leaf removed to reveal a series of stunted, non-flushed inner leaves

This is also a good pro-serotina feature. Dasineura hyperici should have larger terminal leaves and they should be more readily visible as a loose fitting bunch of leaves resembling a cabbage. I wish I could find images of dissected galls for both species to compare against, just for peace of mind, but I can't. 

Unexpected hitch-hikers inside the gall

This batch of eggs (?) has been laid upon the first layer of hidden leaves, I don't know what they are but I suspect they are of hymenopteran origin. The genus Macrolabis (gall-midges) are known to occur within Dasineura galls, but I don't think that's what we're seeing here. It's sometimes quite boggling what you can discover whilst looking down the barrel of a microscope.

And these are the larvae of the gall-causer itself

These larvae are very small, though they may still be immature with a fair bit of growing to do, but importantly they are more-or-less transparent rather than white, pink or orange. Note the uppermost one is the same opaque colour as the background leaf whereas the (partly obscured) right hand grub is far greener, clearly matching the leaf surface behind it. In each you can discern a darker patch, which I'm guessing is something to do with the gut. The given descriptions for serotina larvae, depending on the website visited, are translucent, clear or hyaline whereas Dasineura hyperici larvae are white or whitish when immature, becoming pink and finally turning orange when mature. At no point are they described as clear or transparent. 

One website mentioned that the larvae of Dasineura serotina are 'surprisingly small' for the size of the gall they dwell within, but this is difficult to objectively judge. They could be surprisingly small because they are not yet fully grown. And anyway, how do you quantify 'surprisingly small' when the adult gall-midges are all of 1.5 - 2.5mm in length!

So there you have it, Dasineura serotina (fairly) safely in the bag and Dasineura hyperici (fairly) safely ruled out. Except that one of the galls had a more 'open' look to it. Almost cabbage like, one could say. And the grub seemed a little whitish to my eye...

 

Hmmmm.......

I feel the chances of finding both species of Dasineura that gall Slender St John's-wort side by side on adjacent plants is remote. I'm happy that at least two of the galls contain Dasineura serotina, I'll just overlook the third one. What I need to do next is inspect more galls in a couple of weeks time when the larvae will be mature, see if any have turned pink or orange. These are multivoltine midges, so there should be a succession of broods present as the "summer" (pah!) progresses.  

And the last Midge of this post is....