The Effect of Interrow Plant Cover on Populations of the Cabbage Root Fly, Delia brassicae (Wiedemann)

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Abstract
The effect of cover (clover, lettuce and artificial material) on egg populations of the cabbage root fly was investigated with special reference to predation and egg-laying. Clover decreased egg numbers by a mean 36% (range 26-65%); larval numbers, adequately assessed in 1 generation only, were decreased 24% by clover and 56% by lettuce. Clover decreased egg laying by 11% in the field and by up to 81% for the first 6 days in the laboratory. Green paper and transparent polythene also diminished egg numbers. Clover enhanced predation by 34% in 1 experiment but there was no association between increased cover and predator numbers, as judged by pitfall catches. Cover by clover (33%) increased yield of cabbage in 1974 by 57% and 15% (marketable heads and average head weight, respectively) and increased soil K and P levels, but in the very dry years 1975 and 1976 clover decreased yields. The weather conditions precluding competition between clover and brassica must be established; lettuce substantially decreased yield. The need is emphasized for detailed observations on the effect of cover on host detection and egg-laying by the fly and predatory behavior of carabids.