IDENTIFY, PREVENT AND GET RID OF ONION FLY
By David Marks
One of the more common onion pests, the onion fly attacks onions, spring onions, shallots, garlic and leeks. The lifecycle begins when flies emerge in May / June from grubs which have over-wintered in the soil or debris on the ground.
The flies are slightly smaller compared to a normal house fly but in reality they look like any other fly to the naked eye. They lay eggs on the leaves of young onion plants near ground level. The eggs are hard to spot because they are very small.
The eggs hatch after three days into maggots which are about 10mm long and whitish (see picture below). It is these maggots, not the flies, which do the damage.
When the maggots emerge they tunnel into the developing onion bulbs and feed on them. After three weeks or so the maggots then move a small distance away from the onion bulb, pupate and produce a second generation of maggots. There may be several generations of maggots in a growing season, the last one occurring around the end of August. This last generation overwinters in the soil and plant debris to start the lifecycle again next spring. The Latin name of this pest, for those researching further, is Delia antiqua. A very detailed article on the onion fly, by Cornell University, can befound here.
SYMPTOMS OF ONION FLY
The first signs become visible on younger developing onions. The leaves wilt and turn brown, these symptoms also then occur on more mature plants. The top of the bulb may smell slightly and will begin to rot. Maggots may well be visible in it. Gardeners sometimes confuse onion fly with white rot because the above ground symptoms are similar. However onion fly has has maggots in the the top of the bulb whereas white rot does not and white rot damage first affects the bottom of the bulb not the top.
HOW TO TREAT ONION FLY
There are no chemicals to treat affected bulbs. Nemasys sell a product called Fruit and Veg Protection which contains a mixture of Nematodes. They claim this can help prevent onion fly although we have no direct experience of it. It needs to be used exactly as described on the packet instructions.
Dig affected plants up as soon as the pest is identified and burn them. Affected onions are inedible. Watch out for maggots in the surrounding soil as you dig the bulbs up, remove and destroy the maggots.
HOW TO PREVENT ONION FLY
It is the maggots which do the damage however there are no chemicals or treatments available for that stage of the lifecycle. The weak point in the life of the onion fly is the fly itself. They can be prevented from landing on newly growing onions with the use of insect proof fleece or mesh when new onion seeds / sets are planted. This will of course only work if you sow /plant new onions in a different soil to the one used in previous years for onions and other members of the onion family.
Other common pests and diseases which affect onions include:
COMMENTS / QUESTIONS LEFT BY OUR READERS
|Date: 13 July
|I have previously been affected with growing leeks with the onion fly pest and I have covered them up this year with fleece. When do you think I should remove the fleece?
ANSWER: Onion fly have two or more generations within a growing season. The last generation tends to be in August when the onions are ready for harvest. This means that the fleece needs to stay on all season for maximum protection. The last generation in the year overwinters in the ground so if you have completely eradicated them in one year they will not re-appear from your ground the next year. Unfortunately the flies can spread from neighbouring crops.