[Dunn and Parrack treat Ectropis crepescularia and Ectropis bistortata separately as follows. Both are now recorded as Ectropis crepescularia.] [Ectropis bistortata] The difference between this and the following species [Ectropis crepuscularia], is apparently, still not fully understood. Both species vary to a considerable extent and variation of one in the direction of the other is frequent. In the north east we find that E. bistortata is usually slightly the larger and generally darker brown in colour against the lighter grey colour of E. crepuscularia. Both are single brooded here, robbing us of a distinguishing character which makes life easier in the south of the county where E. bistortata is double brooded. Even here, the overlap in flying times is only slight, depending upon the early spring weather. E. bistortata can emerge early in March but is more often seen siting on tree trunks in April and the early part of May. E. crepuscularia is seldom seen before the middle of May and goes right through Hune to early July. There is, however, a slight overlap in May, when one may have to resort to looking at genitalia. Many of the records we have for E. bistortata are dated June and July (one even for August, but this may be of a partial second brood), and these are most probably E. crepuscularia. In spite of these doubts about identification it is obvious from the mass records submitted to us that E. bistortata is well distributed and fairly common in birch and larch woods. [Ectropis crepescularia] Here we have fewer records and even with a few of the bistortata records possibly incorrectly allocated, it still seems to be a very much rarer species than the previous one [Ectropis bistortata]. The similarities and differences were first commented upon and discussed in the 'Vasculun' by the late Professor J. W. H. Harrison. He undoubtedly was one of the first workers to fully understand the situation. We have no hesitation in accepting all his records from Whittle Dene, Chopwell Wood, Dinsdale Woods and the coastal denes in Durham. [...] The rest must wait for a further more critical investigation. TCD has recently seen a good specimen captured in Sacriston Wood by R. Woods in 1982.