Moths of the season: Challenging chestnuts

With the majority of the winter now behind us and with a mild spell of weather forecast in the last week of February, many moth-ers will be thinking of getting out their light traps for the first time this year. Many of the early spring species such as Pale Brindled Beauty, Spring Usher, Dotted Border and Early Moth should now be the expected targets in the garden. Several species that have overwintered as adults may also be flying again before laying eggs to complete their breeding cycle. This is a good time to see the quite variable yet easy-to-identify Satellite with its many combinations of orange and white spots.

Satellite at Warndon (Worcs), December 2011 (Roger Wasley).

From following threads on various moth chat newsgroups, and reading comments on blogs and websites in the last five months, one species pair seems to provide quite a few identification problems, especially for those who are relatively new to the game. The Chestnut Conistra vaccinii and Dark Chestnut Conistra ligula are smallish, compact and very closely related noctuids, both of which overwinter as adults. They both emerge in the autumn, with vaccinii on the wing from late September and ligula a little later from October. Both species can hibernate during long cold periods but are also able to make the most of any milder midwinter conditions and can be expected to light at any time. Dark Chestnut numbers usually taper off by mid-March whereas Chestnut can be seen as late as mid-May, which make those adults some of the longest living of the British moths. Both species have a wide range of foodplants.

Chestnut is certainly the more abundant of the two, especially in prime woodland; however, Dark Chestnut can be almost as regular in occurrence in gardens, parks and paddocks. In the autumn, when both moths are fresh, there is not too much of a problem with identification. Both species have variable colour and patterning, and Waring & Townsend depict three examples of each, illustrating the basic range of colour forms, pattern and general structure. A simple but effective plate showing a comparison of the two species with useful visual pointer can be seen here.

In addition, the following images and notes are intended to help show the range of variation and potential identification features as well as some natural pitfalls that some moths can exhibit.

Chestnut — typical medium rufous-brown type at Howick (Northumberland), late autumn 2011 (Stewart Sexton).

Chestnut — plain but darker rufous-brown type at Warndon (Worcs), late December 2011 (Roger Wasley).

Chestnut — medium patterned lighter rufous type at Howick (Northumberland), late autumn 2011 (Stewart Sexton).

Chestnut — extreme patterned with lighter brown type at Warndon (Worcs), late December 2011 (Roger Wasley).

These four show some classic Chestnut features. These are, in the author’s opinion, and on relatively fresh moths, in order of importance:

• A rounded apex to the outer edge of the forewing, producing a nice continuous curving contour.
• The overall base colour of the forewing being a variation of 'chestnut' or light, medium or rich rufous-brown. A large dark grey in-fill 'spot' to one side of the lower, kidney-shaped stigma is often present.
• Often, a more curved or bowed shape to the whole outer or lateral edge of the forewing, as well as the inner (bottom) edge.
• Regular tiny 'notch' marks along the outer (side) edge of the forewing, which tend to be more obscure and usually coloured only slightly paler than the whole base colour of the upperwing; i.e. dull orange, sandy or reddish. This feature is variable and should only be used as an indicator of identification.

Dark Chestnut — typical 'teak'-coloured type at Howick (Northumberland), autumn 2011 (Stewart Sexton).

Dark Chestnut — extreme 'ebony' type at Howick (Northumberland), autumn 2011. If they all looked like this there would be little need for this article! (Stewart Sexton).

Dark Chestnut — 'ebony' subtle patterned type with conspicuous pale notches along outer (side) edge of forewing at Howick (Northumberland), autumn 2011 (Stewart Sexton).

Dark Chestnut — a more rufous-patterned type at Howick (Northumberland), autumn 2011 (Stewart Sexton).

These four images show the classic Dark Chestnut features, which are (again as far as the author is concerned and on fresh moths), in order of importance:

• A very square apex to the outer edge of the forewing. This produces, in some moths, a near right angle with the inner (bottom) edge, which in fresh moths is also very straight. Some individual moths can showed a marked hooked tip to the apex and many also show a concave edge to the lower (bottom) edge. This has been described as 'showing an S shape to the contour of the wingtip'.
• The overall general colour of the forewing a consistently darker brown, ranging from dark ebony to dark reddish-brown in the lightest. These paler ones are usually quite well patterned. A dark oily sheen can be noticeable, especially on fresh, very dark individuals.
• Often, a straighter edge to most of the outer or lateral forewing, as well as the inner (bottom) edge.
• Regular tiny 'notch' marks along the outer (side) edge of the forewing tend to be quite noticeable and usually coloured silver-white, dull white or creamy, and markedly paler than the whole base colour of the upperwing.

