Sitona obsoletus

Formerly known as Sitona lepidus, this species is less common than the familiar Pea-leaf Weevil Sitona lineatus.

Although similar to lineatus, the gap between the fore coxae and the constriction near the foremargin of the prosternum was too great for this species - the gap was a similar size to the gap between the rear margin of the coxae and the rear margin of the prosternum. However, I strugged to reach a satisfactory species ID using either the Duff or Hackston keys. Both keys led me to either obsoletus (as lepidus in Hackston) or cylindricollis, but in both cases the characters differentiating these two species did not appear to be exactly as described for either. The main difference appears to be in the shape of the elytral scales - much longer and narrower in obsoletus compared to short and round in cylindricollis. In practice, before measuring the scales on close-up photos, I didn't find this easy to judge - a few scales were clearly long and thin but many were much less obviously so. Duff also provides an additional character whereby on obsoletus the second striae diverge away from the suture near the apex, so that the second intervals broaden - however he writes that this can be hard to see due to covering scales, and I can vouch for that!

I suspected it was obsoletus, but to record it as such I needed to be much more certain, so dissected it in the hope of finding an aedeagus. Unfortunately it was female, so that didn't help. Mark Gurney's guide didn't really help - indeed the mention that obsoletus often has three pale dots down either side of the centre of the pronotum put me off slightly (as mine didn't have these). But it was Mark Gurney's high quality photos on Flickr that finally gave me confidence. Comparing the shape of the elytral scales on his photo of obsoletus with thoise on his his photo of cylindricollis, the difference was so much clearer than I had appreciated from reading the keys. Mine was clearly obsoletus. I eventually managed to see that the first and second elytral striae diverged towards the apex by removing an elytra, cleaning it in KOH, turning it upside down and examining it through a brightly bottom-lit microscope.

female Sitona obsoletus showing its fore coxae and prosternum, top of the head to show frontal groove, spermatheca, elytral scales and elytral punctures from below (upper edge in photo is the sutural side), North Elmham (Norfolk, UK), 30th December 2021

Although I caught the next one before the last, I examined it later. It had appeared to be darker than Sitona lineatus, and a quick look at the position of the fore coxae in relation to the constriction near the foremargin of the prosternum confirmed that it was not that common species. I had some problems identifying this one too, but different problems to those experienced with the one above. This time it was the length of the frontal groove that was problematic. I examined this carefully and was sure it did not extend back far as the rear margin of the eyes. It seemed to stop in a deepened pit around level with the middle of the eyes. That would make it puncticollis, a rather rare species in Norfolk. I examined the aedeagus which only helped to a degree - the differences between the species are not huge. I went through the key (Duff's) again, and also through Hackston's key, and still ended up at puncticollis, but still had a nagging doubt that I'd somehow gone wrong. Hackston described the frontal groove ending in a round cavity with the appearance of a large pin-prick surrounded by a bare area. On mine it ended in a depression but it was definintely a long thin depression, not a round one. One of the references had suggested that it may be necessary to remove scales to accurately determine where the frontal groove ends, so I did this (not as easy as it sounds - they didn't just brush off). Sure enough, this revealed a faint narrow continuation of the frontal groove right up to level with the back of the eyes. Hard to see, even after removing the scales, but definitely there.

So it wasn't puncticollis, but this presented a new problem. I thought I'd made out that the second stria was parallel with the elytral stria to the tip, which would make it cylindricollis rather than obsoletus, but the scales and aedeagus fitted obsoletus better. But it had been really hard to see the striae at the tip and I wasn't 100% certain. After clearing the elytra in potassium hydroxide solution it was still hard to see - the elytra stria in particular was really faint - but eventually, by examining it from underneath while strongly back-lit, I managed to see that the second stria did in fact diverge slighly from the (barely visible) elytral stria.

male Sitona obsoletus showing its fore coxae and prosternum, top of the head to show frontal groove (first photo with dotted red line indicating apparent extent of line and second photo after removing scales showing its true extent), aedeagus (3 agnles) and elytral punctures from below (lower edge in photo is the sutural side; the elytral punctures are only just visible), North Elmham (Norfolk, UK), 23rd June 2021

The next one I encountered (attracted to LED lights in parkland) wasn't much easier. Again the frontal groove was only obvious up to a point between the middle of the eyes, though this time I could see it continuing to the end of the eyes, albeit much more faintly. Seeing the paths of the sutural and second striae from above was impossible and viewing them from below was challenging simply because of the angle at which you need to hold the elytra to see the rounded tip in a relatively flat plain. Once I'd managed to see the two rows of striae it looked for all the world like they wre parallel to the tips. It took a significant amount of effort to position the elytra such that I could photograph it with the striae visible and in focus, but eventually when I did achieve that I could tell that in fact the striae did diverge towards the tip (although they then appeared to converge again slightly at the very end). Unsure of its sex I dissected it, but it was a female (the spermatheca broke when I was preparing to photograph it, but resembled the one shown above).

female Sitona obsoletus showing head shape from rear, top of the head to show frontal groove, fore coxae and prosternum, elytral scales (from above - suture on left - and wettened from side) and elytral punctures from below (including with sutural and second striae highlighted), Wendling Beck Environment Project (Norfolk, UK), 3rd February 2023

Another one, which after the previous experience above, was a lot easier to determine.

male Sitona obsoletus showing head shape from rear, top of the head to show frontal groove, fore coxae and prosternum, elytral scales and median lobe (flat, from the side and close-up of tip), Wendling Beck Environment Project (Norfolk, UK), 29th March 2023