The effect of plant density on populations of the cabbage root fly (Erioischia brassicae (Bch.)) and the cabbage stem weevil (Ceutorhynchus quadridens (Panz.)) on cauliflowers

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Abstract
To study the effects of plant density on populations of the cabbage root fly (Erioischia brassicae (Bch.) ) and the cabbage stem weevil (Ceutorhynchus quadridens (Panz.)), cauliflowers were planted in 24 concentric circles to achieve spacings of 10–90 cm at 22 plant densities (1·5–83/m2). Some plants were treated with a root drench of chlorfenvinphos. Each week female cabbage root flies laid approximately three times as many eggs per individual plant at the lowest than at the highest plant densities tested. This was equivalent to approximately 350 and 5000 eggs/m2, respectively. The numbers of cabbage root fly pupae produced ranged from 11/m2 at the lowest to 210/m2 at the highest plant density. In the absence of an insecticide, increasing the plant density considerably increased the absolute population of the pest without affecting cauliflower yield. Approximately seven times as many flies were produced per unit area of untreated mini-cauliflowers as from an equivalent area of plants growing at a conventional density. When chlorfenvinphos was not applied, damage by the cabbage stem weevil occurred in 30% and 70% of the plants grown at the lowest and highest densities, respectively.