S. Keller et C. Schweizer, INVESTIGATIONS ON THE POPULATION-DYNAMICS OF THE PEA MIDGE CONTARINIA-PISI WINN (DIPT, CECIDOMYIIDAE) AND THEIR PARASITOIDS, Journal of applied entomology, 118(3), 1994, pp. 281-299
Over a period of 10 years, the population development of the pea midge
in northern Switzerland was followed. The investigations started at a
bout the peak of the gradation and ended after the collapse of the pop
ulation. The data are supplemented by a multi-year study in a second p
ea-midge population. During these field studies, the density of the ga
lls and the appearance of the pea midge and its main parasitoid, Piren
e chalybea Hal., in emergence traps and in pea fields, were monitored.
Investigations in the laboratory provided data on C. pisi and P. chal
ybea on the parts of the population emerged without diapause after the
first and subsequent overwinterings, on the species composition of th
e parasitoids, and on the rates of parasitisation and the longevity. D
uring the investigations in the field, the number of galls per 100 til
lers 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 hatc
hed in the same period. The sex ratio of both species in the laborator
y populations was balanced in contrast to the populations caught in th
e emergence traps. In the partial population, however, there were diff
erences. With increasing duration of development or overlying (diapaus
e lasting more than one hibernation), the proportion of C. pisi female
s 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. cha
lybea. 24% of C. pisi but only 8% of P. chalybea hatched after two or
more simulated hibernations. Consequently, the corresponding rate of p
arasitisation 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 the
ir 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 prolonge
d to 147 hours by feeding them with honeywater.