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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 use of beni seed, Hyptis spicigera Lam. (Labiatae), to protect stored cowpeas from bruchid attacks is a widespread practice in West Africa. More knowledge about its actual effects on storage pests is needed to enhance its effectiveness. In this laboratory study, repellent and lethal effects of powder and essential oil from H. spicigera on Callosobruchus maculatus, the main pest of stored cowpeas, were studied. Repellency tests undertaken with a Y-tube olfactometer showed that powder was repulsive for C. maculatus adults. On the other hand, this product had no insecticidal activity. Essential oil had a dose-dependent insecticidal effect while sublethal doses repelled the adults. Both products reduced oviposition by 15 – 19 and 7 – 78% according to the doses of powder and essential oil, respectively, tested. Similarly, egg viability decreased with increasing doses of powder and essential oil used in a range of 40 – 75 and 24 – 86%, respectively. Only essential oil was lethal to C. maculatus lar...

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.

Callosobruchus maculatus (F.) (Coleoptera : Bruchidae) is a tropical beetle that develops in the seeds of Vigna unguiculata. C. maculatus adults show an imaginal polymorphism with differences in morphological, behavioral and reproductive characteristics. Adults of the flight morph that emerge in cowpea storage systems were studied under natural climatic conditions. A large number of the flight morph females were in reproductive diapause and had a long imaginal life. These females did not synthesize vitellogenin, produced a specific diapause protein and possessed significant protein reserves. This suggests that the beetles survived in the tropical ecosystem for a long time and colonized the crops during the cowpea growing and flowering phases. Analysis of reproductive activity in females captured in the V. unguiculata crops indicates that they terminated their reproductive diapause and began to lay eggs as soon as the pods were formed. Few females of the flight morph were sexually active at the beginning of imaginal life. In this paper we discuss the adaptive significance of these two reproductive strategies in females of C. maculatus.

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.

Dinarmus basalis (Rond) and Epelmus vuilleti (Crw) are two Hymenopteran species, which are solitary ectoparasitoids of bruchid larvae. In the presence of seeds of Vigna unguiculata (Walp) containing hosts parasitized by E. vuilleti, a high percentage of D. basalis females avoided multiparasitism whatever the age of the eggs or the larvae present on the host. The least avoidance was observed when the hosts were parasitized by E. vuilleti 30 min beforehand. This avoidance behavior is adaptive and is related to the low survival chances of the D. basalis larvae when they are in interspecific competition with E. vuilleti larvae. The analysis of the behavior of D. basalis demonstrated that the avoidance of multiparasitism could be due to the perception of two signals; an external signal deposited on the surface of the seeds during the E. vuilleti oviposition phase and an internal signal due to the presence of the eggs and larvae at the surface of the hosts. E. vuilleti females did not avoid multiparasitism and multiparasitized the hosts bearing D. basalis eggs or larvae. The behavior of E. vuilleti females was not disturbed by the presence of its competitor. Under these conditions of interspecific competition, the survival chances of E. vuilleti larvae were very high whatever the age of its competitor D. basalis. The two species of parasitoids could move in a column containing healthy seeds of V. unguiculata and patches with seeds containing parasitized or unparasitized larvae. The distribution of D. basalis females introduced into these columns depended on the host quality. They avoided the patches containing the hosts parasitized E. vuilleti and were found in the patches with healthy hosts. The behavior of E. vuilleti females was very different; the distribution of the females and the parasitism and multiparasitism rates were not affected by the quality of the hosts present in the patches. The adaptive significance of the behaviors of these two species was analyzed in relation to the survival chances of their offspring.

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.

