Hemerocallis gall midge is a tiny fly that lays eggs on the developing flowers buds of day lilies. The feeding activities of the larvae inside the buds cause abnormal bud development and these buds fail to open. 

Gall midges are a family of flies, the Cecidomyidae, there are more than 600 species found in Britain.  As adults most are small brown or black flies, they do not bite.  As the name suggests many species feed as larvae within plant tissues causing galling and distortion. Some are however, predatory on aphids and mites whilst others feed on rust fungi.
 

Seen the hemerocallis gall midge or its effects? We would like to know.

As part of RHS research we would like to know where the hemerocallis gall midge has been seen.

Please submit your records via our hemerocallis gall midge survey (expected time to complete survey = two minutes).

Submissions to these surveys are stored permanently in an anonymised form in order to monitor the spread of the organism . We may contact you within 2 months of your submission in order to verify your sighting but your personal data will not be permanently stored in connection with your submission and will be deleted after 1 year. We publish and share only non-identifiable data from survey submissions (such as a six figure grid reference) with third parties and the public for the purposes of scientific research and advancing understanding among gardeners.

Thank you to everyone who has submitted records so far – read a blog about the surveys 

If the foliage of daylilies appears normal but the flowers abnormal in the ways described below, hemerocallis gall midge is almost certainly to blame;

• Affected flower buds are shorter and much fatter than healthy daylily flower buds
• Such buds fail to open and either dry up or rot
• Numerous almost transparent maggots, up to 3mm long, may be found inside the buds, crawling around in a watery liquid

Hemerocallis gall midge affects the flowers but not general health of host plants.

Pick off and destroy galled buds as soon as they are seen. Encourage other gardeners who grow daylilies in nearby gardens to do the same. Damage by this insect comes to an end by mid-July.

Hemerocallis cultivars that have all or most of their flowering period after the gall midge’s egg-laying period (May-July) is over are often unaffected. 

Hemerocallis gall midge has one generation a year. Adult midges emerge in May-June and lay eggs on the developing flower buds of daylilies.

The larvae feed inside the buds causing them to develop in an abnormally. Instead of being long and slender, infested flower buds are shortened and have an enlarged conical shape. The larvae are up to 3mm long and almost transparent, which can make them difficult to see in the watery liquid that accumulates between the petals in the base of the bud. Nearly 400 larvae have been found in a single flower bud; this is likely to be the progeny of more than one female midge.

When fully fed, the larvae go into the soil where they overwinter inside silk cocoons. Galled flower buds either rot or dry up without opening.