Carrot Rust Fly

Pull a carrot. Narrow rusty tunnels wander the lower half of the root. Cream maggots sit in rusty mush. Cover the seedbed from the day you sow, and bury the edges. Do not cover last year's carrot ground. A spray will not kill the maggot already in the root.

Carrot rust fly is a small black fly with an orange head and yellow legs. Gardeners often call it carrot fly. The adult does not chew the carrot. The maggot does. Tops can look fine while the root is already tunneled. Parsnip, celery, and parsley in the same bed feed the same fly.

• Pull a carrot. Rusty winding tunnels in the lower half confirm it.
• Cover from sowing. Bury the edges so the fly cannot crawl under.
• A spray will not kill the maggot already in the root.

Cover the bed before the fly lays eggs

The fly lays eggs on the soil at the crown. Maggots hatch and enter the root. Once they are inside, you cannot spray them out. Stop the fly from landing, then harvest the row as a block.

1. Confirm. Pull one carrot. Look for narrow winding tunnels in the lower two-thirds, filled with rusty mush, and a cream, legless maggot with no brown head.
2. Cover at sowing. Floating row cover or insect netting goes on the same day as the seed, or as soon as seedlings show. Pin or bury the edges a couple of inches so the fly cannot crawl under. Leave the cover on until you harvest.
3. Move the bed. Pupae overwinter in last year's carrot soil. Covering that same ground traps the flies that crawl out underneath. Fail if the fabric sits on last year's row.
4. Sow thin. Bruised foliage and yanked seedlings release the carrot smell that draws the fly. Snip extras at the soil line instead of pulling them.
5. Harvest the row as a block when roots are ready. Do not leave carrots in the ground over winter. Bag infested roots. Do not home-compost live maggots.

Pull a carrot: rusty tunnels sit in the lower half

The first thing you notice is often nothing aboveground. Young plants can wilt or yellow if maggots prune the taproot early. Older tops can stay green. Pull a carrot. Surface tunnels ring the root with rusty brown scars. Cut it. Narrow winding galleries sit mostly in the lower half. The maggot is cream to yellow, stiff, about a third of an inch, and legless, with no head capsule. The rust-colored mush in the tunnels is frass. That is how the fly got its name.

The adult is about a quarter inch long, with a shiny black body, an orange head, yellow legs, and clear wings. It flies low, often under two feet. Females lay small white eggs on the soil at the crown. One to three generations a year is normal. The second generation in August and September does the most damage to carrots you meant to store. On parsnip the feeding often sits higher, on the shoulder. On carrot it sits lower.

A tunnel at the crown is a different insect

Carrot weevil chews the crown and the upper third of the root. The larva is a legless grub with a reddish-brown head, not a headless maggot. The tunnel is darker and often partly open after the grub leaves. See carrot weevil. Do not run a weevil trap calendar here.

Wireworms are hard, yellow click-beetle larvae. Holes are larger, straighter, and often hollow. See wireworm. Maggots in a cabbage root that flags at noon are a different fly — see cabbage maggot. Tunnels that turn mushy are rot moving in after the feeding; see bacterial soft rot. Knobby swellings without rusty frass are root-knot nematode.

Sow after the first flight, or harvest before the second

In cold gardens, wait until early summer, after the first flight, often after mid-June, so the young roots miss those eggs. In mild gardens, sow in late winter or late summer. Harvesting the spring row before late August can skip the second generation. Carrots that sit into fall take the worst tunnels.

An early row next to a later sowing feeds the second flight. Keep later carrots farther away if you can. Ask a local extension desk when the first flies show in your county. Do not build a climate table from a national map. Queen Anne's lace and volunteer carrots at the fence still feed this fly, so pull those too.

A spray will not kill the maggot in the root

The maggot feeds inside the carrot. A foliage spray does not reach it. Adults are tiny and do not linger after they lay. Skip the brand aisle. Commercial rows may use labeled products on adults. That is not the home plan after you already have tunnels. If you missed the cover this year, peel what you can eat, then cover the next sowing on clean ground. Fail if a bottle on the foliage is the whole plan after tunnels are already in the root.

Beneficial nematodes watered into the soil are sold for young larvae. They are not a rewind once the maggot is deep in the taproot, and they are not the first move. A complete cover on a clean bed still beats a soil drench.

Related reading

• Carrot weevil — darker tunnels in the upper third, and a grub with a brown head.
• Cabbage maggot — wilt at noon on brassicas, not a rusty carrot tunnel.
• Wireworm — larger, straighter holes in roots and tubers.
• Bacterial soft rot — mush after a tunnel or a bruise.
• How to grow carrots — a bed that can make a root worth covering.
• Planting carrots — sow thin so you do not bruise the row.
• Harvesting carrots — lift the row as a block, not one root at a time.