This species-group includes only L. secedens, an aberrant species within Lasionycta with a nearly filiform male antenna and several unique features of male and female genitalia (Figs 138 and 196). Th e uncus is short with a pointed tip, the valves taper from base to apex due to a large sacculus and small cucullus, and it is the only Lasionycta lacking a postsaccular flap. The aedeagus has a unique stout distal spine and long serpentine vesica with a long distal field of small setae. Females are the only members of the genus with an asymmetric widening of the left side of the posterior ductus bursae, a strongly spiraled appendix bursa, and fewer than three signa on the corpus bursae.

The rationale for including L. secedens in the genus is outlined in the Lasionycta Remarks section. Th e weakly spiraled appendix bursae and narrow male antennae of L. phaea suggest that it is an intermediate between L. secedens and more typical members of the genus. Th is is supported by the fact that L. secedens and L. phaea are placed adjacent to each other on CO1 distance analysis when all species-groups are included. Similarly, L. mutilata species-group males have a raised patch of multiple spines on the apical aedeagus and females have a corresponding widening in the posterior ductus bursae intermediate between the L. secedens species-group and other Lasionycta speciesgroups that lack both characters. We consider the absence of the male postsaccular flap in the L. secedens species-group to be a secondary loss.

Lasionycta secedens is widely distributed in the boreal zones of Eurasia and North America.