The larvae of I. plena feed on herbaceous plants including Fuchsia excorticata, Coprosma species, and introduced species such as garden fuchsia as well as crops such as apple trees.

In 1880 Arthur Gardiner Butler, thinking he was describing a new species, named it Dianthoecia viridis.

[3] In 2019 Robert Hoare undertook a major review of New Zealand Noctuidae species.

Its general colour is pale pinkish-brown becoming pale green on the ventral surface; there is an obscure wavy blackish lateral line, stronger near the middle of each segment; a wavy blackish subdorsal line stronger near the middle of the posterior segments and appearing from above as a series of very slightly oblique blackish marks.

Palpi hardly extending beyond the head; second joint densely pilose; third conical, less than one-fourth of the length of the second.

Abdomen brown, extending rather beyond the hind wings; apical tuft of moderate size.

Fore wings partly black-shaded, with several denticulated transverse black lines; orbicular and reniform marks large, pale green, black-bordered, slightly tinged with red, of the usual form; some pale cinereous points on the exterior part of the costa; marginal points black.