Etude de l'adaptation de Bruchus affinisàLathyrus sylvestris: analyse de la mortalité avant l'installation des larves dans les graines

SUMMARY: Study of adaptation of Bruchus affinis to Lathyrus sylvestris: analysis of larval mortality before establishment in the seeds Bruchus affinis (Coleoptera, Bruchidae), is an univoltin species selectively attracted to flowers of Lathyrus sylvestris: one of the three plants of which seeds are consumed by the larvae. This insect is a typical specialist in its feeding preference. However, females distribute their eggs on young pods of L. sylvestris without taking into account how many seeds are available or how many eggs are already present, whereas the development of the larvae is solitary. Neonate larvae perforate the pods and go into the ovules. Several larvae can reach the same ovule, but only one grows out completely in the developing seed. The number of eggs laid on the first pods exceed largely the number of seeds growing in these pods. The surplus of eggs on first pods leads to a high mortality. The movements of the larvae inside the pods and their penetration behaviour into the growing seeds do not suppress intraspecific larval competition. No adaptation process could be found that adjusted the distribution of eggs and larvae to an optimal use of the available seeds, nor an adaptative strategy that limited the waste of progeny. Apparently, in this case trophic specialization has not led to a reproductive behaviour of optimal foraging of resources and survival of offspring. Although the period studied took less than 10% of the annual cycle length of B. affinis, the very high mortality observed between egg-laying and establishment in the seeds probably constitutes the key-factor of the annual fluctuation of the population. The mortality factors showed much variation from one observation station to the other, making it impossible to arrange them into a hierarchic order.