New to Britain in 1989, hemerocallis gall midge is now widespread in England and Wales. It can have a serious effect on the ability of daylilies to produce flowers in early summer.
What is hemerocallis gall midge?
Hemerocallis gall midge is a tiny fly that lays eggs on the developing flowers buds. The feeding activities of the larvae inside the buds cause abnormal bud development and infested buds fail to open during late May to early July.
If the foliage of your daylilies appears normal but the flowers abnormal in the ways described below, hemerocallis gall midge is almost certainly to blame;
- Infested flower buds are shorter and much fatter than normal daylily flower buds
- Such buds fail to open and either dry up or go rotten
- Numerous almost transparent maggots, up to 3mm long, may be seen inside the buds, where they crawl around in a watery liquid
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 pest comes to an end by mid-July.
Some hemerocallis species and cultivars have all or most of their flowering period after the gall midge’s egg-laying period is over and so most of their flower production is unaffected.
None of the pesticides currently available to gardeners has a label recommendation for use against hemerocallis gall midge. However, RHS research has shown that systemic pesticides based on thiacloprid or acetamiprid (e.g Bug clear ultra or Bayer Provado ultimate bug killer) applied according to manufacturer's instructions during June can reduce (but not eliminate) infestations. Insecticides should not be sprayed onto open flowers due to the risk to pollinating insects.
Note: these sprays are not suitable for use on plants where it is intended to use the flowers for culinary purposes.
Hemerocallis gall midge has one generation a year. Adult midges emerge in May-June and lay eggs on the developing flower buds on daylilies.
The larvae feed inside the buds and those that are infested develop in an abnormal manner. 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. As many as 382 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 down into the soil where they overwinter inside silk cocoons. Galled flower buds either rot or dry up without opening.
Common name Hemerocallis gall midge, daylily gall midge
Latin name Contarinia quinquenotata
Plants affected Daylilies, Hemerocallis species and cultivars
Main symptoms Abnormally swollen flower buds that fail to open
Caused by Larvae of a small fly
Timing Late May-early July