Carrot root fly (Psila rosae)

Carrot root fly Psila rosae is one of the most irritating pests on the allotment.

I think that it is safe to say that adults are flying and laying eggs from May to November. The only time that there are no flies is from November to April. There are two peak times when the flies are flying and this suggests that there are usually two generations during the summer. The first generation is from the end of April to the end of May and the eggs are laid from the end of May to the beginning of July. The second generation begins hatching from the beginning of July to the middle of August. Most eggs are laid from the beginning of July to the beginning of August. A third generation hatches from late September to late October and the larva from these overwinter in carrot roots and other wild members of the carrot family. Most wild species are less prone to carrot fly than the average cultivated carrot and sustain less carrot flies per plant. Fools parsley Aethusa cynapium is an important host so it is worth getting rid of it if you have it on or near the allotment. They can lay eggs until the middle of November so either the carrots have to be lifted or kept covered with enviromesh even at this late date. This third generation starts to cause damage as the weather becomes warmer in the spring and can lead to very high fly populations the following April. Really, any carrots still in the ground must be dug up before the end of March to avoid the larva from becoming flies.

Trying to avoid planting at high concentration times seems to be impractical for the amateur gardener. We need to plant when we are able rather than at the most appropriate time. So it seems that the most effective way of preventing flies from laying eggs around carrots is to have a physical barrier such as fleece or my favorite enviromesh. Evidence suggests that carrot root flies are weak fliers; not flying particularly high. Putting a barrier about 60 cm tall around the carrots, they suggest, will prevent Psila rosae from reaching your carrots. Not in my experience. The research shows that there is only 15% drop in damage within fences. I would suggest the little beggars get over fences easily. Other people have suggested growing in pots lifted up on some kind of support. Tried this and putting carrots in a big blue barrel full of sifted soil. Still got carrot root fly.


The only way I have prevented carrot root fly and got reasonable carrots is by completely covering them with enviromesh. The wind blows and even squalls and eddies would carry the flies over any open topped barrier.   I put in wire hoops and then cover with the enviromesh. When I put on the enviromesh I bury the edges and ends in the soil to form a really good seal. However, there are drawbacks with the method; it is expensive, and conditions under the netting encourage the development of fungal pathogens and weeds, which is why I thin and weed under the mesh.

Carrots under the enviromesh

The mesh is supported by old cloche wires.  I bury the sides and the ends in the soil and this tends to keep out other pests like slugs and snails.  There are some rips and tears in the enviromesh but the fly does not seem to be able to find these.  They are quite small.  If the holes get any bigger, I might invest in a new net.

I am also going to experiment with cloches next year. I don't think that carrots like the heat generated by cloches though so it may not be as successful as I would like.

As I thought, cloches were not such a good idea because they were too warm for the carrots and also  they did not produce the complete seal that enviromesh could.  I will continue to use the enviromesh.  

Carrot root flies are most active in the late afternoon or early evening. They are crepuscular. There is a distinct crepuscular peak particularly in the evening. I emphasize this because I have seen the morning suggested on various web sites. So if you are weeding or thinning then it may be best to do this before the afternoon. However, I have done these jobs in the afternoon and if you get the enviromesh back on the carrots fairly quickly then the flies do not have time to lay eggs. As this suggests, I take off the enviromesh along one edge to weed and thin.
There is some evidence that carrot root flies use the scent of the carrots to hone in on them. This has led to experts to suggest that you do not thin carrots. Just sow the seeds thinly. I do not agree. If you want reasonable sized carrots then you need to weed and thin them and this is what I do. It probably does bruise the carrot foliage and produce more attractants but believe me the little beggars are going to find your carrots in any case, whatever you do unless you protect them. There are many different theories about how the flies identify their food plant and foliage shape is another one.

I usually water the carrots with comfrey liquid after weeding and thinning.  The comfrey liquid fertilizer smell particularly foul and I am hoping that this masks the smell of the bruised carrot foliage.  Hope springs eternal as old Alex says.

I use the fly resistant varieties such as Flyaway. These varieties have been bred from Vertou, Nandor and Sytan but amateur gardeners do not seem to be able to get these varieties. Researchers do not seem to know exactly why these varieties are resistant but Flyaway seems to work well under my enviromesh.

