Very few pests and diseases trouble Hemerocallis. The major pest is the Hemerocallis Gall Midge, which seems to affect only the early-flowering buds of some Daylilies. If the gall midge has been active in your area, you will notice that some buds are swollen and distended, which indicates the presence of gall midge larvae. Picking off infected buds and destroying them is probably the best method of control. It may also be worth growing later flowering daylilies as they are unlikely to be affected.
The picture on the left shows the typical shape of a hemerocallis bud infected with the Hemerocallis Gall Midge larvae. Such buds should be removed and destroyed. The picture on the right shows an infected bud broken open to show Gall Midge larvae.

The RHS website has a map of areas where this pest has been reported, which is helpful.

Slugs and snails are usually a minor problem even though they can damage new divisions and seedlings in longer periods of rainy weather. Daylilies are not troubled by Lily Beetle or any other problems of bulbous lilies.