Preliminary Investigations on the Biology and Ecology of the Parasites and Predators ofBrevicoryne brassicae(L.)

Abstract

During 1953 and 1954, experimental plots were set up on two sites in eastern England and regular fortnightly samples were taken in an attempt to evaluate the degree of control of the Cabbage Aphid, Brevicoryne brassicae (L.), exerted by its parasites and predators.The plot at each site consisted of a block of 5 rows of 40 plants and each main sample consisted of a sample of three leaves (one upper, one middle and one lower leaf) from 25 per cent. of the plants. Two such samples (A & B) were taken at each visit and were so arranged that the plants were sampled in strict rotation. The leaves were taken back to the laboratory and sample A was used to assess the number of Aphids, the other for an assessment of parasitism. The Aphids were removed from sample A and put into 95 per cent. alcohol and counted. The mummies were removed from sample B and put into labelled tubes. The leaves from sample B were then kept in insect-proof containers and examined daily for the next fortnight. All mummies which formed were removed and sorted. The insects which emerged from the mummies were killed, sexed and recorded. Predators taken from this sample were identified and recorded and used for further experiments. The validity of counting Aphids from one sample and parasites and predators from another was investigated by a series of “check-samples” taken from arbitrary plots in the fields. A shorter method for assessing parasitism was tried, using a sub-sample of the main sample, and an explanation is given for the disparity between the two sets of results.