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What Do Organic Growers Do About Carrot Fly?
Organic gardeners would probably agree that the carrot fly is by far the worst of all pests. Mainly because once they have arrived, there is little you can do to remove them.
The fly is attracted by the scent of the carrots which is why they frequently attack just after you have thinned the crop. Small birds including the robin will feed on their larvae when your soil is turned over. But as the flies can follow the scent of carrots for miles, it may not be enough just to rely on your feathered predators.
Do remove any cow parsley from your garden as this weed is a favourite source of food for the fly.
Resistant varieties
While no variety is totally safe from carrot fly, many varieties are advertised as being less attractive to them. The latest of these is 'Fly Away'(T&M) Unlike many 'resistant' varieties this one has a good flavour.
Timing
The first attack comes in late May or early June and the second in August and September. Delaying sowing until late May, can help to avoid this first generation. Alternatively, sow very thinly or use pelleted seed, thus eliminating the need for thinning.
Prevention
As the carrot fly is attracted by smell, one form of prevention is to mask the scent by planting wormwood, sage or tansy around the beds or laying branches of these plants there at critical times. In Germany, powders made from wormwood and tansy are commercially available for preventing carrot fly. Some gardeners prefer to use their onions. They alternate their rows of carrots with rows of onions to keep the fly away. Or mix together onion and carrot seed. Gardeners who use this method say it works as long as the onions remain green. Once the stems begin to dry, there is no protection. If you are not keen on onions you can use leeks or salsify instead.
Grass cuttings
An old gardening trick was to deter the fly with the scent of newly-mown grass. Gardeners simply scattered grass between the rows of carrots every time they mowed the lawn. Of course they cut the lawn regularly. If you are the sort that waits until the lawn is capable of swallowing up a small child, then this method will not be so effective!
Barriers
Perhaps the surest way to protect your carrots is to use some form of a barrier that they can't get past. Building a 'wall' two feet six inches high around the carrot plot will stop the pest. Despite their name, the carrot fly is not great at flying and frequently depends on a favourable wind to get around. The wall can be made of various materials, but strips of polythene are probably the easiest to erect.
Completely covering the crop with horticultural fleece is an simpler barrier to apply and the higher temperatures under the fleece plus the protection from heavy rain damage will produce a better crop.
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the carrot fly is territorial, and wont lay eggs if competition is in sight, the fly will continuously attack the mirrors eventually exhausting itself,