Description: The Hemerocallis gall midge is a tiny fly about 0.08 inch (2 mm) in size and greyish-brown in colour with translucent pink wings. Very difficult to see with the naked eye. They emerge from the soil in late April or early May to begin mating. The female has a retractable ovipositor with which she can pierce newly forming daylily flower buds to deposit her eggs inside.
Once hatched, the small, legless, white maggots feed and develop inside the flower bud. This causes the bud to become misshapen and deformed, failing to open. If you have this pest in your garden, you will see these affected buds from early May through to early July…usually amongst the earlier flowering daylily varieties.
The maggots get to about 0.12 inch (3 mm) long and drop to the ground in July where they spin their silk cocoons in the soil and pupate over the winter. Adults emerge the following spring in late April or early May.
There is one generation per year.
Treatment and Control: Yellow sticky traps set up in and amongst the hemerocallis plants have proven somewhat effective against the adult gall midges in catching them before they have a chance to lay their eggs. Once the eggs have been laid, there is no effective treatment, organic or otherwise, to deal with the larvae as they are “cocooned” within the developing flower bud. Best line of defense at this stage is due diligence in keeping a close watch for any deformed buds and removing them for disposal into the garbage. By eradicating the affected buds, the gall midge numbers are at least kept in check…hopefully wiped out in due course. But this may be too much to hope for…especially if your neighbours are not picking off the affected hemerocallis buds in their gardens as well.
Additional information re Hemerocallis gall midge
Jump ahead a few years and observation of the gall midge pest has provided some more useful information. Of particular note is the fact that most of the gall midge damage appears to be on the early flowering hemerocallis plants. And yellow seems to be the predominant flower colour under attack, although other colours of daylily flowers can suffer bud damage too.
Thank goodness for the good plants people and scientists at the Royal Horticultural Society’s (RHS) Garden at Wisley. There is a large Hemerocallis collection at this garden which came under attack of Contarinia quinquenotata in the 1990s. Meticulous records were kept for a number of years on everything to do with daylilies and the gall midge. One of the results: they were able to assemble an admirable list of daylilies species and cultivars which are able to provide a worthwhile floral display in the garden after the gall midge attacks run their course in late July.