Caryocolum species generally have a similar wing patterning but vary, even within species, in the extent of darker markings. Where dark forms of any species are encountered it is unlikely that a determination can be made on forewing features alone. Caution should therefore be applied when attempting to identify non-bred specimens and in many cases dissection is recommended - in all but one pair of species (alsinella/viscariella) the genitalia are distinctively different.
Caryocolum alsinella is our smallest Caryocolum although larger specimens overlap to some extent or other with C. viscariella, C. proxima and C. junctella all of which can have fairly similar forewing markings. When fresh specimens are encountered, it may be possible to use the range of features detailed below to name the moth, especially if comparative material is available. However, netted or light-trapped specimens are nearly always likely to require retention of a voucher specimen allowing closer examination and often dissection. A note has also been added below about potential confusion with a dark from of Caryocolum blandulella known to occur in Europe.
Comparison with C. viscariella
C. alsinella (wingspan 7.5 - 11mm) is smaller than C. viscariella (10 - 14mm) but with a slight overlap, has more white scales in the centre of the forewing, lacks the small orange-ochreous patch in the basal area and the angulated fascia from costal to tornal spot is white (light yellowish-brown in C. viscariella). Although the genitalia are very similar the foodplant, distribution and habitat preferences can assist with the identification process.
Comparison with C. proxima
C. alsinella is very similar to C. proxima which differs in the shape of the fuscous blotch and the angle (60-80 degrees) it makes with the costa (c.45 degrees in C. alsinella) and has pinkish scales on the tibia of the forelegs and especially of the midlegs; the latter feature can also be found in C. junctella (see that species for further information). Historically there has been considerable biological and distributional confusion between C. alsinella and C. proxima with many early records of C. proxima (under the name C. maculiferella) referring to C. alsinella.
Comparison with C. junctella
C. junctella can be quite a variable species but has the first discal spot included in the bar from the costa to the fold and the second distal spot not, or only slightly L-shaped. The patches of yellowish scales in the fold and before the white costa are characteristic of C. junctella. The frons (front of the face) is silvery and the head and thorax which are metallic shiny in C. junctella as opposed to shining pale ochreous grey frons and duller head and thorax in C. alsinella. The tarsi of the forelegs and mid-legs in C. junctella often have pinkish scales (although it should be noted they are sometimes whitish and that in Denmark this feature is always lacking), a feature only otherwise seen in C. proxima. C. junctella is the only species of this genus known to hibernate as an adult so date of capture may assist with the identification.
Comparrison with C. blandulella
On the continent there is a dark form of C. blandulella which could cause identification problems if present in the British Isles. C. alsinella has a less distinct costal block and the costal and tornal spot are often separated by orange-brown.
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