The effect of interrow plant cover on populations of the cabbage root fly, Delia brassicae (Wiedemann).
Abstract
The effect of cover (intersown clover or lettuce or artificial material) on egg populations of Delia brassicae (Wied.) on cabbage crops was investigated in field studies in Lullymore, County Kildare, Irish Republic, in 1973-76 with special reference to predation by carabids and oviposition. Clover decreased egg numbers by a mean of 36% (ranging from 26 to 65%); larval numbers, adequately assessed in one generation only, were decreased 24% by clover and 56% by lettuce. Clover decreased oviposition by 11% in the field and by up to 81% for the first 6 days in the laboratory. Green paper and transparent polythene also decreased egg numbers. Clover enhanced predation by 34% in one experiment but there was no association between increased cover and predator numbers, as judged by pitfall-trap catches. Thirty-three percent cover by clover increased the yield of marketable heads and average head weight in 1974 by 57 and 15% respectively, and increased soil K and P levels; however, in the very dry years of 1975 and 1976, clover decreased yields. It was stressed therefore that the weather conditions precluding competition between clover and brassica have to be established. Lettuce substantially decreased yields. The need is emphasised for detailed observations on the effect of cover on food-plant detection and oviposition by the fly and on predatory behaviour of carabids.<new para>ADDITIONAL ABSTRACT:<new para>Clover sown between the rows at about the same time as cabbages, cv. Greyhound, reduced Delia brassicae egg numbers in the field by 36% and larval numbers by 24%. Lettuce, cv. All-the-year-round, reduced larval numbers by 56%. The number of marketable cabbages produced in the field was increased by 57% and head weight by 15% by clover in one year, but in the same year lettuce depressed yields and in two dry years clover depressed yields. Clover enhanced predation by 34% in one experiment but there was no association between increased cover and predator numbers.