The Natural Mortality of Erioischia brassicae (Bouche) (Diptera, Anthomyiidae) During the Egg Stage of the First Generation

Abstract

Evidence is given of predation of E. brassicae eggs on Brassica crops by the ground beetle fauna at Wellesbourne. When counts are made of the number of eggs laid, this predation results in an under-estimate of the total. A method is described of deriving the average survival of eggs, over a short period of time, by counting the eggs at different time intervals. Predation losses over the short periods are related to contemporary daily trap-catches of predators. This relationship of predation to the daily trap-catch of beetles is used to estimate the probability of egg survival on each day. The probability of egg survival on each day is used in conjunction with the number of eggs counted to estimate the total number of eggs laid. Calculations show that egg predation is the major cause of egg mortality, accounting for over 90% of the losses in the 1957 and 1958 first generations of E. brassicae. The role of the weather in modifying predation efficiency is discussed.