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International audience; Biological control of bruchid beetles, Callosobruchus maculatus (Fabricius) (Coleoptera: Bruchidae), infesting cowpea seeds, Vigna unguiculata (L.)Walpers (Fabaceae), can be performed via augmentative releases of Dinarmus basalis Rondani (Hymenoptera: Pteromalidae) parasitoid wasps. Females of the latter species are therefore likely to experience intense intraspecific competition: they should encounter numerous previously parasitized hosts but also conspecific competitors, with which they may fight to secure hosts on which to lay their eggs. Such contests might therefore disrupt biological control programs. Here, we studied aggressive behavior that D. basalis females show toward conspecific competitors and subsequent host exploitation strategies. We further investigated factors that classically affect contest intensity and outcomes in animals, such as the effect of ownership status, by manipulating the residency period before the intruder's arrival. In addition, we tested the effect of the size of female reproductive tissue (measured in terms of egg load) and the quality of the habitat previously experienced by females (either rich or poor in hosts). These two factors are expected to influence the value that females place on the host and therefore the costs they are willing to pay to win it. Finally, we discussed the consequences of agonistic behaviors on females' host exploitation strategies. Our results suggest that contest competition may actually enhance host control by favoring parasitoid dispersion, rather than disrupting it.

The solitary larval parasitoid Dinarmus basalis Rondani is a promising biological control of Callosobruchus maculatus F, a major insect pest of stored cowpeas in West Africa. Farmers traditionally introduce Hyptis suaveolens L. Poit. leaves in their granaries for protection of cowpea seeds against bruchid damage. However, effects of botanical treatment using this plant on host location behavior and reproduction of D. basalis remain unknown. Olfactometer studies showed that sublethal doses of volatiles emitted by the crushed leaves and the essential oils were repellent for naive females D. basalis, which had previously developed in the absence of H. suaveolens volatiles. These females were able to move in a three-dimensional device and to avoid the host patches associated with H. suaveolens volatiles. Their reproductive activity was consequently reduced in such patches. Females, which had been exposed to sublethal doses of H. suaveolens volatiles during their postembryonic development, were no lon...

The eupelmid Eupelmus vuilleti CRW. and the pteromalid Dinarmus basalis Rond. are sympatric in West Africa. The reproduction of E. vuilleti and D. basalis females was analyzed in 10, 20, and 40 host patches when present alone or together. In 10- and 20-hosts patches, the presence of E. vuilleti affected the reproduction and offspring survival chances of D. basalis. In 40-hosts patch, only the offspring survival of D. basalis was affected by the presence of E. vuilleti. In contrast, the presence of D. basalis did not influence the reproduction of E. vuilleti and the survival chances of its offspring at all host densities tested. Multiparasitism did not occur at random. In a choice test, E. vuilleti showed a preference for hosts previously parasitized by D. basalis on healthy hosts. This attraction is mediated by chemical markers deposited by D. basalis females on the surface of the seed. At high host density, the likelihood of encountering a seed containing a host previously parasitized by D. basalis was lower for E. vuilleti, affecting moderately D. basalis reproductive success.

The parasitoid species, Dinarmus basalis, attacks bruchid beetle larvae concealed within hard seeds. Female wasps are able to discriminate the quality of their host but the position in the concealed host system where this discrimination takes place remains unclear. Under laboratory conditions, we investigated whether the cues were detectable: (1) at a medium or short distance from the seed, (2) on the seed, or (3) within the seed, at the point of contact with the host. In particular, we investigated whether internal or external markings are applied by the parasitoid and then recognized in a subsequent encounter or by a conspecific encountering the host. Detailed behavioral observations did not provide evidence of markings being applied onto the seed or used, but showed that host discrimination occurred on the basis of internal cues. The lack of evidence of external markings is unexpected and raises the question as to which circumstances in the parasitoid's ecology and life history could result in such a lack not causing a reduction of fitness.

