Daylilies are relatively care-free members of the perennial garden, but even these easygoing plants have habits that make them attractive to garden pests that grow in the cool soil around their bulbous roots. Fungus gnats are true pests--a bother rather than a threat to life for the resilient daylily. Left untreated though, they can invade not only garden plants like daylilies but get indoors to plague houseplants.
Hemerocallis, once considered weeds, are valued members of today's most refined gardens. Perennial natives of North America and Asia, they reproduce prolifically by division as well as seed. Their mounds of grass-like leaves and cluttered clumps create a cool, damp forest of plants that protects their bulbous roots and provides an ideal breeding ground for certain insects. To dayliliy's credit, it is not attractive to many pests and diseases.
Adult fungus gnats are black-winged insects similar in appearance to mosquitoes but less than 1/10 inch long. Juvenile gnat "larvae" are slightly longer---about 1/4 inch long. Their black heads are at the end of a long white body. Fungus gnats have a life span of about one month; they spend up to 2 weeks in the larval stage and slightly less as winged adults. The female gnat will lay as many as 300 eggs before she dies. The clumps are scattered in soil near sources of food; soil rich in organic or decaying matter and fungus. It is the larval form that is found feeding on daylily roots and stalks. Because the length of each stage and transition period vary by a few days, a colony of fungus gnats, once established have insects in every phase co-existing at once.
The main danger from fungus gnats has always been that simply of nuisance, but they are becoming more than that in greenhouses and indoor gardens where they can reproduce all year around. Larvae travel in unsterilized garden and potting soil and live in wet areas of trays and potting benches where fungi grow. They will eat young plants and burrow into stalks. The daylily, with its tightly-wound stalk of leaves provides a convenient apartment building for fungus gnat larvae. Since the gnat's primary diet consists of fungus, it can distribute fungal pathogens as it deposits eggs on top of larvae throughout soil and on plant roots.
Fungus gnat invasions are unsettling. Adult gnats emerge as if evolving from the soil itself and rise away from their larval homes. The damage they do to daylilies is found around the crown of the fan (the part of the stalk that turns white about an inch below the surface) and in the fat swellings along the roots. Both areas may have holes where larvae have eaten their way. Roots may be mushy instead of firm. Flour bug traps can be placed near plants to monitor populations.
Prevention and Treatment
Plants should be divided when they become crowded and affected plants destroyed. Predatory and parasitic insect controls are available as well as soil drenches. Avoid over-watering daylilies and never plant them in or near areas where water stands after heavy rains. Use only sterilized soil for container garden daylilies. Provide drainage so water cannot stand in the bottom of pots or planter boxes and allow soil to dry between watering to discourage return infestations.