Sycamore Aphid Numbers and Population Density: II. Some Processes

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Abstract
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 behavior is not observed in the field populations where immigration and density-disturbing weather conditions may be important. Analysis of the data from the laboratory populations identified 2 density-dependent processes: flight by adult 1st generation aphids and reduced fecundity of 2nd 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 behavioral 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.