The behavioural and physiological responses of the synovigenic parasitoid, Dinarmus basalis (Rond.) (Hymenoptera: Pteromalidae), were investigated whilst the wasps were submitted to conflicting oviposition stimuli: an environment which induced them to restrict egg-laying and a physiological factor, egg-load pressure, which induced them to lay eggs. When individual females depleted patches containing hosts parasitized 24 h or 72 h beforehand, they laid preferentially on unparasitized hosts and the number of eggs laid decreased significantly. Although they remained for several days in such patches, egg-load pressure did not lead them to modify their strategy (i.e. to accept all the available hosts including the already parasitized ones). In this synovigenic species, the oviposition constraint resulted in the retention of oocytes. Dissection and histological observation showed that the unlaid eggs are resorbed, a reproductive regulatory process which has seldom previously been reported in the presence of unparasitized hosts. This physiological response was induced by the mere presence of the parasitized hosts rather than their density. The females’ response was also influenced by the presence of conspecifics. They still avoided laying on parasitized hosts but the number of eggs laid daily became comparable to that observed in patches containing only unparasitized hosts. Thus, D. basalis females adapted their strategy in response to their environment, in which the most significant factor seemed to be the quality of the hosts provided, and their physiology. The originality and the importance of these results are discussed.

The sex-allocation behaviour of the solitary ectoparasitoid Dinarmus basalis (Hymenoptera: Pteromalidae) was investigated by examining the female's response to two proximal factors: the host-patch characteristics and the conspecific female density in the patch. The offspring sex-ratio of single females presented with unparasitised hosts was female biased (approximately three daughters to one son in the progeny) whatever the host density tested. The sex-ratio in the presence of hosts parasitised by a conspecific 48 h beforehand was male biased. The proportion of male offspring also increased when the number of conspecific females exploiting the patch changed from a single female to a group of females, whatever the group size. No sexual differences in mortality as a result of larval competition on superparasitised hosts were observed. The offspring sex-ratios observed in response to the host-patch characteristics were the result of manipulation of the egg fertilisation by the females during the oviposition phase. Thus, the variability of the sex-ratio in response to an increase in the number of conspecific females in the same patch represented a reduction in egg fertilisation. These results are analysed with regard to the predictions of Local Mate Competition theory and the females' host discrimination ability.

Callosobruchus maculatus (Fabricius) develops in the seeds of Vigna unguiculata in the Sahel region of Africa. The reproductive activity of this species and the development of its offspring were studied during the dry season at Ouagadougou and Bobo Dioulasso (Burkina Faso), two zones with different climatic conditions. The length of imaginal life of the females and the developmental time varied with changes in climatic conditions, the lowest values for both parameters being found at the end of the dry season when temperature and humidity increased. In the two zones, variations in the density of the bruchid populations have been studied in experimental stores. The density of the bruchid population was low at the beginning of the study and increased when temperature and humidity rose. The introduction of Dinarmus basalis (Rondani) adults to the stores caused a significant reduction in the numbers of C. maculatus adults. The density of the D. basalis populations was low from December to February and increased when temperatures and then humidity became higher. The increase of the D. basalis population and the rate of parasitism were greater at Bobo Dioulasso than at Ouagadougou. Laboratory studies have demonstrated that fecundity, duration of adult life and developmental times depend on humidity. This study has shown the importance of local climatic conditions on parasitoid reproduction and development and also in the control of bruchid populations in stores.

In the solitary ectoparasitoid, Dinarmus basalis (Hymenoptera: Pteromalidae), the occurrence of superparasitism according to the unparasitised host density, and the nature of the host(s) provided was investigated in laboratory studies. In this species superparasitism was observed whatever the experimental conditions used, but the degree of superparasitism depended on the density of its host, Bruchidius atrolineatus (Coleoptera: Bruchidae). Superparasitism was due to successive egg-laying phases on the same host. However, females were able to discriminate between unparasitised hosts and hosts parasitised from 8 h to 72 h beforehand by themselves or by conspecifics. There was no conclusive evidence that superparasitism in the presence of a host parasitised 30 min before was linked to an absence of host discrimination. Host discrimination in this species is achieved by host-quality markers. These are individual-specific markers since conspecific superparasitism rates were often higher than self superparasitism rates. One deterrent substance is emitted by the females during oviposition onto the egg or released by the 16 to 24 h-old egg itself. Another host-quality marker is associated with the presence of a larva on its host. On the other hand, host discrimination ability did not always imply avoidance of superparasitism. In D. basalis there exists a positive relationship between the survival probability of the second egg and the tendency to superparasitise, and superparasitism could therefore result in a significant fitness gain. Under our experimental conditions, D. basalis females exhibited a wide range of oviposition behavioural plasticity in relation to the parasitoid developmental stage, the type of superparasitism, and the encounter rate with unparasitised hosts.

