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.
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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