Eupelmus vuilleti (Crw) is a solitary ectoparasitoid of several bruchid species which develop inside Leguminosae seeds. In this hymenopteran a preemergence learning process is responsible for a specific response of females toward the host and host-plant volatiles. This learning occurs after the imaginal molt, while the adult remains in the larval chamber of its host. The persistence of the response toward host and host-plant volatiles of females reared onBruchidius atrolineatus (Pic) larvae developing inVigna unguiculata (Walp) seed was investigated in this study. In the absence of any reinforcement, the response towardB. atrolineatus larvae persisted for 6 days after female emergence from the seeds, whereas the response towardV. unguiculata seed volatiles was maintained for 7 days. Our experiments showed that a later learning process also takes place inE. vuilleti. An ovipositional experience on a given plant-host complex enabled females to learn the stimuli from that complex. This learning occurred even if the complex experienced during oviposition differed from the one on which females were reared. After an ovipositional experience on a new plant-host complex, the response acquired through preemergence learning was maintained. These experiments demonstrate that theE. vuilleti females can memorize cues from several species during successive learning processes. The adaptive significance of these learning capacities, which confer an important behavioral plasticity to this generalist parasitoid, is discussed.

The response to different host and plant species odours was investigated in Eupelmus vuilleti (Crw). This hymenopteran is a solitary ectoparasitoid of several species of bruchids developing inside Leguminosae seeds. The locomotor behaviour of females reared on Bruchidius atrolineatus (Pic) larvae developing in Vigna unguiculata (Walp) seeds was analysed using a tubular olfactometer. Females showed a specific sensitivity to the semiochemicals emanating from the host and the seed species on which they had developed. Odours from V. unguiculata seeds were attractive to the parasitoid and stimulated their locomotor activity. Odours from Vigna radiata (Wil) seeds had no effect on the locomotor behaviour. Odours from B. atrolineatus larvae were attractive to the females whereas odours from Callosobruchus maculatus (Fab), another bruchid species, had no effect. By isolating the females from the seed and the host at different developmental stages, we found that the specific sensitivity observed resulted from an early adult learning. This learning which occurs before the emergence from the seed while the imago is in the larval chamber of its host is dependent on contact with the seed and the host larval remains.

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.

Eupelmus vuilleti (CRW) is a solitary ectoparasitoid which, under our experimental conditions, developed at the expense of the larvae and pupae of Bruchidius atrolineatus (Pic) (Coleoptera, Bruchidae). The larvae of this beetle are found inside the seeds of cowpea, Vigna unguiculata (Walp). Using a tubular olfactometer, we determined the olfactory stimuli involved in the orientation of females of the hymenopterous species E. vuilleti towards one of its hosts; B. atrolineatus. Several odours were tested including those from host plants of B. atrolineatus, from non-host plants, from the host itself and from the waste products of the host. The odours of pods or dry seeds of V. unguiculata (host plant of the phytophagous insect) affected the locomotor behaviour of females of E. vuilleti. The odours from the plant in the vegetative stage had no effect. Kairomones from larvae, pupae or faeces of B. atrolineatus were attractive to the parasitoid females. Odours from adult male or female B. atrolineatus, as well as the sex pheromone emitted by females were not attractive to E. vuilleti females. This sex pheromone did however stimulate the locomotor activity of the parasitoid. The adaptive significance of these olfactory stimuli is discussed. Resume Reponse du parasitoide Eupelmus vuilleti aux odeurs de son hote phytophage et de la plante-hote dans un olfactometre Eupelmus vuilleti (CRW) est un ectoparasitoide solitaire qui se developpe dans nos conditions experimentales aux depens des larves et des nymphes de Bruchidius atrolineatus (Pic) (Coleoptere: Bruchidae). Les larves de ce Coleoptere se trouvent a l'interieur des graines d'une Legumineuse Vigna unguiculata (Walp). Grâce a un olfactometre tubulaire, nous avons determine les differents stimuli olfactifs intervenant dans l'orientation des femelles de cet hymenoptere, vers l'un de ses hotes; Bruchidius atrolineatus. Plusieurs odeurs emanant soit de plantes-hotes ou non-hotes de B. atrolineatus, soit de l'hote lui-meme ou resultant de son metabolisme ont ete testees. Les odeurs de gousses ou de graines seches de Vigna unguiculata Walp (plante-hote du phytophage) influencent le comportement locomoteur des femelles. Par contre, les odeurs de plantes au stade vegetatif n'ont pas d'effet. Les kairomones emanant de larves, nymphes ou feces de larves de B. atrolineatus exercent un effect attractif sur les femelles parasitoides. Les odeurs emanant d'adultes mâles ou femelles de B. atrolineatus ainsi que la pheromone sexuelle emise par les femelles ne sont pas attractives pour les femelles de E. vuilleti. Cette pheromone sexuelle, par contre, stimule l'activite locomotrice des parasitoides. La signification adaptative de ces differents stimuli olfactifs est discutee.

