Sycamore aphid numbers and population density. II. Some processes.
Laboratory populations of the sycamore aphid tend to reach an asymptotic density in the autumn, irrespective of the initial number of aphids colonizing the trees. This population behaviour is not observed in the field populations (see Wellings et al. 1985), where immigration and density-disturbing weather conditions may be important. Analysis of the data from the laboratory populations identified two density-dependent processes: flight by adult first generation aphids and reduced fecundity of second generation adults as a result of crowding experience. Growth and development rates of individual aphids were influenced by the nutritional value of their host plant and by crowding density. Other laboratory experiments indicated that the effect of crowding was mediated through the host plant, rather than solely through direct behavioural interactions. This effect was shown to act during development and the pre-reproductive period, as well as on the reproductive rate of adults. The role of these intraspecific processes in the regulation of sycamore aphid numbers is discussed.