Adults: Head and thorax are black, with some yellow parts including mouthparts.

[10] Though it was only introduced to North America from Europe in the 1950s, where it naturalised,[4] its range continues to expand.

Monostegia's economic importance lies in the destructive habits of its caterpillar-like larvae, which feed on the leaves of plants of the family Primulaceae, principally Lysimachia (such as yellow loosestrife, (Lysimachia terrestris)), and Anagallis (pimpernel).

[10] Original reports in North America involved Lysimachia nummularia as the host plant, but L. terrestris was identified in the 1960s.

The mature larva (sixth instar) stops feeding and drops to the soil where it burrows and pupates.