Oogenesis of the parasitoid wasp Eupelmus vuilleti is known to be dependent on host availability. However, examination of ovarian dynamics by microscopy showed that oogenesis and vitellogenesis are initiated before female eclosion and proceed 1-2 days after, independent of host presence. Oogenesis continued beyond the 2nd day only in the presence of hosts, otherwise it was replaced by egg resorption. It is thus possible to distinguish between host-independent and host-dependent periods of oogenesis. In the presence of host, each ovariole (three per ovary) contained generally three oocytes: a fully mature oocyte, a nearly mature one and an immature one. However, host deprived-females resorbed their most mature and their smallest oocytes, but kept one almost mature oocyte per ovariole. Comparison of zero, short and long host deprivation periods showed that females always had the ability to quickly lay eggs to exploit any new host. However, increased deprivation led to a reduction in the number and the viability of eggs. Enzymo-immunological measurements of ecdysteroids were made in whole females, in dissected ovaries and in newly laid eggs. Our results indicated that ecdysteroids play a major role as circulating hormones involved in the regulation of oogenesis.

Callosobruchus maculatus (F.) is a tropical insect (Coleoptera: Bruchidae) that develops in Vigna unguiculata (Walp) seeds. In this beetle, there are two distinct adult forms that differ in their morphology, physiology and behaviour, the flight form and the flightless form. The development of C. maculatus is analyzed in a traditional store in Niger. During the dry season, 6 to 7 generations of the flightless form develop in the store. When the atmospheric water content increases during the rainy season, adults of the flight form appear and escape from the store. Experimental studies have shown that the adult polymorphism is induced during post-embryonic development and depends on prevailing abiotic factors during this period. The proportion of flight from adults is high when larvae develop in moist seeds (14–15% of water content). Under dry conditions (6–7% of water content) only flightless adult forms emerge from the seeds. Temperatures during development also influence adult polymorphism. In thermoperiodic conditions with 12–13% seed water content, the proportion of flight from adults increases with increasing duration of the thermophase. However, at low temperatures, a high proportion of flightless adult forms emerge, regardless of seed water content. The adaptive importance of these two abiotic factors on polymorphism induction is examined in this study. Resume Importance de la temperature et de la teneur en eau des graines sur l'induction du polymorphisme imaginal chez Callosobruchus maculatus F. Coleoptere Bruchidae Callosobruchus maculatus (F.) est un Coleoptere Bruchidae tropical qui se developpe aux depens des graines de Vigna unguiculata. Les adultes presentent un polymorphisme; il y a en effet deux formes, la forme voiliere et la forme non voiliere, distinctes par des criteres morphologiques, physiologiques et comportementaux. L'evolution des populations de C. maculatus a ete suivie pendant toute la saison seche et le debut de la saison des pluies dans la region de Niamey au Niger (13° LN) a l'interieur d'un grenier traditionnel. Pendant toute la saison seche, 6 a 7 generations de C. maculatus de la forme non voiliere se succedent a l'interieur du grenier. Les adultes de la forme voiliere, tres actifs, apparaissent surtout au cours de la saison des pluies lorsque la teneur en eau de l'atmosphere et la teneur en eau des graines augmentent. Les etudes experimentales realisees au laboratoire montrent que le polymorphisme imaginal est induit au cours du developpement post-embryonnaire et depend des conditions climatiques dans lesquelles celui-ci a lieu. Le taux d'adultes de la forme voiliere est eleve lorsque les larves se developpent dans des graines a forte teneur en eau (14 ou 15%). Lorsque la teneur en eau des graines est faible (6 a 7%), il n'y a emergence que d'adultes de la forme non voiliere. Lorsque la teneur en eau des graines est comprise entre 12 et 13%, le taux d'adultes de la forme voiliere s'accroit et varie avec la duree de la thermophase. A basse temperature il y a toujours emergence d'adultes de la forme non voiliere, quelle que soit la teneur en eau des graines. L'influence de ces deux facteurs abiotiques sur l'induction du polymorphisme imaginal joue probablement un role important dans les mecanismes adaptatifs permettant le maintien de cette espece en zone tropicale.

