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Résumé :
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Over a period of 10 years, the population development of the pea midge in northern Switzerland was followed. The investigations started at about the peak of the gradation and ended after the collapse of the population. The data are supplemented by a multi-year study in a second pea-midge population. During these field studies, the density of the galls and the appearance of the pea midge and its main parasitoid, Pirene chalybea Hal., in emergence traps and in pea fields, were monitored. Investigations in the laboratory provided data on C. pisi and P. chalybea on the parts of the population emerged without diapause after the first and subsequent overwinterings, on the species composition of the parasitoids, and on the rates of parasitisation and the longevity. During the investigations in the field, the number of galls per 100 tillers increased from 256 to 278 and then dropped continuously to 8. The rate of parasitisation increased from 5% to a maximum of 63% 3 years after the estimated maximum of the population density of the pea midge. The emergence of C. pisi and P. chalybea was well synchronised. The appearance of females of C. pisi in pea fields corresponded to that in emergence traps. In the laboratory, too, C. pisi and P. chalybea hatched in the same period. The sex ratio of both species in the laboratory populations was balanced in contrast to the populations caught in the emergence traps. In the partial population, however, there were differences. With increasing duration of development or overlying (diapause lasting more than one hibernation), the proportion of C. pisi females increased from 41 to 67% while that of P. chalybea decreased from 61 to 50%. C. pisi had a more pronounced tendency to overlie than P. chalybea. 24% of C. pisi but only 8% of P. chalybea hatched after two or more simulated hibernations. Consequently, the corresponding rate of parasitisation changed: it was highest after the first hibernation and gradually decreased after the following hibernations. It is discussed whether prolonged overlying is a strategy of gall midges to escape their parasitoids. Females of C. pisi lived, on average, for 158 hours at 20-degrees-C in the presence of tap water while those of P. chalybea lived for only 50 hours. The longevity of the latter could be prolonged to 147 hours by feeding them with honeywater.
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