Problem type: Insect
Name of problem: Onion maggot
Plant name(s): Onion (white, yellow and red), bunching onion (green onion), shallot, chives, garlic, leek
Symptoms / Characteristics:
In Manitoba, the first symptoms may appear in early summer. Damage is due to larval tunneling and feeding and may continue throughout the growing season. Symptoms vary according to the maturity of the onion. If the plant is attacked shortly after emergence (during the loop stage) the seedling withers and dies. Damage during the seedling stage can be particularly devastating. If seedling death occurs before the larva is full grown, it will proceed to feed on adjacent plants, potentially killing dozens of seedlings. If an attack occurs after the plant has developed two or three leaves, the leaves become chlorotic and wilt, as the stem begins to rot just below the soil surface. Plants that are pulled at this stage will break off at the stem, exposing the feeding larvae. Plants that are attacked after bulb formation seldom die. Although aerial symptoms may go unnoticed, underground bulbs will likely be deformed. Larval feeding may even cause the complete hollowing of smaller onions. Secondary rots typically accompany onion maggot feeding damage. Onion maggot damage is commonly confused with cutworm damage and Fusarium basal rot.
The onion maggot, Delia antiqua, overwinters in the soil as a pupa. Adult flies, resembling the common housefly, emerge in late spring and are often seen hovering around dandelions. As the adults mature, they become sensitive to the onion odor and mating typically occurs in or near onion plantings. Later, the female deposits batches of white eggs in the soil adjacent to onion stems. When the larvae hatch, they proceed to tunnel into and feed on the stems and roots. Feeding continues until they reach a mature length of approximately 1 cm, at which point they return to the soil to pupate. These legless larvae (maggots) are whitish in color and narrow at one end, with black feeding hooks. In Manitoba, there are usually two generations of onion maggot per year. Other provinces in Canada may have up to three.
Control / Preventions:
First of all, it is important to confirm that the onion maggot is responsible for the damage. Begin monitoring for adults in late spring. Registered insecticides are available if adult populations are extremely high. Distribute the onions throughout the garden instead of planting in high-density groupings. At the time of planting, a granular insecticide may be applied with the seed to help control early generation maggots and reduce future outbreaks. Diseased or damaged plants are extremely susceptible to onion maggot damage. During the growing season, keep the plants as healthy and vigorous as possible. Good sanitation is the best method of control. In the fall, make sure that all harvested onions, cull piles and crop residues are removed from the field so that any larvae preparing to overwinter will starve and die. Avoid planting onions in the same place in the garden year after year. If space permits, move them to a different place in the garden, or otherwise, rotate them with non-host crops. There are several flies, beetles and wasps that are natural predators of the onion maggot.