Overwintered adults of Nanophyes marmoratus appear on purple loosestrife in mid-spring. The beetles start feeding on the youngest leaves but as soon as flower buds develop, beetles move to upper parts of flower spikes where they mate and feed on flower parts. Egg laying starts soon thereafter and continues into August. Eggs are laid singly into the tips of flower buds before petals are fully developed. Larvae first consume stamens and, in most cases, petals, followed by the ovary. Mature larvae use grass to form pupae chambers at the bottom of the bud. Attacked buds remain closed and often drop to the ground. The new generation of beetles appear mainly in August and feed on the remaining green leaves of purple loosestrife before overwintering in the leaf litter. Attacked buds that remain on the stem often show characteristic exit holes. Complete development from egg to adult takes about one month. There is one generation a year. Adult and larval feeding causes flower-bud abortion, thus reducing the seed output of purple loosestrife. Attack rates can reach over 70% in Europe.
Nanophyes damage to purple loosestrife flowers.