The seed maggot generally overwinters as a mature larva in a puparium (pupal case) 8-15cm (3-6 in.) deep in the soil. Adults occasionally hibernate and become active very early in the spring. The adult is a greyish-brown fly, similar to a housefly, but only one half the size. They become active in late April, and their activity peaks through May and June. The generations overlap so that adults are present from late April to early December. Seed maggots infesting early seeded crops such as peas, corn, early cole crops, radishes, onions and spring cereals are usually those from the overwintering generation. When the season is wet and cold, the first generation is delayed and can cause serious damage to beans.
In normal years the second generation has the greatest impact on bean culture and is the most destructive. Neither the third nor the fourth generations are of economic importance, although fall seeded cereals and legumes are sometimes slightly infested. Seed maggots are attracted to freshly cultivated soil. The warm, moist earth that has been exposed attracts the flies and stimulates them to lay eggs. Often before seeding, eggs or maggots are present in the soil as a result of egg lying during seedbed preparation.
Decaying organic matter has also been associated with seed maggot egg lying. Often, high infestations follow spring plough-down of green manure or early crops such as spinach. Other egg-laying attractants are certain germinating seeds especially legumes, corn and cucurbits. The stage of germination also has an effect on the degree of attraction.
Even with heavy populations of seed maggots, infestations are often sporadic and difficult to predict, since weather and soil conditions influence the degree of injury. This pest has the potential of causing severe economic losses, necessitating protective seed treatment at planting time.