Eupelmus vuilleti (Hymenoptera; Eupelmidae) is a solitary ectoparasitoid producing yolk-rich eggs. The female oviposits mainly on the fourth larval instar of Callosobruchus maculatus (Coleoptera; Bruchidae), which develop within pods and seeds of Vigna unguiculata (Fabacae). Parasitoid females are synovigenic, i.e., they are born with immature eggs and need to feed from the host to sustain egg production during their entire lifetime. However, eggs are rapidly resorbed in unfavourable conditions and an efficient stop-and-go mechanism controls oogenesis in such animals. In this study, the possible involvement of ecdysteroids in the regulation of parasitoid oogenesis is examined. In a first step, the identity and titre of ecdysteroids in reproductively active and inactive female parasitoids were investigated by high performance liquid chromatography followed by enzyme immuno-assay (EIA/HPLC). A larger secretion of ecdysone was found in female during their reproductive period compared with inactive females. In a second step, both the secretion of ecdysteroids into the medium of in vitro incubated ovaries and the ecdysteroid content of females reared with or without host were measured (EIA). The presence of the host, which represents both the oviposition site and the nutritional source, induced an active biosynthesis of ecdysone. This synthesis started at a slow rate after host introduction and reached a maximum after 48 h. When hosts were available, this synthesis was cyclic and continuous during the entire female lifetime. These results showed that host presence triggered ovarian synthesis of ecdysteroids, which are involved in a stop-and-go regulation of egg production linked to host availability.

Eupelmus vuilleti (Hymenoptera; Eupelmidae) is a host feeding ectoparasitoid of fourth-instar larvae or pupae of Callosobruchus maculatus (Coleoptera; Bruchidae) infecting Vigna unguiculata seed and pods (Fabacae). Parasitoid females are synovigenic, i.e. they are born with immature eggs and need to feed from the host in order to sustain egg production. In this study, the role of sterols obtained through host feeding in parasitoid oogenesis are examined. Quantitative and qualitative analyses of the sterol contents in each partner of the tritrophic interaction show that a parasitoid female's larval sterol contents is sufficient to produce only 30% of the total number of eggs laid throughout a female's life cycle. In a second step, by manipulating the composition of the sterols hemolymph in the host, it is shown that cholesterol obtained through adult nutrition plays a crucial role in the eggs viability but does not affect the egg production quantitatively. This result has important implications for understanding both the nutrient allocation strategy in this species and the impact of cholesterol in parasitoid reproduction.

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.

Eupelmus vuilleti ( Crawford) is an ectoparasitoid of the seed-eating beetle Bruchidius atrolineatus (Pic), which is an important pest of stored cowpea, Vigna unguiculata Walp, seeds in West Africa. Herein, we investigated the dispersal abilities of females within columns of seeds to assess the potential of E. vuilleti as a biological control agent of bruchids in cowpea granaries. The influence of host presence together with the 2 abiotic factors light and gravity on parasitoid movement and parasitization efficiency were analyzed. E. vuilleti females were able to travel through large seed masses and parasitize hosts located at the end of the seed column opposed to their introduction zone. Parasitoid movement was stimulated by light. E. vuilleti females exhibited a negative geotropism. Females introduced at the bottom of the seed column dispersed more and parasitized more hosts than females introduced at the top. Host presence had some influence on the dispersal of the parasitoids within the seed column at a host density of 10 infested seeds for 16,000-18,000 uninfested seeds. This depended on female introduction zone because gravity was the major factor influencing dispersal. The possible applications of these results for biological control of bruchids in cowpea granaries are discussed.

