Skip a Year to Control Carrot Flies

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You rarely see carrot rust flies themselves or even the larvae, but the damage to the roots is very obvious. Source: gardening.which.co.uk

Sometimes the easiest way to regain control over carrot rust flies, Psila rosae, also called carrot root maggots, is simply to stop growing carrots (and other plants that can host carrot flies, like parsnip, celery, celeriac and dill) for a year.

Once the carrot fly has found your garden, it will come back again and again. The flies overwinter as pupae and adults emerge in late spring and start to lay eggs in the soil near carrots and their relatives, then the larvae burrow into the soil and start to chew on and tunnel into carrot roots, leaving your harvest criss-crossed with blackened, rotting tunnels. But if you don’t grow any carrots or carrot relatives that year, the poor flies have to wander off elsewhere to look for food. Being weak fliers, they don’t go far and likely won’t find a host plant. Most will simply die in transit.

The following spring, there will be no carrot flies in the sector and therefore no damage.

It tends to take carrot flies at least 5 to 6 years to find your garden again, often a decade … at which time you can again thwart them by not sowing anything they can eat for a year.

Insect control by starvation? Why not!

Won’t Work in Community Gardens

Note that this method will probably not work in community gardens, as there will almost certainly be other gardeners growing carrots just a few lots from yours … unless you can convince the whole garden community to not grow carrots for a year. But it works wonderfully in individual home gardens.

Carrots Resistant to Carrot Fly

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Carrots damaged by carrot fly larvae.

The carrot fly (Chamaepsila rosae, formerly Psila rosea) is a major annoyance for vegetable gardeners. It’s found pretty much around the world and can make growing carrots difficult or even impossible. The fly itself is rarely seen and quite harmless. It’s its larva you have to worry about. The tiny white grub traces galleries in carrot roots, which then begin to rot, rendering them unusable. But there are ways of keeping carrot flies at bay.

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Carrot fly larva.

Carrot flies can’t attack your carrots if they can’t find them… and they track the presence of carrots by a chemical component carrots give off: chlorogenic acid. Carrot flies can detect the presence of chlorogenic acid from miles away, especially when carrot leaves are damaged. Not only does chlorogenic acid attract carrot flies, but fly larvae need this acid for their survival. Without the presence of chlorogenic acid, the larvae die quickly.

And that’s why the right choice of carrot varieties can make all the difference, because there are carrots that are very low in chlorogenic acid or even totally free of it. Such is the case with ‘Fly Away’, ‘Flyfree’, ‘Resistafly’, ‘Healthmaster’, ‘Ibiza’, ‘Parano’, ‘Maestro F1’ and ‘Sytan’. These carrots don’t attract carrot flies and even if the fly does find them, they suffer little damage, because the larvae, unable to find enough chlorogenic acid for their development, die shortly after hatching.

Several seed catalogs offer seeds of carrots resistant to carrot fly. Here are just a few I’ve noted: Veseys (www.veseys.com), Halifax Seed Company (www.halifaxseed.ca), West Coast Seeds (www.westcoastseeds.com), T T & Seeds Ltd. (www.ttseeds.com) and Thompson and Morgan (www.thompson-morgan.com).