Two Dark Chestnut on the left, a probable Chestnut and more typical Chestnut on the right, at Howick (Northumberland), autumn 2011 (Stewart Sexton).

This image shows two rather reddish Dark Chestnuts alongside two presumed Chestnuts and helps to illustrate, by direct comparison, some of the points outlined above. To be really safe it is best to rely on the whole suite of characters being present in a particular moth rather than one or another.

So it seems that most fresh moths, even when exhibiting quite a wide variation of colour and pattern, can still fit nicely into one camp or the other on structure alone. As each winter progresses, adult moths of both species will undoubtedly have periods of activity and inactivity depending how long the colder spells may be. During these active times, each moth is prone to the vagaries of wear, especially in milder winters like this one. Wear is especially noticeable on the forewing which, being an exposed and well-used part of the moth, will come into contact with foreign bodies more frequently. The apex of the forewing will often wear considerably by late winter and the nice rounded apex of the Chestnut can become more square-looking, albeit often looking more ragged or perforated. Conversely, the nice angled or hooked tip to the Dark Chestnut can wear away completely and may even look slightly rounded on some individuals.

A tricky chestnut sp. at Howick (Northumberland), late autumn 2011. This rather dark greyish individual still retains the rounded forewing apex, grey in-fill in kidney stigma and is a Chestnut. (Stewart Sexton).

A tricky chestnut sp. at Warndon (Worcs), late December 2011. A dark worn moth with contrasting broad pale sub-terminal band, distinct pale notches and still a suggestion of pointed apex point towards Dark Chestnut. (Steve Whitehouse).

A tricky chestnut sp. at Warndon (Worcs), late January 2012. A plain, two-tone chestnut moth with some pale notches and a suggestion of an oily sheen but still showing a curved wing shape and round apex must be a Chestnut. (Steve Whitehouse).

A tricky chestnut sp. at Warndon (Worcs) late December 2011. A very dumpy, well-patterned moth also exhibiting a pale sub-terminal area and pale notches. Although appearing to have a square apex, this is probably due to wear and it must be a Chestnut. (Steve Whitehouse).

A tricky chestnut sp. at Gretton (Glos), late January 2012. A dark, rich chestnut colour and suggestion of pointed wing apex understandably at first indicated a Dark Chestnut. However, the slightly bowed outer wing shape and rather plain costa gives some credence to alternative views and indeed it could be either species. (Roger Wasley).

A very tricky chestnut sp. at Warndon (Worcs), late December 2011. This difficult moth appears to show good features of both species. It is dumpy, quite reddish, has bowed but apparently nicely pointed wings, some pale notches, medium-grey kidney stigma in-fill and a paler sub-terminal band. The original thought was Dark Chestnut but it is now possibly best left as unidentifiable (Steve Whitehouse).

These final six images show just how difficult some moths are to judge. Some moths that readers may come across will undoubtedly have to be left as unidentified. Some of us may prefer to get a definite outcome to a particular moth by doing a genital analysis. This should only be done where the identification is critical, for example when producing a full species list for a site that has not recorded a particular species before. This particular method is still regarded by experts as difficult and it seems the best way forward is to look at lots of live moths in both woodland and man-made habitats between October and April and get one’s eye in. There will inevitably be some specimens that cannot be identified in the field and in these cases it is preferable to record “Chestnut aggregate” (i.e. chestnut sp.), rather than a doubtful determination to species level.

Recently, an as-yet-unproven extra identification feature was kindly explained to me by Martin Kennard from Warwickshire. This involves the examination of the under-forewing of the moth. If you turn a normally docile moth such as a chestnut upside down, it can be difficult to see, as the folded hindwing is tucked up in front of it. However ,by carefully examining a cooled moth with a hand lens, it may be possible to see enough of the forewing to detect that, on the Chestnut, the wavy, pale sub-terminal line on the underside (i.e. the one close to the edge) is quite distinctive. The corresponding line on Dark Chestnut is vague, diffuse and difficult to make out. Images that actually show this can be seen within this article, which explains the discovery of a new species of Conistra moth on Sicily. Martin Kennard put this observation to the test in late autumn 2011 and found it to be conclusive on a small sample of fresh moths that he examined. He is now waiting to see whether it is true for moths of both sexes captured on recent milder nights.