“What’s wrong with my garlic?” my friend Jason posted in our local gardening group last summer. “I’ve been gardening for years, and I’ve never seen anything like it.” Many of his garlic heads were riddled with tiny white worms, which was a familiar sight to me and to many other local gardeners. His garlic was infested with onion root maggots, and so was mine.
A worldwide pest of allium family crops, onion fly larvae or onion root maggots (Delia antiqua) are most common in temperate climates with moist, moderate summers. High heat slows them down, so they are less problematic in climates where summer is hot and dry. Onion flies feed on all members of the onion family, including chives, garlic, leeks and all types of onions. In a pinch they can feed on carrots and parsnips. Damage is often most severe on garlic because after being planted in autumn, it is ready to be invaded when the onion flies emerge in spring.
![Onion fly]()
Known for its humped back, the adult onion fly resembles a small housefly
In most climates there are three generations each year. The first and most important batch emerges in mid to late spring, when the weed called yellow rocket (Barbarea vulgaris) blooms. Resembling small houseflies, onion flies promptly mate and then females lay 50-200 eggs at the base of plant stems and in nearby soil. Eggs hatch in only a week, and the whitish maggots feed on the roots and stems of host plants. After feeding for about a month, maggots become rice-size brown pupae that transform into adults in two weeks. A complete life cycle lasts about 50 days.
Controlling Onion Flies Organically
Commercial growers depend on pesticides to grow maggot-free garlic and onions, but there are other ways to control onion root maggots in an organic garden, including these nine simple methods:
1. Rotate allium crops
Rotating crops so that alliums are planted in the same place no more frequently than every three years can thwart attacks in spring and prevent the buildup of diseases in the soil. Be aware though that it will not cure the problem completely, because adult flies can travel a half mile (1 km) or more in search of host plants.
![Onion fly larva]()
Onion root fly larvae can quickly make their way into alliums in spring
2. Clear and mulch beds where onion flies have been a problem
After removing onions or garlic that hosted onion root maggots, mulch can make the soil more hospitable to ground beetles, which are important natural predators of onion fly pupae.
3. Grow more than one variety
Research from China indicates that garlic varieties with a high allicin content are non-preferred by onion flies. Variety and growing conditions affect allicin content, and I often have seen differences in maggot activity in different varieties. In my garden, big, vigorous hardneck garlic varieties like ‘Music’ and ‘Purple Glazer’ tend to show less damage than strains that produce smaller heads. I also have noticed that leeks are the last allium to show damage in bad years.
![Garlic varieties]()
Growing different garlic varieties can reduce overall crop losses due to onion root maggots
4. Exclude onion flies with insect netting or row cover
About two weeks before your last frost date, cover autumn-planted garlic and spring-planted onions with row cover or insect netting to keep onion flies from finding them. Every few days, lift the covers to release the many flies, gnats and other creatures that emerge from the soil and become trapped under the cover.
The first generation is active from mid spring to early summer. You can remove covers in midsummer when the plants are large and temperatures are warm. Late maturing long-day onions must be covered again in early autumn, when more onion flies emerge in response to cooler soil.
5. Trap onion flies under row cover
After last year’s garlic disaster, next spring I will cover the previously infested garlic bed with row cover and arm the interior of the tunnel with yellow sticky traps. This should reduce the number of flies looking for fresh victims.
![Garlic chives]()
Perennial alliums like garlic chives can host onion flies from year to year
6. Pot up perennial alliums
Where onion flies are truly terrible, consider limiting perennial onions such as chives and garlic chives to containers, because perennial alliums can host small populations year-round. The flies seem to avoid plants grown in containers, perhaps because of warmer root conditions.
7. Start with onion sets or robust seedlings
Onion flies seek out struggling plants, so the faster onions become established, the less damage you will see from this pest. If spring stays cold, delay planting until the soil warms, which will promote strong, fast growth.
![Onion fly pupae]()
Onion fly pupae can survive winter in soil, or in culls thrown on the ground
8. Pull out troubled plants
Should you notice that certain onion or garlic plants simply refuse to grow, carefully lift a sample plant to examine the roots and base. Chop up and compost young plants that host root maggots, because they rarely recover after significant loss of roots or basal stem.
9. Compost or bury damaged bulbs
Shrivelled cloves and hopelessly mushy bulbs are likely to harbour onion fly pupae, which can survive winter if culls are thrown on the ground or left in a neglected pile. Dispose of culls in an active compost bin or bury them under at least 8 inches (20 cm) of soil.
