December 5 2013 Latest news:
Martyn Davey, Head of Horticulture and Design, Easton College
Tuesday, November 20, 2012
Question: Please could you identify a small maggot-like pest, white in colour, which attacks my potatoes and also I have found them in carrots too. They congregate in large numbers and really decimate the crop. I rotate my crops but they appear all over my plot. Could they be eel worm and what is the remedy? (Mr J Prior, Newton St Faith)
The problems are almost certainly not linked and I am sure if you can see the maggots with your bare eyes they are not eel worm. The problem on the carrots will be the dreaded carrot fly. Carrot fly is a small black-bodied fly whose larvae feed on the roots of carrots and related plants. They cause rusty brown scars ring the tap roots of carrot and other susceptible vegetables, making them inedible. When the roots are cut through, tunnels are revealed, often inhabited by slender creamy-yellow maggots up to 9mm long. Maggots hatch from eggs laid in late May–June and in August-September. Newly-hatched larvae feed on the fine roots but later bore into the tap roots. The brown scars are where tunnels near the root surface have collapsed. Two or three generations of carrot fly can develop between late spring and autumn, with the pest overwintering.
The best methods of control are to sow sparsely to avoid thinning the seedlings. Late sown carrots (after mid-May) avoid the first generation of this pest; similarly carrots harvested before late August avoid the second generation. Protect vulnerable crops by surrounding them with 60cm (2ft) high barriers made of clear polythene to exclude the low-flying female flies, or cover the plants with horticultural fleece, such as Enviromesh. You must practise crop rotation with these methods. Choose carrot cultivars that are less susceptible to carrot fly, such as Fly Away, Maestro, Resistafly and Sytan – but none is fully resistant. A mixture of pathogenic nematodes, sold as Nemasys Grow Your Own, can be watered into the soil.
Chemicals can be used but these are limited, Lambda cyhalothrin (Westland Plant Rescue Fruit & Vegetable Bug Killer) can be sprayed against the adult pest.
The potato problem is more likely to be wireworms, the larvae of click beetles (Elateridae). These are soil-inhabiting pests, typically found in grassland but also capable of attacking a wide range of crops. Carrot, and leeks may also be affected. This is a problem that can be controlled with good crop rotation. There are traps available to catch the adults that can be used to help affect some control.