Bean seed fly (seedcorn maggot)
Adult: 3 to 6 mm long. The grey to yellowish thorax and abdomen bear a brownish median longitudinal band, the legs are black.
Egg: pearly white, elongated and about 1 mm long.
Larva: white maggot which reaches 5 to 8 mm in length at complete development; the last segment is obliquely truncated.
Puparium: red-brown, it measures about 5 mm.
The larvae destroy the shoots of seeds or bulbs in course of development and thinned-out plants. They often cause the plant to rot, leading to the more or less total destruction of seedlings and beds. On asparagus, the attacked spears are deformed, often split, and have a bitter taste.
The damage depends on the size of the larval population, which is governed by the attractiveness of the ground to the adults (freshly overturned soil, rich in recently buried organic matter). In addition, any factors which slow down the speed of germination and the shoot's vigour increases the risk of damage, in particular, low temperatures and excessively deep seedlings.