Carrot root fly

What carrot root fly is

Carrot root fly (Psila rosae) is a small, shiny black fly, only about 8mm long, that you'll rarely spot in action. The adult itself does no damage. The trouble comes from its larvae — slender, creamy-yellow maggots that hatch from eggs laid in the soil at the base of your plants and burrow straight into the roots.

Despite the name, it isn't fussy about carrots alone. Parsnips, celery, celeriac and parsley all belong to the same plant family and can be attacked too, though carrots take the worst of it in most UK gardens.

The rusty tunnels

The first sign is usually above ground: the foliage takes on a reddish or bronzed tinge and the plants look stunted and sickly. The real evidence is hidden until you lift the crop. Pull an affected root and you'll find it riddled with thin, rust-brown tunnels running just beneath the skin, often with the maggots still inside.

Light damage can be trimmed away, but a badly tunnelled root is fit only for the compost heap. The wounds also let in rot, so stored carrots can collapse over winter even if they looked passable at lifting.

The scent trigger

Here's the part that catches beginners out. The female fly finds your row largely by smell. Crushed or bruised carrot leaves release a strong scent that she can detect from a long way off, and that scent is exactly what disturbing the foliage produces. Thinning a crowded row, brushing past the tops, or weeding all send up a plume of it and effectively ring the dinner bell.

How to prevent it

You can't easily kill the larvae once they're in the soil, so prevention is everything.

• Cover with fine mesh. The flies travel low to the ground, so draping crops with insect-proof mesh or horticultural fleece is the single most reliable defence. Tuck the edges in well — gaps undo the whole effort.
• Use a low barrier. The adults fly close to the soil, so a vertical screen about 60–70cm high around the bed forces them up and over. Fine netting or even fleece on canes works.
• Thin carefully, or not at all. Thin on a still, dry evening when the flies are least active, sow thinly so you barely need to, remove every thinning from the plot, and firm and water the soil afterwards to settle the scent.

It also pays to move your carrots around the plot each year as part of crop rotation, since overwintering pupae lie in wait in soil that grew carrots last season.