The population dynamics of the cabbage aphid, Brevicoryne brassicae (L.), in Central Otago, New Zealand

Abstract

The field method developed by McLaren (1968) for making life table studies of the cabbage aphid, Brevicoryne brassicae (L.), was used to record details of survival, causes of mortality, daily reproductive output and total natality on a range of cruciferous crops growing at Earnscleugh, Central Otago, over the summer-autumn period of 1968 and 1969. The information was used to investigate the effect of the different mortality factors. Predation, parasitism and environmental factors were found to cause relatively low levels of mortality. The largest and most variable losses were attributed at first to a "missing" category, since these aphids disappeared without trace. But it was later shown that at least 75% of those that went missing were falling off the plant onto the ground. Falling off was found to be the most significant cause of changes in population numbers in the subsequent generation. Losses from falling off occurred in all stages of the insect's development and were shown to be influenced largely by the plant, particularly the variety of host plant, the age of the leaves and of the plant itself. It was suggested that falling off arose through disruption of the feeding activity of the aphid on unsuitable host leaves resulting in the loss of an important means of anchorage, the aphid's stylets embedded in the leaf tissues. No evidence was found for any density relationships, mainly because aphid densities were relatively low throughout the period of the study.

The results were used to develop a simple mathematical model to describe changes in B. brassicae numbers, but its application was thought to be severely limited through the absence of density relationships and lack of knowledge of the plant factor(s) responsible for falling off.

Several discrepancies were found between the assumptions in the Hughes and Gilbert (1968) model and the results of this study, the most important being that the plant could not be treated as a constant.

On the contrary, it was the conclusion of this study that the plant was the most important variable in the population dynamics of B. Brassicae in Central Otago.