Intercropping with the clover family helps to prevent infection but undersowing produces too higher level of competition between carrots and companion plant, which lowers the yield and produces split carrots.

Intercropping with one or more rows of onions planted between each row of carrots has led to a modest decrease in carrot fly damage, the strength of which depended on the ratio of carrots to onions. The best effect was achieved when four rows of onions were planted between each row of carrot. This is fine until you get the onion miner fly too as I have. Also, if you are trying to keep to a ridgid rotation of crops, onions do not really go with roots.

What you are trying to do here is put a crop, carrots, that do not need a lot of manure with onions that do like manure. I still question whether carrots fork and split if you use manure with them but maybe you can circumvent all of this by applying compost to the bed. That should satisfy both of them.

I tried marigolds and chamomile around both the onions and the carrots but I could not see a noticeable effect in pest prevention.

That might just be me and my conditions. Brightens up the allotment, though.

Rotating carrots each year is another way of reducing carrot root fly damage. The distance that the amateur allotment holder can move the carrot bed is limited so this may not be as effective for gardeners as it is to farmers. The fact that there is more damage to carrots growing near to hedges and ditches shows that planting as far away from them as possible may reduce damage. It is a shame that my allotment is so close to the hedge.

I have grown a great many different varieties over the years, however carrot root fly seems to have become worse over the last ten.
After resisting for several years, I eventually had to grow flyaway.
I was pleasantly surprised by the taste. We used it both in salads and steamed.
It just shows you how much better fresh vegetables taste, regardless of the variety.
If you do not have too much carrot root fly you have to go for Nants - it is a special taste, and for autumn and winter with big roots I would have to go for Autumn King too.

I have been growing Flyaway for several years now but I also cover the carrots completely with enviromesh using piping to keep the enviromesh off the plants. I have tried planting Flyaway with no protection and they do get damaged by carrot root fly.
I gave the carrots a couple of ounces of blood fish and bone when I planted the seeds and have got some really good roots this year. I think that thinning out is possibly a way of preventing too much damage by the carrot root fly. I thinned out to about 8 inches apart. It is a bit of a chore to keep removing the enviromesh to weed and thin but it was worth it to get some decent roots. I left them in the ground so I hope the frost has not done too much damage.

Mulching with lawn mowings or bark chippings did not have much or any effect on carrot root fly damage. There is no connection between the amount of carrot damage and the number of carrot fly predators.
Companion planting with lucerne and clover does reduce the yield of carrots due to competition. However, the effect of companion planting on the reduction of carrot fly damage compensates to some extent for the suppressed yield due to companion planting. There is evidence that carrot root flies have more difficulty in finding carrots to infect when companion planting is used.
If leguminous plants are used as a companion plant, i.e. low growing clovers, tares, vetches etc. there is an additional bonus of fixing atmospheric nitrogen.
Digging in the tops of these legumes will give about 60 grams of nitrogen per 100m2 digging in the roots will give 10 grams of nitrogen per 100m2 .
This is important Tone. You seem to get more nitrogen from the tops of legumes than the roots. So make sure you always dig in the tops of peas and beans too. Do not burn them.
So that is how I grow my carrots.

7 comments:

  1. Every control I've researched seems to cost quite a bit money, so it makes one wonder if growing carrots is cost effective when they are relatively cheap in the greengrocers shops & supermarkets.

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  2. Like virtually all vegetables we grow on allotments, farmers can grow them a lot cheaper. However, they use pesticides to produce them and carrots are particularly adept at absorbing these chemicals. I would rather grow my own using enviromesh. as I have over the years.

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  3. Growing carrots is not about COST but is all about TASTE!

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    1. Carrots taste like carrots to me whatever. I just like to grow fresh, pesticide free carrots that are high in nutrient value. Regardless of the cost, farmers can't do that.

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  4. Thank you very much for covering all the issues that were bothering me. I stopped growing carrots several years ago because of carrot root fly damage. This year tempted to try again I bought a pack of "Fly Away". I didn't know how effective the protection would be. Thanks to you I shall go off and buy some enviromesh and get cracking.

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  5. Cleared all carrots, parsnips, etc. at end of season.
    Bought FlyAway seeds to sow well spaced in June.
    Will cover thoroughly with Enviromesh.
    Was going to stop growing them but your dedication has inspired me. Thanks. Fresh from the garden carrots taste so much better than supermarket.

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