The tiny gall midge lays eggs on the plant and the larvae develop inside the individual flower buds, inside the flower head sheath or in the petals of flowers that have gone over. The larvae can then cause the bud to be deformed and discoloured and often fail to open, as their feeding activities convert the plant material into a gall. The severity of the damage can range from a couple of buds failing to collapse of the entire flower head.
Infestation can be confirmed by opening the buds or flower heads and looking for the presence of small maggots 1-3mm in length which are a creamy yellow colour. The midge larvae leave the flower head to pupate in the soil, which takes around ten days. It is likely that they also overwinter in the soil and pupate the next spring.
The larvae can live in any stage of flower development, including in senesced flowers. Larvae can most commonly be seen inside individual flower buds, but if infestation occurs before the flower head sheath opens then the larvae can live and feed between the developing flowers and cause complete failure of the flower head.
Our research so far has shown that there may be multiple overlapping generations of the midge, as active larvae have been seen between mid-June and early October.
So far no biological or pesticide treatments have proved effective at targetting the larvae in the flowers. This is unsurprising as they develop inside the plant tissue and the long active period of the adults makes targetting egg-laying females difficult. The underground pupation and overwintering life stage is likely to be the most useful target for control.