Department of Pure and Applied Biology, University of Leeds, Leeds LS2 9JT, UNITED KINGDOM.
A natural population of Pleuroptya ruralis has been studied for three years with the following aims: (1) to elucidate its life history; (2) to study the variations in the larval and adult densities; (3) to identify the mortality factors acting upon them; (4) to investigate possible relationships between the intensity of the mortality factors and the spatial distribution (horizontal and vertical) of P. ruralis; (5) to describe the role of the major mortality factors in determining the size of the population studied; (6) to assess the potential of the system for further studies about the mechanism of population regulation.
P. ruralis caterpillars were individually labelled and followed throughout their development on a natural growth of their food plant, Urtica dioica (the nettle), near Leeds, Yorkshire, United Kingdom. Parasitism and predation as well as the distribution and movements of the larvae on the host plant were assessed.
Caterpillars were also collected and reared in the laboratory for identification of the parasitoid species and to provide support for the field results.
Adults were studied in the field by the mark- release- recapture method. Potential female fertility was investigated in the laboratory.
The population suffered a dramatic decline in numbers during the period of this study. The main factors determining the observed population levels were the interactions between the weather, time spent in development, deviations in the sex ratio, bird predation towards the completion of pre-adult development and parasitization by a braconid (Macrocentrus grandii). Other parasitoids also had some effect, notably Diadegma sp. Predation also had a strong impact on the population of M. grandii. Birds appear to concentrate their attack in the part of the plant in which they are more likely to find the prey.
Weather also influenced adult activity and availability of nectar sources in the field, and starvation of females reduced fecundity in the laboratory. Thus, egg shortfall may also have been important in the decrease of population size.
The field method used with the caterpillars provided detailed and valuable information about individual and spatial variation and it is suggested that the system 'nettles / P. ruralis / natural enemies' can be a powerful tool for studies on the mechanisms of population regulation.