Pest Management Fact Sheet #5031
James F. Dill, Pest Management Specialist
Clay A. Kirby, Insect Diagnostician
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Description & Biology
The onion maggot (Delia antiqua) is one of the most destructive insect pest of onions and related plants. Injured seedlings wilt and die. Larger bulbs may survive some injury, but are often poor keepers. Once onion maggots infest an area, they tend to be a problem every year. White onion varieties are more susceptible to attack than other varieties.
Onion flies and maggots are very similar to the seedcorn maggot, Delia platura. The grayish brown adult is less than a 1/3″ and lacks the cabbage maggot’s three dark thoracic stripes. These flies will generally stay hidden. When mature, the whitish maggots are about 1/4″ long.
The onion maggot overwinters in the pupal stage under debris in or near the soil surface. In about the third week of May, when onions are up or planted, the flies emerge to lay their eggs near the onion plants. In less than a week, the eggs hatch.
The maggots feed for two to three weeks and then pupate. In about two weeks, the second-generation adults emerge from the pupal stage. Cool, wet weather favors development of three generations per year. A complete life cycle takes 45 to 65 days.
Stunted or wilting onion plants are the first signs of onion maggot damage. At this time, you may find the maggots in putrid, decomposing onion plants. Light infestations may not kill onions, but may make them more susceptible to rots. Onions of all sizes may be attacked, especially in the fall, when cooler weather favors the maggot’s activity. Damaged onions are not marketable and will rot in storage causing other onions to rot.
The onion maggot has many natural enemies such as ground beetles, birds, parasitic wasps, nematodes and a fungus that is most effective in cool, wet weather. There seems to be no resistant varieties at this time except for a Japanese bunching onion that, at times, shows resistance or at least tolerance.
Rotation as a method of maggot control is not practical for the home gardener, but controlling wild onions should help. Volunteer onions and chives may be a source of infestations. The onions should be removed and the wastes burned and not plowed into the soil. Highly organic soil may be more attractive to the flies. Barriers (such as floating row covers) may help protect onions from the adult fly.
Controlling the spring infestation is most important because small or new plants are most easily damaged. Killing flies or maggots in the spring helps to lower their populations in the fall.
When Using Pesticides
ALWAYS FOLLOW LABEL DIRECTIONS!
Pest Management Unit
Cooperative Extension Diagnostic and Research Laboratory
17 Godfrey Drive, Orono, ME 04473
1.800.287.0279 (in Maine)
Information in this publication is provided purely for educational purposes. No responsibility is assumed for any problems associated with the use of products or services mentioned. No endorsement of products or companies is intended, nor is criticism of unnamed products or companies implied.
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