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Scientific Name: Delia antiqua

The onion maggot can be the most damaging insect pest of onions throughout the northern regions where onions are grown. If not controlled, onion maggot can prevent the production of a marketable crop. In Ontario, the onion maggot has become difficult to control. Resistance has developed to many of the insecticides used as granular furrow treatments at seeding. 

Identification

• Small (<5 mm or ¼ in.) translucent white maggots
• Legless and oblong-shaped with a pointed anterior
• Adult is a small (5 mm, ¼ in.) grey-black fly
• Infestations often result in wilted transplants and poor stand establishment

Often Confused with
Damping-off and root rots
Onion smut

Biology
The onion maggot overwinters in soil as a pupa. Adults begin emerging in mid-to-late-May in the onion-growing areas of Ontario. During late May and June, onion maggot flies can be seen in onion fields and often on dandelions around field borders. Adults mate when they are about 6 days old and begin laying eggs after an additional 3 to 4 days.  Each female will live about 30 days and may lay up to 200 eggs around the base of onion plants.  Eggs hatch within a few days and the larvae bore directly into the onion plant. These larvae, tiny, white, legless maggots grow to a length of about 1 cm when mature, and have conspicuous dark feeding "hooks" on the tapered end of their bodies. They may attack the onion at any stage of development and feed within the plant for about 2 to 3 weeks. When larva become full grown, they leave the onion and pupate in the soil.

Period of Activity
In Ontario, there are three generations of onion maggot each year.  Damage from the first generation is usually seen mid-to-late June as wilted onion seedlings.  Second generation damage occurs in July, when the onions have begun to form bulbs and feeding by larvae does not kill the plants.   However, damage at this time will result in grossly misshapen onions that are not suitable for market. In addition, secondary rots may set in following the onion maggot damage and the bulb will be unmarketable.  By the time the third generation of flies is laying eggs, most onions will be in windrows drying and eggs are laid directly on the bulbs or on the soil immediately next to and beneath the drying onions. Maggots hatching from these eggs can bore into the onion bulb at any point, but tend to enter around the root areas. These maggots may be taken into storage with the result that both direct damage from the maggots and secondary storage rots cause heavy losses of marketable onions.

Scouting Notes
First-generation onion maggot thresholds are a combination of trapping and visual scouting.  Place four traps (of 3 sticky boards each) centrally along each field edge.   Check traps weekly.  Assess the percent damage using plots containing 100 onions repeated four times within the field.

Thresholds

	TRAP COUNTS	ACTION
< 2% DAMAGE	--	DO NOT SPRAY
2-5% DAMAGE	AND 5 FLIES/TRAP/DAY	SPRAY
> 5% DAMAGE	AND 2 FLIES/TRAP/DAY	SPRAY

Management Notes

• Good sanitation is extremely important in controlling onion maggots. Removal of onion residue from the field and cull pile management will help reduce feeding and overwintering locations for future generations.
• The most important aspect of chemical control with granular insecticides is correct rate and placement. 
• Maggots do not start to develop unless the temperature is above 4°C. Degree day model thresholds are 210, 1025 and 1772 degree days for first, second and third generation, respectively.

Traps for adult flies do not need to be placed in every onion field on the farm; however, traps are useful tools for gauging the trends in the fly population. Onion flies are not easy to identify on such traps and it is advisable to find someone who is familiar with identification to assist you.