International audience; Dinarmus basalis Rondani (Hymenoptera: Pteromalidae), an ectoparasitoid of Bruchid pest of stored cowpeas, is a potential biological control agent. We investigated whether grain protectants from Hyptis spicigera and H. suaveolens (Lamiaceae) disturb parasitism and post embryonic growth of the parasitoid. When cowpeas containing bruchid larvae were treated before being placed in the presence of D. basalis females, the rate of parasitism decreased on average up to 24 and 47% respectively in the presence of leaves dry powder and essential oils from both plant species. The estimated larval mortality was higher on treated (9.5 to 28.6%) than on non treated parasitized hosts (2.8%), depending on the plant species and the plant extract tested. Additional larval mortality was higher for essential oil, and for H. suaveolens..An in-depth analysis of the pre-imaginal development in capsules adapted for this purpose showed that without treatment, mortality was recorded only for eggs and the two first larval stages. In the presence of treatment, mortality not only significantly increased on eggs but also spanned over all other pre-imaginal stages. Pre-imaginal development was also significantly extended in the presence of treatment, depending on the type of extract and dose used. In conclusion, both plant species exert acute toxicity on D. basalis larvae and also act as growth inhibitors. These results provide additional information that enable to move towards a harmonious use of allelochemical compounds from plants while protecting biocontrol agentsfrom their adverse effects.

Dinarmus basalis Rond. (Hymenoptera : Pteromalidae) et Eupelmus vuilleti Craw. (Hymenoptera : Eupelmidae) sont les deux principales especes d’ectoparasitoides solitaires de larves et de nymphes de Bruchidae en Afrique de l’Ouest. Des etudes anterieures effectuees sur ces parasitoides ont montre que D. basalis et E. vuilleti entraient en competition interspecifique. E. vuilleti est competitivement superieur a D. basalis et l’elimine quand les deux especes se developpent a densite initiale egale. Nous avons montre, dans les conditions du climat subtropical de type guineen que l’issue de la competition pouvait dependre du nombre de femelles de D. basalis exploitant le patch par rapport a celui de E. vuilleti. Quand la proportion initiale des concurrents est en faveur de D. basalis (75 % de D. basalis et 25 % de E. vuilleti), la descendance est majoritairement constituee de D. basalis. Ces resultats permettent d’expliquer en partie l’abondance de D. basalis par rapport a E. vuilleti dans les greniers de la zone subtropicale de type guineen. Dinarmus basalis Rond. (Hymenoptera : Pteromalidae) and Eupelmus vuilleti Craw. (Hymenoptera : Eupelmidae) are the two main solitary ectoparasitoids of larvae and pupae of Bruchidae in West Africa. Former studies carried out on these species showed that D. basalis and E vuilleti entered interspecific competition. E vuilleti is competitively higher than D. basalis and eliminates it when the two species develop with equal initial density. We showed under subtropical climate conditions in guinean zone that the exitence of the competition could depend on the number offemales of D. basalis exploiting the patch compared to that of E vuilleti. When the initial proportion of the competitors is in favour of D. basalis (75 % of D. basalis and 25 % of E vuilleti), the descent mainly consists of D. basalis. These results permitted to explain the abundance of D. basalis compared to E. vuilleti in guinean zone.

Females of Dinarmus basalis, a solitary ectoparasitoid, strongly avoid superparasitising hosts bearing eggs which are 20 or more hours old. Our objective was to determine the mechanism by which the factors inducing host discrimination and such an oviposition deterring effect act, and the origin of the deterrent. This study showed that the information is carried by the parasitised host. It is acquired by a transfer of molecules from the wasp egg onto the host and it requires contact between egg and host for at least 4 h. The factors transferred do not, as commonly reported in other species, come from the female's genital apparatus but from the live egg itself. Consequently, the D. basalis egg directly participates in the success of parasitism by releasing factors which elicit the oviposition host deterring effect. The originality and the ecological importance of these results are discussed.

Host selection in parasitic hymenopterans involves the use of a wide array of chemical signals emanating not only from the host itself but also from its food (Lewis & Martin, 1990; Tumlinson et al., 1993). For each tritrophic combination, only some particular cues which have proved to be reliably associated with the presence of hosts, are used (Vet & Dicke, 1992). Numerous studies show that the use of chemical cues can be genetically fixed but that learning often plays a predominant role (van Alphen & Vet, 1986; Vet & Groenewold, 1992; Tunings et al., 1993). By learning profitable cues, females can adapt to spatial and temporal variations in their foraging environment (Turlings et al., 1989; Lewis et al., 1990). However, little is known about how different learning abilities develop in different species and it is still unknown whether these differences could be better explained by differences in ecological niches or by taxonomic status of the species.

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.