Carrot Root Fly are one of the most persistent perennial pests in the garden. They are active for the entire growing season, and can happily lay eggs on alternative vegetables and herbs even if you don’t have carrots in the first place.
For the last ten years, we have really struggled, and have tried everything under the sun, but there are two tricks that have definitely begun to reduce their numbers.
How to spot Carrot Root Fly
Carrot Root Fly are black flies, with a brownish-orange head, and bodies just over 5mm long from head to tail. Their larvae are a similar length, and less than 2mm wide, with translucent creamy white bodies. Their eggs are almost undetectable, soil coloured, raisin shaped specks that are less than 0.5mm across, laid directly on the soil surface at the base of the plant.
There is one definite way to detect them – put flypaper around your carrots and check it every few days. If there is a carrot fly on it, it’s too late, there are already some larvae laid, but you can avoid an influx by netting the carrots now before more arrive.
Plants affected by carrot root fly
Carrot Root Fly are persistent garden pests, with an insatiable appetite for carrots and everything in the carrot family. Before you head outside and start spotting carrot root fly it’s important to know exactly where to look.
Parsnip, parsley, celeriac and celery are all affected by carrot root fly. If given a choice, the flies and their larvae will choose your carrots every time, but if you’re not growing them, it doesn’t mean you can relax! The symptoms are similar, but harder to spot on parsnips and celeriac, and near impossible on parsley and celery, so if you suspect there are carrot root fly, check for these symptoms:
What does Carrot Root Fly damage look like?
The outside of the carrot will have black blemishes, usually in rings around the carrot’s root. On parsnips, these will be brown and look more like bruising. There are other afflictions that look similar on parsnips so the best way to tell is to snap the root open and check for holes.
The clearest sign of carrot root fly is holes in the centre of the carrot. They will start from outside, and eat channels right through the root.
Misdiagnosing Carrot Root Fly can cause all sorts of headaches, not least treating one thing in a way that will encourage another.
If you have tunnels in your carrot roots, with small black patches (rather than rings), these are likely the work of wireworm. Black spots, rings or patches by themselves with no sign of tunnelling are usually black root rot.
All are signs of overwatering, or over damp soil, and exacerbated by covering carrots, thus over humidifying them. If you have no sign of Carrot Root Fly other than these symptoms, simply stick to companion planting to discourage the pests, otherwise, you can encourage other problems.
- Tip: Carrots search for their water, and if you are growing in pots, watering from a tray under the pot will not only produce longer carrots, and reduce these other pests, but also limit foliage disturbance when watering, which can attract carrot root fly.
When do Carrot Root Fly lay eggs, and when are Carrot Root Fly active?
Carrot flies are persistent and depending on where you live can lay up to three times per year, with a pupal stage that tends to overwinter. Adult Carrot Root Fly are active from spring to late autumn and typically live for the entire year, laying up to three times in spring, summer and autumn, with offspring which lay as soon as they reach maturity.
Carrot Root Fly come in two to three generations per year:
- Generation 1: Pupae emerge from the soil in spring as adults. These adults will lay new eggs on any early sown crop in mid-spring (March-April for me in the UK). Their larvae will hatch within 1 week, and begin eating your carrots, where they will stay for up to three months.
- Generation 2: The first generation are still active, along with their new offspring, and will lay a second batch of eggs in mid-summer (July-August here), which again, hatch in one week, and mature into adults within three months.
- Generation 3: In a mild autumn, a third generation is possible, as the offspring of generation 2 will leave the soil to lay a further batch. The best way to avoid this is to skip mulching any carrot patch, which will help cold temperatures penetrate the soil.
- Over winter: Carrot Root Fly will survive the winter as even if generation 3 didn’t mature, they will remain in the soil as pupae, hatching in spring to start the cycle. Any adults from generation 2 or 3 will survive as long as they find somewhere warm to over-winter (a shed, greenhouse, or a gap behind a compost bin are ideal places for them to hide). Another way to help reduce soil temperatures is to disturb the top layer of soil, or dig over the carrot patch. This will dramatically speed up temperature drops.
How to deter Carrot Root Fly
Prevention is always better than cure, a rule for life and for Carrot Root Fly. Carrot Root Fly can smell your carrots from over a mile away, by detecting the presence of chlorogenic acid. Worse still, carrots affected with Carrot Root Fly, actually produce higher concentrations of the acid, meaning the next generation will find them even easier. So, it is a much better use of your time to prevent them from finding your crop in the first place.
Sowing carrots to avoid carrot root fly
The first, and most basic thing to know, is that every time you touch your carrot tops, you bruise the leaves, and release chlorogenic acid (which smells glorious to us, and to the flies). The highest concentrations are released when thinning carrots, so to avoid this mass attraction event, start by sowing thinly in the first place.
Sow rows of carrots, at least 10cm apart, and no more than 1 seed every 2 inches. Yes, it’s fiddly, but you’ll have a generous crop without the risk of thinning later on.
Considering that a Carrot Root Fly hatches in early spring, and immediately looks for carrots, parsnips, celeriac and parsley to lay eggs on, by far the most effective control method is to cut out the first generation of reproduction. By sowing your first carrots in July, you’ll have a later crop, but reduce the Carrot Root Fly population in your garden by at least half.
