Bruchidius atrolineatus (Pic) is a tropical beetle (Coleoptera Bruchidae) that develops during the larval and pupal stages in the seeds of a legume Vigna unguiculata (Walp). Two species of Hymenoptera, Dinarmus basalis (Rond) and Eupelmus vuilleti (Craw), solitary ectoparasitoids of the larvae and pupae of B. atrolineatus, were introduced successively in the presence of their hosts, varying the interval between the two introductions. When D. basalis females were introduced 24 h, 3 days or 7 days after E. vuilleti, multiparasitism was low. The females had low fecundity, and their eggs were not distributed randomly over the different available hosts. When E. vuilleti females were introduced second, they oviposited on the different hosts availabe and did not avoid multiparasitism. The presence of hosts already parasitised by D. basalis increased the reproduction of E. vuilleti, and the fecundity of the females was higher than in control batches with E. vuilleti alone. E. vuilleti seems capable of detecting the ovipositor shafts drilled by the D. basalis females, and by introducing its own ovipositors killing the D. basalis eggs or larvae. When interspecific competition was occurring the number of E. vuilleti adults emerging from the seeds was no different from that observed in control batches with E. vuilleti alone, and there were always fewer D. basalis adults than in control batches (D. basalis alone). This interspecific competition reduces the influence of the two parasitoids in the biological control of bruchid populations.

No abstract

Eupelmus orientalis (Crawford) (Hymenoptera: Eupelmidae) is a solitary ectoparasitoid of the larvae and pupae of West African bruchids. Since the biology of this species was unknown, studies were carried out on its reproductive biology. The age-dependent fecundity and offspring production of virgin and mated females of E. orientalis were determined under laboratory conditions at 33°C:23°C, 50%:80%r.h., L:D 12:12 on its host Callosobruchus maculatus (Fabricius) (Coleoptera: Bruchidae) during the first 15 days of the life cycle. No differences were found between virgin and mated females for any parameter. A study of the age-dependent fecundity and offspring production was also carried out under the same conditions over the entire life cycle of mated females. The resulting data were used to determine the longevity and reproductive potential of the species, particularly through the calculated life tables and estimation of the intrinsic rate of increase. With ten hosts per female per day, E. orientalis females had a mean longevity of 45.75 + 5.09 days. They laid an average of 219.91 + 39.08 eggs, parasitized 154.27 + 26.17 hosts, and produced 166.77 + 9.41 offspring, consisting of 80.46+14.10 males and 86.31 + 10.04 females. The total mean developmental time was 18.58 + 0.22 days for males and 20.03 + 0.21 days for females. The net reproductive rate (R o ) was 62.33 female progeny per adult female, and the intrinsic rate of population increase (r m ) was 0.1389 per day. The generation time (G) was 29.76 days and the doubling time (DT) 4.99 days. These results constitute the first step to determine the potential of E. orientalis as an agent for the biological control of bruchidae, especially C. maculatus.

Coexistence of two solitary ectoparasitoids of bruchids may be the result of counter-balanced competition. Some strategies in interspecific competition at the extrinsic level of female behaviour are identified. Dinarmus basalis (Rondani) (Hymenoptera; Pteromalidae) has adapted an evasion strategy to avoid competition. Females of D. basalis show interspecific discrimination against hosts parasitized by Eupelmus vuilleti Crawford (Hymenoptera; Eupelmidae) and lay fewer eggs in the presence of females or hosts parasitized by the latter. Eupelmus vuilleti has adapted an aggressive strategy. In contrast to D. basalis, E. vuilleti concentrates her ovipositions on hosts already parasitized by D. basalis. Females of E. vuilleti preferably use oviposition holes made by other parasitoids, and are able to kill eggs and larvae of D. basalis selectively by thrusts of their ovipositor (ovicide and larvicide). Furthermore, E. vuilleti can act as a facultative hyperparasitoid on older larvae of D. basalis. The number of E. vuilleti offspring is not affected by the presence of D. basalis on a host. Our study does not provide insight in the process of larval competition.

Lorsque 2 femelles ou 3 femelles de l'ectoparasite D. vagabundus parasitent conjointement un même lot d'hôtes l'intensité du grégarisme larvaire s'accroît par rapport à celui observé en présence d'une seule femelle pondeuse. Cela se traduit par une augmentation significative du nombre moyen de descendants des 2 sexes mais la production moyenne en fils est toujours très inférieure à celle des filles. Cependant, lorsque la densité de femelles pondeuses passe de 1 à 3, le rapport des sexes mâles/femelles des descendants évolue vers l'unité traduisant une augmentation plus rapide de l'effectif des fils que des filles.