Gardening Advice

Spring cabbage… and the cabbage fly

We starting sowing spring cabbage in our heated greenhouses on 7 February 2012. Two varieties this year, starting with ‘Précoce de Louviers, a traditional variety with an elongated pointed head. It can be eaten raw or cooked, and originates in Normandy (Eure). Organic seeds are available from Germinance. Secondly, the Primero red cabbage is a very early maturing bolt-resistant variety. The compact head can be harvested after eleven weeks’ growth (seeds available from Voltz).
After four weeks, the young seedlings are transferred to small pots and placed in the garden for Easter weekend at the beginning of April 2012.
Cabbage is a vegetable that attracts many pests; in May, when temperatures are starting to climb, an invisible enemy already has its eye on the seedlings.
The cabbage fly, or Delia radicum to give it its Latin name, is attracted by the sulphurous compounds that characterise the brassica family. This tiny fly, 6 to 8 mm long and hard to spot, lays its eggs at the base of the cabbage. These quickly hatch into larvae that cause irreversible damage. After hatching, the larvae burrow into the soil, hollowing tunnels in the roots to feed; from this point on, the first symptoms can be observed: withering, slowed growth, leaves wilting – as if they lacked water – and turning a reddish colour. Young seedlings die, and stronger specimens stop growing. Digging around the cabbage stems, we can see tunnels, white larvae and often mould around the base and roots.

Mechanical defences

First of all, we need strong seedlings planted deeply in mounds of soil. Regular hoeing and spray watering can disrupt the cabbage fly’s laying cycle. You can also install anti-insect mesh or cut out notched cardboard or plastic collars that you can place around the stem.

Technical methods used at Villandry

As anti-insect mesh is out of the question at Villandry for aesthetic reasons, we spray insect-parasitic nematodes (Steinernema carpocapsae) on the seedlings in the greenhouse a week before we plant the cabbages in the garden. These microscopic worms need to be introduced as close to the roots as possible. The nematodes penetrate the substrate and actively seek their prey. Once they find a larva, they infiltrate it via its natural orifices, perforate its intestinal wall and release symbiotic bacteria that cause the death of the larva within 48 hours. A second generation of nematodes leaves the larva’s body after 7 to 14 days of development. The nematodes come in powdered form in sachets, and a sachet of 25 million nematodes covers 50 m² of cabbages in pots.
After planting, we spray the whole garden again with Vectobac, an organic insecticide based on Bacillus thuringiensis var. Israelensis. The bacteria are ingested by the larva, and once they enter its digestive tract the larva stops feeding and dies within 24 hours. Vectobac comes in the form of a concentrated liquid, and we use 40 ml in 20 litres of water.

As a curative measure, we have found an effective organic insecticide based on soil bacteria, Spinozad, which is available commercially to fight ants. Direct application, diluted in the watering can, to the stems of affected cabbages can eliminate the larvae and save the cabbages.

Finally, we can say that after three years’ treatment with no chemical products we have succeeded in protecting our crops against cabbage fly. But instead we are under heavy attack from flea beetles, little black coleoptera that adore brassicas and feast on our cabbages’ wide leaves. Fighting flea beetles with soap, oils and soil conditioners should also have an effect on the cabbage fly. We have yet to find the ecological counter-attack for this beetle, but when we do we will describe it in another gardening tip!

Laurent Portuguez
head gardener 

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