Recently I experienced one of the most disheartening crop failures a gardener can suffer. I noticed that some of my carrots had developed bronze foliage, so I pulled a few to see what was up. I dug up carrot after carrot, feeling more and more dismayed as I discovered that every single root was riddled with rusty and blackened tunnels.
I was sure that I knew what was responsible for the damage, but just to confirm I broke two or three carrots in half. Each root was honeycombed with tunnels, from which several drowsy, yellowish maggots wiggled.
My worst fears were confirmed: it was carrot fly.
The Curse of the Carrot Fly
Carrot flies themselves don’t cause any damage, but they lay their eggs near carrots and other susceptible crops such as parsnips, celery, dill and parsley. The ravenous larvae then wriggle down through the soil to the nearest available root – and begin to feed.
At first they may only nibble on the fine root hairs, but they soon progress to the roots themselves. They munch higgledy paths on the surface of the roots before boring into the heart of the carrot. This doesn’t usually kill the plant, but it can stunt growth and cause the bronze foliage I’d noticed. And, of course, depending on the extent of the damage, the roots become inedible.
Carrot flies produce two, and sometimes three generations per year. The first eggs are laid in late spring. These were the ones that hatched into the maggots that took a fancy to my carrots. Had I not interrupted the growth cycle by hoicking out the infested roots, they would then pupate and hatch into adult flies, ready to spawn a second generation.
How to Deter Carrot Fly
Carrot fly is a widespread problem, and gardeners and farmers have developed a range of techniques to deter them.
Companion planting. Completely encircling your carrot crop with allium family plants such as onions, leeks or chives is believed to literally throw carrot fly off the scent. As with most companion planting advice there is conflicting evidence on its effectiveness, so it’s worth experimenting in your own garden.
Strategic sowing times. Carrot flies are active from late spring until autumn. By delaying sowing until early summer, you can sidestep the first generation of the pest. My mistake this year was sowing too soon in my eagerness to get growing!
Avoid thinning. The process of thinning out carrots bruises the foliage and intensifies its scent, which is said to attract carrot flies from some distance away. Sow sparsely instead, so that thinning is not required.
Harvest susceptible crops promptly. In milder areas maincrop carrots can often be left in the ground over winter, but if your garden is prone to carrot fly lift them all in autumn. After an infestation turn the soil over occasionally – this will make it easier for birds to snap up overwintering larvae and pupae.
Crop rotation. Avoid growing carrots in the same spot every year. While this won’t stop carrot flies from flying in afresh each year to lay their eggs, it will avoid the risk of overwintering pupae emerging right in the middle of your crop.
Vertical fences. Carrot flies are low-flying insects, so a vertical barrier of horticultural fleece, fine mesh or polythene at least 36cm (3 feet) high should, in theory, stop them in their tracks. But – and there’s always a but – carrot flies are tiny, lightweight creatures which, even on a calm day, are bound to be buffeted around and lifted up over obstacles by the breeze. If you do try vertical barriers, make your carrot bed long and narrow to make it harder for the flies to land within them.
Grow in containers. Similarly, tall containers or high raised beds may help to reduce the incidence of carrot fly, but aren’t guaranteed to keep them away. If you grow containers of carrots on a second floor balcony however you may never see this pest!
Resistant varieties. There are some resistant varieties of carrot available – for instance, ‘Flyaway’ (which came out tops in Which? Gardening trials), ‘Resistafly’, ‘Maestro’ and ‘Parano’. Remember, they are merely resistant to fly attacks, not immune, so use these varieties in conjunction with other techniques.
The Only Surefire Way to Prevent Carrot Fly
The only guaranteed way to prevent carrot fly is to stop them from getting anywhere close to your carrots and other susceptible crops. In practice, this means draping row covers of horticultural fleece or very fine insect-proof netting over your carrots, either directly on top of the crop or suspended on hoops of wire or plastic. This must be done as soon as the crop is sown, and should only be removed very briefly to weed and harvest. Make sure to secure the edges of the cover so there are no gaps. Ideally, dig the edges 5cm (2in) into the soil.
Pest attacks are always disheartening, none more so than when they completely ruin the crop you’ve been so looking forward to harvesting and eating. But with good pest prevention techniques, carrot fly will soon be a thing of the past in my garden – and hopefully yours, too.
Don’t forget to report any carrot fly – or any other pests you see – to the Big Bug Hunt and help us to develop a pest prediction service so you can take action to protect your crops before pests strike.