According to the American Hemerocallis Society (the worldwide governing body for hemerocallis):

Contarinia quinquenotata is a small fly which has is one of the few pests of daylilies in Europe and Canada. Maggots develop inside daylily flower buds causing them to become inflated, distorted and unable to open properly.  Some buds may dry up.   Reports indicate that in some cases clumps may be so badly affected that few buds open normally.  Early flowering daylilies are typically the most heavily infested and egg-laying may have ceased by the time later flowering daylilies are forming buds, allowing them to escape much of the damage. Infested buds may contain from one or two up to a hundred or more small white legless larvae up to around 0.12" in length which, when sufficiently mature, fall to the ground where they overwinter.  Thus far it appears this insect only has one generation each year.  In spring they emerge as adults and fly to daylily buds to lay their eggs.  Because they are inside the galled buds, larvae are protected from contact insecticides.  Also, since the adults are flying for several weeks each season, it will be difficult to provide sufficiently continuous contact insecticide coverage to prevent egg-laying. Treatment involves removing and collecting affected buds as soon as it is obvious that they have been attacked.  These buds must then be destroyed so that the maggots within them cannot continue their life cycle.  Do not compost infested buds unless the larvae have already been killed by some means.  Daylilies purchased in pots or with intact budded scapes are more likely to introduce the pest to a new area than daylilies acquired bare-root and without scapes. Some gardeners destroy the maggots by burning the infested buds, but other alternatives need to be determined where burning is not possible.  

What does Gall Midge/Galled buds look like?

Galled Buds look different on different cultivars and will show differences dependent on when in their development they were infected. A general clue is to look at the buds on a plant as a whole. The infected buds will have a different shape than the healthy buds.  They will appear swollen at the mid base of the bud while remaining constricted just below the tip. They often show a scalloping at the petal lines and flaring at the tip of the petals.  See photos below.

When does Gall Midge infection occur?

In the UK the infestation occurs early-ish in bloom season. In a 'normal' year (do we have those anymore?) it usually does not affect the Early Early blooms but starts with the Early and Early Mid season blooms.This is totally weather dependent so it can occur in Early bloomers one year and not until Early Mid bloomers another year. It only lasts a few weeks and will not recur during the rest of the season.

Are some cultivars more resistant?

We have found that some cultivars are very badly affected one year and not at all the next. This inconsistency seems pretty consistent. However, we have found that there is a much higher infestation of blooms that have a darker colour on the outside of the sepals even though the bloom colour may be a lighter colour. Also, can the midge see ultraviolet or other wavelengths unseen by human eyes as do bees and butterflies?

What can I do to lessen the infection of Gall Midge?

Firstly, if you already have an early season blooming hemerocallis cultivar that you can not live without then the answer is 'diligence'! You must pick the infected buds as early as you suspect a problem. That means inspecting your plants every few days (preferrably every day) for galled buds. Do not wait until the bud drys and shrivels as 'Elvis has already left the building' at that point. I have seen many articles stating that the galled bud drops off and then the maggots move into the soil to pupate. NOT true. I have also heard that they 'climb' down the scape to the soil. I have 'bagged' galled buds on plants with organza bags to study how and when they escape from the bud. The bud cracks slightly and they literally ping out. They get caught in the bag and are never at the bottom but at bud height or higher. Usually this occurs way before bud drop which can occur several days later or not at all. So pick them early and without hesitation if unsure if the bud is affected or not. If you have a severe infestation try cutting the entire scapes off of the early hemerocallis for one year.

Secondly, buy and grow Mid and Mid Late season bloomers.

Thirdly, do not buy hemerocallis in pots or with scapes. This is an invitation to establish an infection in your garden. Buy from established hemerocallis nurseries that are bare rooted, trimmed and washed.

What do I do with the galled buds I collect?

Bag them securely (zip lock type bags) and place them in the freezer for a few days. DO NOT dispose in your garden waste bin or your compost pile prior to making sure they are dead. Make sure they are dead prior to disposing in your black bin. You can also place in paper bags and burn them immediately upon collection.