Kilerobbins!
Where are you located? In America we have iris borer (Macronoctua onusta) that feeds within the leaves in May and later moves to the rhizome (July-August).
Cleaning up dead leaves and stalks in late autumn and in spring helps get rid of the eggs.
Some people apply a systemic insecticide to the soil in spring (imidacloprid).
Do you have pictures of the pest?
