Rusty tunnels in carrots, wilting cabbages and rotting leeks are often the work of flies and moths whose larvae feed on roots or inside leaves and stems. This guide summarises the Royal Horticultural Society’s (RHS) advice pages for carrot fly, cabbage root fly, onion fly, allium leaf miner and leek moth. The advice is written for gardens in the United Kingdom. It concerns edible crops grown at home, where the RHS pages describe cultural controls and mesh rather than sprays, and it is general information, not a treatment recommendation.
Carrot fly
The RHS says carrot fly larvae can make a large proportion of carrots and related crops inedible. Affected crops include carrot, parsnip, parsley, celeriac and celery, and the adults are most active from May to October. Symptoms are rusty brown scars on the tap roots, and tunnels containing slender creamy-yellow maggots up to 9mm long.
The RHS advises sowing sparsely to avoid thinning, because female flies are attracted by the smell released when surplus plants are removed. Carrots sown after mid-May avoid the first generation of the fly, and those harvested before late August avoid the second. Insect-proof netting is an option, and the RHS says its research showed that a complete covering is more effective than a barrier. Crop rotation is essential under netting, because adults can emerge from overwintered pupae in the soil. Cultivars such as ‘Fly Away’, ‘Maestro’, ‘Resistafly’ and ‘Sytan’ are less susceptible, but the RHS stresses that they are not fully resistant.
Cabbage root fly
According to the RHS, cabbage root fly maggots eat the roots of cabbages and other brassicas, and can tunnel into swedes, turnips and radishes. The timing runs from April to September. There are three generations in the summer, but the first, in late spring and early summer, is often the most damaging, and later generations are less harmful to leafy brassicas because older plants have bigger root systems. Root crops such as radish, turnip and swede are damaged by any generation.
Recent transplants can be protected with a brassica collar of about 8 to 15cm, bought or made from carpet underlay, roofing felt or cardboard, which stops females laying eggs beside the stem. Insect-proof mesh or horticultural fleece kept in place through the growing season is also advised, again with crop rotation so that flies do not emerge from pupae under the cover. The RHS encourages natural enemies such as birds, hedgehogs and ground beetles.
Onion fly
The RHS says onion fly maggots feed on the roots and bulbs of onions, shallots and leeks, making them inedible and killing seedlings. It is most often met in central and eastern England and is most damaging in June and July. Growing from sets rather than seed can help plants survive the first generation, and some protection comes from insect-proof mesh. Badly damaged plants found towards the end of summer should be lifted carefully and burnt so that the maggots cannot finish developing.
The RHS notes that onion fly can be told apart from allium leaf miner and leek moth because it feeds on roots, not within the leaves.
Allium leaf miner and leek moth
Allium leaf miner, first detected in Britain in 2002 and now widespread in England and parts of Wales according to the RHS, attacks leeks, onions, chives, shallots and garlic. Adult flies puncture the leaves, leaving lines of white dots, and the maggots tunnel within foliage, stems and bulbs, with rotting often most obvious in autumn. Peak adult activity is March to April and September to November, and the second generation is usually the most damaging. The RHS says plants can be effectively protected by insect-proof mesh while adults are active and laying eggs, with crop rotation so flies do not emerge under the covering.
Leek moth causes similar damage, but its caterpillars are creamy white with brown heads and small legs, whereas the leaf miner larvae are white, headless, legless maggots. The RHS gives April to October as the leek moth season, with the second generation more numerous. Mesh should stay on for the whole growing season, and the white net-like silk cocoons on foliage can be found and squashed.
Nematodes and natural enemies
For carrot fly, cabbage root fly and onion fly, the RHS mentions a mixture of pathogenic nematodes sold as ‘Fruit and Vegetable Protection’ for watering into the soil against young larvae. It warns that these nematodes are not specific, may affect other soil insects and can kill non-target species. The site’s guide to buying predators and nematodes explains the timing and handling points the RHS raises.
FAQs
Does the RHS recommend spraying for these pests?
The RHS pages summarised here describe mesh, collars, timing, crop rotation and, for some pests, nematodes. They do not recommend a spray.
Why does the RHS stress crop rotation under mesh?
It says adults can emerge from overwintered pupae in the soil under the covering if susceptible plants are grown in the same ground in successive years.
How long should mesh stay on?
The RHS says leek moth and cabbage root fly mesh should stay in place for the whole growing season, while allium leaf miner mesh is needed when adults are active and laying eggs.
The bottom line
The RHS treats these vegetable pests mainly as problems to be avoided by timing, tidy sowing, fine insect-proof mesh and strict crop rotation. Carrot fly can be dodged by late sowing and early harvesting, cabbage root fly by collars and mesh, and the allium pests by covering leeks and onions when adults fly in spring and autumn. Nematodes are described as non-specific. This is general information, not a treatment recommendation.
Sources
- Carrot fly (rhs.org.uk)
- Cabbage root fly (rhs.org.uk)
- Onion fly (rhs.org.uk)
- Allium leaf miner (rhs.org.uk)
- Leek moth (rhs.org.uk)