Carrot Root Fly barriers
Carrot Root Fly are weak fliers. They tend to fly lower than 75cm from the ground and climb walls and fences to move between gardens. Because of this, mosquito net fences are usually a very effective control measure. If they can’t get to your carrots, they can’t lay eggs. Sadly, like all flies, they walk too, so you have to take the extra measure of burying the edges of your net at least 5cm deep.
In colder climates, or for late crops, garden fleece can do the trick too, but avoid fleece in summer, as it will almost definitely overheat your crops and lead them to wilt.
- Tip: Shorter carrot varieties, like the globe shaped Paris Market, will grow in shallower pots. The only time we have ever gone an entire year without Carrot Root Fly, was when we grew these in window boxes. The windows were 1.5m from ground level, right outside the kitchen, and while it’s not the most exciting window display, it was a generous harvest when we got to it.
Companion plants to deter Carrot Root Fly
Companion plants for carrots are almost always selected with Carrot Root Fly in mind, and all of them provide a distraction rather than a cure, so the stronger the scent, the more effective it will be.
The first group of carrot companions are alliums. Garlic chives and spring onion are effective deterrents that take up a small amount of a room, with the convenient bonus of being harvestable at the same time. They will not provide 100% protection but do typically have some positive impact by masking the scent of carrots.
Oily herbs, like sage, or rosemary, can have a similar effect. Particularly Rosemary ‘Miss Jessop’s Upright’ for its compact shape, and the option to grow a full hedge around your carrot patch.
Marigolds and carrots are often suggested as a strong partnership, but we’ve found they have no impact at all here. However, I must admit that we grow marigolds from seed by the ton as a slug distraction, so perhaps we should move to a variety with a heavier scent and less foliage.
- Tip: Avoid Dill and Parsley, as both will actively attract Carrot Root Fly.
How to get rid of Carrot Root Fly
Can you spray for Carrot Root Fly?
There are no pesticides available for residential use that work effectively on Carrot Root Fly. Insecticides, despite any claims on packaging, are dangerous for any insect that comes into contact with them until they dry completely. We also grow masses of cabbage for pet rabbits, so any drift from pesticides could be fatal.
Most commercial bug sprays claim to be effective, but as an inorganic pesticide are likely to cause damage to other pollinators, and from most online reviews, are highly ineffective against Carrot Root Fly, as they only tackle the adult, leaving the larvae underground to continue wreaking havoc.
One tip I plan on trying this year is garlic spray, the same as you would use on Hosta to deter slugs. In theory, if planting garlic is a reasonably effective deterrent, maybe the spray I already have will help ward them off too. It’s worth a try (1tsp of vegetable oil, 1tsp of grated hand soap, 1 pint of water, and 1tbsp of crushed garlic: Leave to steep, and then sieve into a spray bottle).
How do I get rid of Carrot Root Fly larvae?
Crop rotation is the most tried and tested measure against Carrot Root Fly (allow three years between growing carrots in the same space), and sow later in the year to limit reproduction. While we practice crop rotation more generally, I am doubtful of its impact on Carrot Root Fly larvae, knowing that they can find crops from over a mile away, and hatch from pupae in the soil in spring.
There are resistant carrot varieties, like Resistafly, Fly Away or Sytan, but almost all of them are resistant for one simple reason; they smell less, which means they taste less. Eaten raw they will give some crunch and sweetness, but lose flavour when cooked. The problem with resistant carrots is that they can only be resistant by reducing the thing that makes them good in the first place – taste.
Nematodes, a parasitic naturally occurring species, are effective against Carrot Root Fly larvae, and while I am wary of nematodes impact on soil diversity and soil structure in most circumstances, our allotment may need a treatment if Carrot Root Fly continues with the same veracity as in recent years.
Can you compost carrots with Carrot Root Fly?
No, it is not advised to compost any carrots, parsnips, celeriac, or parsley, that have been impacted by Carrot Root Fly. Because they have such straightforward lifecycles, it is definitely not advised to compost them.
Carrot Root Fly are susceptible to frost, so you can dig the ground over in winter to let the cold in, which will reduce their pupal form before they have a chance to hatch, but in doing so, you will reduce the fertility of the soil, and it will take longer to warm through in spring.
Conclusion
So, Carrot Root Fly might be almost impossible to eradicate in your garden, but you can effectively manage their effects by planting carrots at higher levels, covering and combination planting. If you grow on a shared allotment though, it’s never too late to ask neighbours to join forces. If you all agree to sow carrots later in the year, you might just thin down the population enough to get a decent crop.
In a similar vein, citizen science projects like Big Bug Hunt help gardeners know when a particular pest is at large in their area. You can sign up for updates about specific species, or keep in the loop about everything going on in your area, so if you’re lucky get a head start on Carrot Root Fly by netting them before they arrive from neighbouring sites.
My two definitive tips for growing carrots without carrot root fly? Grow them higher up, using shorter varieties, and only sow after July. Eventually, you’ll reduce the population or carrot root fly in your garden down to something almost unnoticeable.
