Delia florilega Zetterstedt Diptera Anthomyiidae

Natural History

Distribution. The distribution of bean seed maggot is not precisely known because it is often confused with seedcorn maggot, Delia platura (Meigen). However, it is known as a pest in the northeastern and midwestern areas of the United States, and in Canada as far west as the Prairie Provinces. It also is found in Europe, its likely source, though its date of introduction to North America is unknown.

Host Plants. Bean seed maggot has a wide host range, though it is usually confused with seedcorn maggot, so its host preferences are not well known. It attacks and develops successfully on such vegetables as cantaloupe, corn, kidney bean, pea, snap bean, squash, and probably onion, potato, and pepper. It also attacks field corn, soybean, and perhaps others. Hosts are more attractive when bacteria and yeast are present (Kim and Eckenrode, 1987). Adults feed on nectar from such flowers as dandelion, Taraxacum officinale, and also on aphid honeydew.

Life Cycle and Description. The temperature threshold for development is about 5°C. The optimal temperature for development, as determined by rapid development and maximal survival, is about 25-30°C. The time required for its development from the egg to the adult stage is 19.2 days at 30°C. Unlike the case with seedcorn maggot, which apparently aestivates at high temperatures, rearing temperatures of up to 35°C do not induce aestivation (Kim and Eckenrode, 1987). There likely are 2-3 generations annually in most locations, though Miller and McClanahan (1960) suggested four generations for southwestern Ontario.

  1. The bean seed maggot egg is elongate, and white in color. It measures about 0.9 mm long and 0.3 mm wide. The egg is convex on one side and slightly concave on the other side. It is indistinguishable from that of seedcorn maggot. They are deposited near the surface of the soil in the proximity of food. The egg stage requires 1.4 days for hatching at 30°C, but development time is little influenced by temperatures in the range of 10-40°C. Oviposition potential varies considerably, and depends significantly on temperature. Mean egg production per female was 193,99, and 47 for flies reared at 20, 25, and 30°C. The eggs are deposited throughout the life of the female. Oviposi-tion can occur at any time of day, but occurs predominantly during the afternoon.
  2. There are three instars, with average development time of 1.4,1.4, and 6.1 days, respectively, for instars 1-3 at 30°C. The larva is white in color, and bears anterior spiracles with 5-8 lobes. The larva attains a length of about 7 mm at maturity. The larva feeds on the embryo of the plant or on the below-ground portion of the seedling.
  3. The oval puparium is reddish or dark-brown, and measures about 5 mm long. Pupation occurs in the soil adjacent to the food plant. Duration of the pupal stage averages 9.7 days at 30°C.
  4. The adults are grayish-green flies, measuring 4-5 mm long. They closely resemble seedcorn maggot, and are separated using the bristles on the legs. In males, the dorsally directed bristles of the middle metatarsus are twice as long as the width of the tarsus, whereas those of seedcorn maggot are no longer than the width of the tarsus. In females, there are four or fewer bristles on the mesothoracic tibia, whereas in seedcorn maggot the number is five or greater (Kim and Eckenrode, 1983; 1984). Mated females aged 4-6 days are capable of oviposition. In laboratory studies conducted by Kim and Eckenrode (1987), the adult females lived an average of 22.8 days, but males lived only 7.1 days when reared at 30°C.
Diptera Caudal Spiracles
Bean seed maggot larva, posterior view showing caudal spiracles. * Color figure 263 is referred to on page 208.

Useful accounts of bean seed maggot biology were given by Throne and Eckenrode (1986), and Kim and Eckenrode (1987). Miles (1952), and Miller and McCla-nahan (1960) provided information, but these authors treated bean seed maggot and seedcorn maggot together. An interesting account of Delia ecology, and implications for management from a British perspective, is found in Finch (1989).


Bean seed maggot damages seeds and young plants in the same manner as seedcorn maggot. Basically, larvae burrow into developing seeds and the below-ground portions of seedlings. Stand density is often reduced, though plants sometimes recover from injury. Bean seed maggot often occur in mixed populations with seedcorn maggot. In a study conducted in Ontario, Miller and McClanahan (1960) found that about 11% of maggots were bean seed maggot, and the balance were seedcorn maggot. Bean seed maggot has also been implicated in the transmission of Erwinia bacteria. See the section on seedcorn maggot for a more complete discussion of damage.


Management consideration discussed in the section on seedcorn maggot are also applicable for bean seed maggot. In one of the few management studies directed solely at bean seed maggot, Kim et al. (1985) reported that rapidly-germinating, colored-seed bean lines were more resistant to attack.

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