Lesser Bulb Flies

Adult - Lesser bulb flies are 5 to 9 mm dark blue insects that have a metallic bronze sheen (they appear black at a distance). There are three pairs of grayish white crescents on the abdomen and yellowish hairs at the tip. The thorax has 2 lengthwise pale stripes. The hind legs have wide segments that look as though the legs are adapted for jumping.

Egg - Lesser bulb fly eggs are very small (0.72 by 0.24 mm), slender, and somewhat pointed on one end. They are usually laid in small clusters.

Larva - Maggots of the lesser bulb fly are tiny (0.75 mm) to small (7 to 10 mm) and white to grayish yellow depending on the quality of the host bulb. The larvae are wrinkled and covered with minute spines. They are slightly flattened (2.5 mm wide and 1.8 mm high) and are more slender than those of the narcissus bulb fly. The breathing tube is brick red or brown. There is a fleshy tubercle on either side of the breathing tube (no other maggot pests of bulbs have these tubercles).

Pupa - The pupa develops within the last larval skin, which becomes a tough protective covering about 6 to 8 mm long, 3 mm wide, and 2.8 mm high (the puparium). The puparium is light gray to reddish brown, but when the maggot pupates in the soil, the puparium is completely covered with fine particles of soil.

Fig. 99: Lesser bulb flies. Eumerus tuberculatus Rondani (the most abundant), Eumerus strigatus Fallen, and Eumerus narcissi Smith; Syrphidae; DIPTERA

A, Adult. B, Larva. C, Puparium.

Distribution - Lesser bulb flies were introduced into the United States from Europe. These flies occur wherever narcissus are grown.

Host Plants - Lesser bulb flies are sometimes damaging to onion, narcissus, and hyacinth. Other hosts include shallots, garlic, iris, lilies, parsnips, potato tubers, amaryllis, cabbage roots, Calla elliottiana, Eurycies, Galtonia, Gladiolus, Scilla, Sprekelia formosissima, and vallota.

Damage - Up to 25 percent of some varieties of narcissus and 10 percent of hyacinths have been infested by lesser bulb flies in some landscapes. The maggots scrape away the bulb tissue and tunnel in. The infested bulbs begin to decay and the interior of the bulb fills with a semi-liquid mass. The bulb may be killed completely, or damaged to the point that only stunted leaves appear the following year.

Life History - The winter is spent as maggots in the bulbs. In spring, the maggots migrate toward the soil surface and pupate. The first generation of flies emerges in April and May. These flies live about 3 weeks. The second generation emerges in midsummer, and a small third generation emerges in late summer. Female lesser bulb flies crawl down into cracks in the soil and lay eggs near, on or under the dead skin covering the bulbs. Some eggs are laid on the leaves at the neck of the bulbs and some are laid on the soil surface. Females lay their eggs singly or in small masses of 2 to 40. More than 100 eggs have been found around one bulb. The eggs hatch in 5 to 10 days. Newly hatched maggots emerge from the pointed end of the egg. Ten to 30 maggots may develop in a bulb (947 is the maximum number recorded from a single bulb). The tiny maggots usually attack the base of the bulb although sometimes only the upper portion of the bulb is infested. Infested bulbs begin to decay and the interior of the bulb fills with a semiliquid mass. Although the maggots can successfully attack a healthy bulb, the maggots cannot complete their evelopment in the absence of certain decay organisms. Bulbs infested with stem nematodes (Tylenchus dipsaci Kuehn) or infected with a root rot fungus are especialy ly vulnerable to attack. After about 30 days, the maggots mature and some of them crawl to the surface where they pupate inside the last larval skin. One to 4 weeks later, adult flies emerge from the puparia. If a source of nectar is available and temperatures are not extreme, some of the lesser bulb flies may live up to 36 days.

If the narcissus are planted as winter annuals and the bulb discarded after blooming in the spring, there will be no problem with lesser bulb fly maggot damage as the flies oviposit only during the warm growing season. If the bulbs are left in the landscape semi-permanently and only lifted every three or four years, then injury caused by the lesser bulb fly may become excessive if the flies are active in the neighborhood. Because of the extended flight period of the lesser bulb fly, it may be necessary to resort to an application of pesticides in neighborhoods in which the flies are active. Over-planting beds of narcissus with summer annuals may greatly reduce lesser bulb fly damage. Tilling the soil destroys the holes left by the dying narcissus leaves that the flies often use to reach the bulbs. Many of the tiny maggots may perish before they can reach the bulbs.

When digging, all bulbs should be removed. Soft, decaying bulbs should be destroyed. Bulbs in marginal condition can be treated in hot water (43 to 44°C) for 3 hours to control lesser bulb flies as well as stem nematodes.