How to Prevent Carrot Flies

Carrots damaged by carrot fly maggots. Photo: gardening.which.co.uk

The carrot fly (Chamaepsila rosae, formerly Psila rosae) is a major annoyance for many home gardeners. The larva of this insect, the carrot fly maggot, pierces rusty-brown tunnels in the roots of carrots (Daucus carota) and then rot sets in, making them unusable. But fortunately there are ways of preventing this pest. Here are a few:

Note that the carrot also attacks other umbellifers (plants in the carrot family, Apiaceae), such as celery, parsley and parsnip. Most of the techniques shown here will help protect them as well.

Sow Late and Avoid the Rush

First, you can simply delay sowing. The first generation of carrot fly is out early, in mid-May in most climate, and these flies lay their eggs in the soil near any carrot seedlings they can find. At first, the young larvae feed on the root hairs and secondary roots of the seedlings and only later do they start tunneling into the main root as it thickens.

However, if you sow your carrots late, about mid-June rather than in early May, by the time the seeds germinate, in about 2 weeks, there’ll no longer be any adult flies around to lay eggs near them, so you can avoid the problem! Of course, that will delay your carrot harvest, but still, should give them plenty of time to reach maturity.

Of course, there is a second generation of carrot fly that appears towards the end of July and into August and even a third generation in the fall in mild climates, but if the important first generation didn’t hatch in your garden, there will be very few latecomers around to even find your carrots. And even if a few flies do, this generation rarely causes any significant damage. It’s the first generation that’s the real problem.

Keep Wildlings Away
It is always wise to keep wild carrots (Daucus carota) away from any vegetable garden; otherwise there will always be carrot flies (and other carrot pests) right nearby. So, get out there and yank them out!

Pest Exclusion Barriers

Another possibility is to sow your carrots early, as usual, but then set up an exclusion barrier by means of a sheet of floating row cover, also called pest-exclusion net or frost cover (in the latter case, because it can also protect against the cold). It’s a very light, translucent, whitish material that lets in rainwater, air and, of course, sunlight, but not insect pests.

For this barrier to be effective, though, you have to carry out crop rotation. It won’t work if you sow your carrots in the same place as the previous year, because the carrot fly overwinters as a pupa in the soil near the roots that it ate the previous summer. So, if you sow in the same spot and adult flies awaken and dig their way out of the ground, they’ll end up inside the barrier, ready to pounce on your carrots! However, if you sow your carrots in a new location, then protect with row cover, flies will find themselves outside of the barrier, unable to reach their favorite food.

The idea is to loosely cover the row or spot where you sowed the carrots with floating row cover and to hold it in place with stakes, bricks, earth or stones so that it doesn’t blow away. No staking or other support is necessary: it’s called floating row cover because it “floats” above the leaves, rising with them as they grow. (That said, many gardeners do install stakes, hoops, etc., probably because it makes them feel more useful.) The border of the row cover, however, must, however, be pressed against the ground or even buried so it doesn’t open in the wind: if the flies find an opening, they will readily enter it.

You can remove the row cover, its work done, when the first generation of carrot flies dies at the end of June.

Skip a Year

Even more easily, if you had trouble with the carrot fly one year, simply avoid sowing them (and other umbellifers) the following year. When the flies emerge from the ground in late spring, they won’t find carrots nearby on which to lay their eggs. Since they’re poor fliers and rarely go very far, most will be unable to find a suitable host and will simply die.

That way, the following spring, there will be no overwintering pupae in the garden, so no flies, no maggots and no damage. Problem solved!

It often takes 4 to 7 years of carrot growing before the fly finds your vegetable garden again.

Make Your Carrots Indigestible … to Maggots, That Is

But the most laidback method of controlling the carrot fly is to grow carrots the flies don’t like.

Carrot flies find carrot plants through the smell of chlorogenic acid they give off. And moreover, maggots actually need this acid for their survival. Without the presence of chlorogenic acid, they soon die.

However, some carrots have a very low chlorogenic acid content or even contain none, like the following:

• ‘Flyaway’
• ‘Flyfree’
• ‘Healthmaster’
• ‘Ibiza’
• ’Maestro F1’
• “Paranoid”
• ‘Resistafly’
• ‘Sytan’

These carrots really don’t attract flies and even if they do find them, will suffer little damage, because the larvae die soon after hatching.

You will find carrot seeds resistant to the carrot fly in many seed catalogs, sometimes even in garden centers.

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And there you go! By using the above methods, you can harvest delicious and healthy maggot-free carrots!