Why do my carrots have rusty brown tunnels through them?

Carrot fly is the reason a lot of people stop growing carrots. The damage is done underground by larvae that tunnel just under the skin, so the first you know is when you lift the crop. The good news is that this is one of the few garden pests with a genuinely reliable physical answer, because the insect's behaviour is predictable.

Step by step

1. Confirm the damage.Fine rusty-brown tunnels running just under the skin and through the flesh, often starting near the shoulder, sometimes with tiny cream larvae still present. Foliage on badly attacked plants goes reddish and stunted.
2. Put a barrier up before you sow, not after.Timing matters. The adults are on the wing from late spring and again later in summer. A barrier put up after they have laid is decoration.
3. Build a vertical barrier at 60–75cm.Fine insect mesh or horticultural fleece stretched round posts, right to the ground with no gaps at the base. The fly flies low and follows the ground; it does not climb over. This is the classic answer and it works.
4. Or cover the bed with fine mesh over hoops.Insect mesh held clear of the foliage on hoops, with all four edges buried or weighted down. It excludes the fly completely and also keeps the row moister, which suits carrots. The mesh needs to be fine — a coarse bird net is useless.
5. Sow thinly enough that you hardly need to thin.Every thinning session releases the scent that draws adults in from a distance. Sowing thinly, or using seed tape, is a real reduction in risk before any barrier goes up.
6. Thin in the evening and take the thinnings away.Adults are active during the day. Thin at dusk, remove every scrap of foliage from the plot, and firm the soil back around the remaining plants so no roots are exposed.
7. Move the bed every year.Pupae overwinter in the soil where carrots grew. A bed that grew carrots last year has a resident population that emerges inside your new barrier.
8. Time sowings around the flight periods.There are usually two main generations. Sowing in early summer, after the first flight, and lifting before the second generation's larvae do much damage, dodges a lot of the problem in many gardens.
9. Lift the crop by late autumn.Roots left in the ground over winter feed larvae and support the next generation. Lift and store, or lift as you need them and clear the bed by the end of the season.

Tips

• Damage is worst on carrots left in the ground longest, so an early crop lifted young often escapes almost untouched while the maincrop beside it is ruined.
• Companion planting with onions is widely repeated and the evidence is weak. Mesh works; onions do not reliably.
• Some varieties are marketed as carrot-fly resistant. They are less damaged rather than immune, and are best combined with mesh rather than used instead of it.
• Parsnips, celery, celeriac and parsley are all attacked by the same fly, so rotate them as one group and mesh them the same way.
• Lightly damaged carrots are perfectly usable — cut the tunnels out. Heavily tunnelled ones rot fast and should not be stored.

Common mistakes

• Using a bird net instead of insect mesh — The fly walks straight through anything with a mesh larger than a couple of millimetres. The mesh grade is the whole point.
• Leaving a gap at the bottom of a vertical barrier — The fly follows the ground. A 5cm gap at the base makes the entire barrier pointless.
• Letting mesh rest on the foliage — The fly lays through mesh touching leaves. Hold it clear on hoops.
• Thinning in the middle of a warm afternoon and leaving the thinnings on the path — That is broadcasting an invitation. The crushed-foliage scent carries a long way.

Questions people ask