Carrot fly
Psila rosae
also called Carrot rust fly
Low-flying flies whose larvae tunnel rusty mines through carrot and parsnip roots, drawn in by the scent of bruised foliage. Sow thinly, thin in the evening, and screen crops with 60 cm insect-mesh barriers.
How to know it
A small shiny black fly with an orange head and long yellow legs, flying low over the soil surface and rarely seen. Its larva is a slim creamy-yellow maggot about 8 mm long, found in a rust-lined tunnel just under the skin of the root.
What you'll see
- Rusty-brown tunnels running through and around the root, just below the skin.
- Foliage tinged red and bronze, then yellowing, on plants that are otherwise well fed.
- Roots rotting and splitting from the tunnels once wet weather sets in.
- Damage worst at the ends of the row and on the sheltered side of the bed.
- A whole sowing spoiled while a covered sowing alongside is clean.
Where it comes from
Two, sometimes three generations a year: the first flies in May, the second and most damaging from July into September. Eggs are laid in the soil beside the plants, the maggots feed for a month in the roots, and the pest overwinters as pupae in the soil or as maggots in roots left in the ground.
The females hunt by scent and fly low — under about 60 cm — so anything that releases the smell of carrot foliage brings them in: thinning, weeding, brushing past. Sheltered gardens with still air suffer more than windy open sites.
Dealing with it
- Surround the bed with a 60–75 cm barrier of fine insect mesh or fleece, or cover the crop entirely — the flies simply cannot get over it.
- Sow thinly enough that you never have to thin, and if you must thin, do it on a still evening and remove every scrap of foliage.
- Water after thinning to settle the soil and damp down the scent.
- Lift the crop by late autumn rather than leaving roots in the ground for the pest to overwinter in.
- Cut off and bin affected tops and roots; do not compost them.
Keeping it out
- Time sowings to miss the flights: sow after early June and lift before late September where you can.
- Grow partially resistant varieties such as Flyaway, Resistafly and Maestro.
- Rotate — pupae overwinter in the soil where the crop stood.
- Interplant with alliums, or grow carrots in a windy, open position rather than a sheltered one.
- Keep the mesh on from sowing to harvest, and check the edges are sealed to the ground.
What it goes for
Carrots above all; also parsnips, celery, celeriac, parsley and other umbellifers.
Trouble it brings with it
Who hunts it for you
See also
The Cottage Garden Companion — the gardener's bestiary at cottagegardencompanion.com