Bruchidius atrolineatus (Pic) (Bruchidae: Coleoptera) is a bruchid beetle infesting Vigna unguiculata (Walp). During the dry season, adults are in reproductive diapause. Studies of adult populations in a traditional store containing contaminated pods of V. unguiculata show that the reproductive diapause is induced during the first larval instars. The temperature decrease from November to January probably is the main climatic factor inducing this diapause. The percentage of diapausing adults depends on the duration of the larval development at low temperatures. Experiments in climatic boxes confirm these results. At 12 h 25 °T–12 h 20° C 11–13 LD, a high proportion of the adults emerging from the V. unguiculata seeds are in reproductive diapause and the duration of their growth is variable. L1 L2 L3 larvae probably are sensitive stages to the thermal variations and the processes allowing emergence of diapausing adults probably are induced during these larval instars. Resume Induction de la diapause reproductrice chez Bruchidius atrolineatus coleoptere bruchidae durant la saison seche en zone Sahelienne Bruchidius atrolineatus (Pic) est un Coleoptere Bruchidae se developpant en zone sahelienne aux depens d'une legumineuse Vigna unguiculata. Pendant la saison seche, les adultes sont en diapause reproductrice. Un suivi des populations d'adultes a ete realise dans un grenier traditionnel en ‘terre sechee’ contenant des gousses recoltees dans la nature et portant des œufs de B. atrolineatus. Il a permis de constater que la diapause reproductrice etait induite durant les premieres phases du developpement larvaire. La decroissance des temperatures de Novembre a Janvier avec des minima de 15 °C et des maxima de 25 °C est probablement le principal facteur permettant l'induction de cette diapause. L'introduction dans le grenier de gousses portant des œufs venant d'etre emis a differentes periodes de la saison seche montre que le pourcentage d'adultes diapausants depend de la duree du developpement larvaire aux temperatures ‘basses’. Des experiences realisees dans des etuves programmees confirment ces resultats. A 12 h 25 °T/12 h 20 °C 11–13 LD, un taux eleve d'adultes emergeant des graines est en diapause reproductrice et la duree de leur developpement post-embryonnaire est tres variable. En conditions plus chaudes 12 h 40 °C/12 h 25 °C 11–13 LD, le taux d'adultes diapausants est plus reduit et la duree du developpement plus courte. L'analyse de la croissance larvaire montre que les premiers stades larvaires sont probablement des stades sensibles aux variations thermiques. Les modifications physiologiques permettant l'emergence d'adultes diapausants ou sexuellement actifs seraient induites durant ces stades.

The development of populations of two species of bruchids, B. atrolineatus and C. maculatus, in traditional stores containing cowpea pods was analysed in the Niamey area of Niger. At the beginning of the storage period, the first generation of B. atrolineatus adults is sexually active, mates and oviposits on the pods. Later, as the population increases dramatically, there is a correspondingly higher percentage of adults exhibiting reproductive diapause. Experiments showed that the diapausing insects leave stores during the dry season. This factor subsequently induces a regression of population growth inside the stores at the end of the dry season. There was no development of the C. maculatus population in the present experiment, because of the low level of infestation in the field before harvest and the fact that climatic conditions were probably unfavourable for this species in the Niamey area. In stores, the rate of B. atrolineatus population increase is very high in a limited number of generations resulting in significant losses of cowpea seeds.

Bruchidius atrolineatus (Pic) presente en zone sahelienne une diapause reproductrice durant la saison seche et une partie de la saison des pluies. Les femelles diapausantes ne produisent pas de vitellogenine et le germarium des ovarioles est seul developpe. Chez les mâles la spermatogenese est tres ralentie et les glandes annexes sont inactives. Lorsque les insectes diapausants sont places en presence d'inflorescences de Vigna unguiculata Walp, leurs organes reproducteurs deviennent fonctionnels apres un temps de latence de 15 a 20 jours. Il n'y a par contre aucune levee de la diapause chez des bruches placees en presence de gousses seches de V. unguiculata dans une atmosphere saturee. Des informations sensorielles issues des pieces florales ou (et) des facteurs nutritionnels sans doute lies a la consommation de nectar semblent etre a l'origine de cette levee de la diapause. Le pollen fort peu consomme n'a par contre aucun role. Cette regulation du cycle reproducteur de B. atrolineatus par les inflorescences de V. unguiculata permet l'emission des oeufs des que les gousses commencent a se former a la fin de la saison des pluies.