Natural Insecticide for Little Flies in Narcissus Bulbs

Natural Insecticides

• Pyrethrin or rotenone dust are commonly available organic insecticides that may deter bulb flies from laying their eggs on or near the bulbs' foliage. Apply these according to directions and timed with the emergence of the flies. The latter step is complicated because the lesser bulb fly has three generations, in spring, midsummer and late summer, respectively. The narcissus bulb fly, on the other hand, has only one generation, in spring. The flies can live up to 3 weeks, requiring frequent rounds of pesticide, both of which also kill beneficial insects.

Cultural Control

• Cultural control of bulb flies is more straightforward than timing pesticide application with the emergence of the flies. Planting bulbs deeper -- to 10 inches if there is good soil drainage -- will deter the larvae from traveling from the soil surface to the bulb. Planting bulbs amidst an annual or perennial cover crop will prevent flies from laying eggs in the bulbs' foliage. Cutting foliage back as soon as it yellows, usually in early summer, is also advisable. Finally, destroying any bulbs with abnormally thin and distorted leaves will kill any larvae harbored within them.

Biological Control

• In some regions of the country, adding beneficial nematodes to the soil may interrupt the larval life cycle. Nematodes are microscopic roundworms commonly available at garden centers. Look specifically for those with the scientific name "Steinernema feltiae," which will invade and destroy the bulb fly larvae near the soil surface. Nematodes can live in the soil for three years but will not survive winters in the northernmost regions of the country. Northern dwellers may have success, however, if they apply the nematodes each spring.

Mechanical Control

• Some gardeners put a physical barrier over the narcissus foliage to prevent adult flies from laying eggs on or near the bulbs and leaves. Floating row cover, a lightweight synthetic fabric, keeps insects out while permitting sunlight and moisture to penetrate. Remove the fabric once the leaves yellow in the heat of early summer. While this method may not enhance the aesthetics of the landscape, it is sometimes the best way to survive an outbreak of the bulb flies, particularly if there is a large planting of bulbs.