Cabbage root fly

Identification

A greyish fly resembling a small housefly, which lays its eggs at the base of brassica plants and on the moist soil around them. The white, legless larvae that hatch feed in the roots and root collar of cabbage, cauliflower, broccoli, radish and turnip. Damaged plants wilt in the sun, grow poorly, take on a bluish tinge and topple over easily or die off completely, while in radish and turnip the tunnels bored by the larvae are visible. The greatest damage occurs on seedlings and young plants shortly after transplanting. The adult fly measures about 5-6 mm, is ashy grey, with dark bristles on its body and legs; males have a dark longitudinal stripe on the upper side of the abdomen. The eggs are white, elongated, about 1 mm long, laid singly or in small groups right by the root collar and in the crevices of moist soil; a single female usually lays over a hundred of them. The larvae are white, legless and headless maggots, tapering towards the front, growing to about 8 mm, which chew irregular tunnels in the roots and root collar, and in the fleshy roots of radish and turnip bore galleries that render them inedible. They pupate in the soil in brown, barrel-shaped puparia. The cabbage root fly overwinters as a puparium in the soil and under our conditions usually produces two to three generations per season; the first, most damaging flight of the flies occurs at the turn of April and May, as the soil warms up (it is sometimes linked phenologically to the flowering of dandelion and fruit trees). Damaged roots become an entry point for bacterial and fungal rots, so infested plants often additionally rot at the base and are easily pulled out of the ground without roots.

Treatment

Discs (collars) of felt or roofing felt placed around the root collar of transplants prevent the larvae from reaching the roots. Covers of fleece or fine mesh, laid on immediately after transplanting, protect the plants from egg-laying. Crop rotation (not growing brassicas after brassicas) and yellow traps for monitoring the flies' flight and timing protection also help. A more reliable indicator of the treatment timing than coloured traps alone, however, is counting the eggs laid by the root collar and observing phenological events - the first flight of the cabbage root fly coincides with the flowering of dandelion and fruit trees; yellow sticky traps are only of auxiliary value here, as they mainly attract aphids and whiteflies. It is also worth earthing up the plants (hilling) - heaping moist soil onto the lower part of the stem stimulates the formation of adventitious roots above the damage and helps the plant survive larval feeding. The sowing and transplanting dates are best chosen so that the most vulnerable seedling stage does not coincide with the peak flight of the first generation. Heavily infested plants should be removed and destroyed together with the root ball, and after harvest a deep digging or ploughing should be carried out, which brings the overwintering puparia to the surface where they die from frost and are eaten by birds. A supporting biological method is watering the soil with a suspension of insect-killing nematodes (Steinernema feltiae), which infect the larvae in the ground; they work best in moist and sufficiently warm soil. Chemical control of the cabbage root fly is difficult and limited, because the larvae feed hidden in the soil and inside the roots - in amateur cultivation the basis remains prevention (collars, covers, crop rotation), while soil-applied insecticides play a secondary role in protecting against this pest.

How to tell it apart from look-alikes

Control — organic and chemical methods

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