Simulation of counts of aphids over two hectares of Brussels sprout plants

Abstract

Abstract Populations of the mealy cabbage aphid, Brevicoryne brassicae (L.), on Brussels sprout plants, Brassica oleracea (L.) were sampled, fortnightly throughout the early season of 1996, in 2 ha 2 blocks. Sampling was done on two spatial scales, firstly over 25, 5×5 m sample areas, and then on the 20 plants within each of these areas. On each occasion, the occurrence of aphids was recorded on all the 500 plants sampled per block (one in 16 plants) and the count of the number of aphids per plant measured on a selected five plants per area, yielding information from 125 plants per block (one in 64 plants). The data were analysed to study various aspects of the aphids’ frequency distribution at each of the two scales, particularly the relationship between variability and population density, and the relationship between the proportion of plants infested and density. Patterns of the incidence of infestation differed radically between the two blocks. Also, for a given aphid density, more plants were infested as the season progressed, and at that density there were therefore, on average, fewer aphids per plant at later dates. Methodology was outlined to simulate realistic aphid counts, conforming to the relationships found for each of the two scales around the time of the achievement of a threshold incidence of 10% infestation. This was used to generate a count for each of the 8000 plants per block, for each of the blocks, representing a 64-fold increase in information. It was verified that these simulations provided realistic sets of counts, that could be used subsequently to study the efficiency of aphid sampling schemes and to replace costly extensive surveys for the development of control strategies.