Eumerus tuberculatus Rondani Diptera Syrphidae

Natural History

Distribution. Onion bulb fly and lesser bulb fly are native to Europe. Apparently they were introduced to North America in the late 1800s—onion bulb fly was first recognized at Ottawa, Canada in 1904. Now, onion bulb fly is widespread in the United States and southern Canada, whereas lesser bulb fly is limited to the northern United States and southern Canada. These similar flies have been confused in the literature, and some damage by E. tuberculatus has been incorrectly attributed to E. strigatus (Latta and Cole, 1933). A third introduced species, Eumerus narcissi Smith, also is easily confused with the aforementioned species, but as it is not found in vegetables it is not to discussed further. Generally, onion bulb fly is more damaging in western North America, with lesser bulb fly more damaging in the east.

Host Plants. Vegetable crops attacked are onion and shallot, but more common hosts are bulbs of flowers such as amaryllis, hyacinth, iris, and narcissus. Reports of these insects attacking carrot and parsnip are almost certainly incorrect.

Natural Enemies. Natural enemies of these flies are not known.

Life Cycle and Description. Apparently there are two generations of these flies annually, although in Oregon a proportion of the second generation continues its development, producing a small third generation. Larvae are the overwintering stage, with adults from the overwintering flies present in April-May, and then again in June-August. There is no evidence that these two species differ in their biology.

  1. The eggs are usually deposited in the soil near bulbs, generally at a depth of less than 1 cm, but sometimes on bulbs or foliage. The eggs measure about 0.7 mm long and 0.2 mm wide. The egg is somewhat elliptical, but one end tapers to a sharper point. The color is white. Eggs may be deposited singly or in clusters of up to 30. Duration of the egg stage is 4-5 days.
  2. Larvae are wrinkled and flattened in appearance. Their general color is variable, ranging from white to greenish or reddish, depending on food substrate. Reddish respiratory processes project from the posterior end. Larvae attain a length of about 710 mm at maturity. Larvae disperse to bulbs, where they typically enter at the juncture of the foliage and bulb if areas of deterioration caused by plant disease are present. Healthy and wounded tissue lacking decay are not suitable for larval survival and growth. Larvae can attempt to feed on healthy tissue, and perhaps create openings that allow fungi or bacteria to gain entry, which in turn makes the damaged bulb suitable for the larvae. Larval development time is dependent not only on temperature, but on suitability of the food substrate. Martin (1934) reported that lesser bulb fly larvae initiating their feeding on diseased tissue completed their development in 22 days, whereas larvae that initially fed on healthy tissue (that later decayed) required an additional five days to complete the larval stage. Apparently not much development occurred until the bulb tissue began to decay.
  3. Pupation occurs in the soil, near the surface, in close association with the bulb. The puparium resembles the larva, except that it is relatively shorter and wider. It typically measures 6-8 mm long, and is gray or brown. Duration of the pupal stage is 9-13 days.
  4. The adult is a robust fly, and is rather short (5.5-7.5 mm) and wide. Even the legs are thickened. The body is metallic bronze or green, and the abdomen bears three white bands that have a gap dorsally. Legs are blackish-green but bear some red and yellow on the base of the tibia. Wings are transparent. Differentiation of the flies is based on genitalia, and generally requires the services of an authority. Copulation normally commences within three days of adult emergence, and egg deposition commences in another
Eumerus Tuberculatus
Adult onion bulb fly.

three days. Adults are observed to feed on flowers of Umbelliferae and Compositae. Adults often live 13-15 days. Total egg production is not well documented, but it is known to be at least 60 eggs per female. In all likelihood, egg production probably numbers in the 100s.

The biology of E. strigatus and E. tuberculatus were presented by Hodson (1927), based on research conducted in England. The biology of lesser bulb fly, based on work conducted in New York, was given by Martin (1934). Wilcox (1926) published a very nice study of bulb fly based on research in Oregon; the species involved is uncertain, but likely is onion bulb fly. Characters for differentiation of the species can be found in Latta and Cole (1933).

Damage

Larvae of both species are associated with bulb decay. Adults clearly prefer to oviposit on bulbs that are injured or infected with plant disease, and larval performance is enhanced by the presence of some disease. The exact role of the insect is uncertain, though there have been several attempts to elucidate the relationship of the flies with bulbs. It appears certain that the larvae require diseased tissue (Creager and Spruijt, 1935), but it is likely that larvae enlarge wounds and retard the normal healing process of the plant. Plants grown from seed do not incur injury, whereas those grown from bulbs are occasionally damaged. Commercial production most often involves growing plants from seed, so these insects tend to be home-garden pests.

Management

Recommendations for management usually involve sanitation. Bulbs displaying signs of decay should be destroyed, not just discarded. A water bath treatment consisting of 3h exposure at 43°C is reported to rid bulbs of both bulb fly and nematodes. Bulbs infected with nematodes are attractive to flies, and nematode injury allows invasion by larvae. The observation by Doane (1983) that onion bulb flies are attracted to, and oviposit on, oatmeal that has been moistened and buried suggests that this technique could be used for population monitoring, or even removal trapping.

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