Natural Mortality of Erioischia brassicae (Bouche) (Diptera, Anthomyiidae) During the Immature Stages of the First Generation

Abstract

Recent observations (Wright 1956, 1957, 1958) have shown that certain chemical control measures may increase the infestation of the roots of brassica crops by the larvae of Erioischia brassicae (the cabbage root fly). This work led to the conclusion that the insecticides were interfering with a system of natural control of the larval population. To aid the interpretation of these results detailed investigations of the population dynamics of E. brassicae were begun in 1955, together with studies of the biology and habits of associated parasites and possible predators. There are many references in the literature to organisms responsible for mortality at different stages in the development of E. brassicae. Predation of the egg stage by trombidiid mites (Schoene 1916, Gibson & Treherne 1916, and Lowry 1915), ants (Schoene 1916) and by carabid and staphylinid beetles (Wishart, Colhoun & Monteith 1957) has been recorded. The last team of workers stained eggs with a solution of Bengal rose and placed groups of marked eggs around plants in the field. Collections made after periods of 24, 48 and 72 hours showed large daily losses of marked eggs. Mortality in the larval stage has been caused by predatory ants (Schoene 1916), beetles (Gibson & Treherne 1916, de Wilde 1947) and other anthomyiid larvae (Smith 1927). Many hymenopterous parasites attack the larval stages but the mortality occurs after the puparium is formed. Five species of the Braconidae, three of the Cynipidae, and four of the Ichneumonidae, have been reared from E. brassicae puparia (Wadsworth 1915, James 1928, Paillot 1914, Schoene 1916, Vodinskaya 1928, Colhoun 1953, de Wilde 1947, Nixon 1939, and Wishart, Colhoun & Monteith 1957). Of these parasites only the cynipid Idiomoryha rapae (Westwood) is of major importance. Parasitism of the pupal stage by species of Aleochara (Coleoptera, Staphylinidae) has been recorded by most of the authors cited above. Predation of the adult flies by anthomyiids (Schoene 1916) and by cordylurids (Hobby 1931, Reid 1958) has been recorded. The adult stage is also attacked by the entomogenous fungus, Empusa musca (Schoene 1916). Various techniques have been developed to estimate the populations of the immature stages of Erioischia brassicae, and while the present paper deals only with the population dynamics of the first generation, the same techniques have also been used in investigations of the later generations of the life-cycle. The