Influence des tanins condensés du tégument de fèves (Vicia faba) sur le développement larvaire de Callosobruchus maculatus

Des tanins condensés sont extraits et isolés à partir du tégument de Vicia faba var. Géante Portugaise. Quatre polymères sont quantifiés et identifiés à quatre fractions: A, B, C et D. Ces fractions sont incorporées, en mélange ou séparées à différentes concentrations, dans un milieu nutritif favorable au développement de Callosobruchus maculatus (F.) (Coleoptera, Bruchidae).Les composés phénoliques provoquent la mortalité d'une partie de la population et retardent le développement des survivants. La mortalité observée varie suivant les fractions, leurs doses et le stade de développement de la bruche.La barrière constituée par le tégument des fèves est bien de nature chimique et concerne les tanins condensés.Dans un système de production agricole, l'utilisation des cultivars de fèves riches en tanins faiblement condensés pourrait permettre de réduire les taux d'infestation des graines de V. faba par C. maculatus.     

The ability of Zabrotes subfasciatus to attack cowpeas

The biological and behavioural responses of two strains of Zabrotes subfasciatus (Boh.) to beans and cowpeas were compared. One strain was collected from beans in Colombia and the other from cowpeas in Uganda. The two strains irrespective of their geographical or nutritional histories laid eggs on both types of seed and the distribution over time of egg laying for the two strains was found to be similar on each commodity. Both strains fed and developed on the two types of seed and no significant differences were found between the numbers of progeny developing on each.Competition between Z. subfasciatus and Callosobruchus maculatus was investigated using replacement series experiments at high and low population densities. Competition coefficients were calculated for the two species and at both densities neither species had a clear competitive advantage over the other. It is concluded that Z. subfascicatus should be regarded as a potential cosmopolitan pest of stored cowpeas.

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Résumé Le Zabrotes subfasciatus est un parasite cosmopolite des haricots stockés (Phaseolus vulgaris). Il s'est installé dans des zones restreintes en tant que parasite des pois sauvages (Vigna unguiculata). Des spécimens de Z. subfasciatus ont été recueillis dans des haricots infestés en Colombie et dans les pois sauvages infestés d'Ouganda afin de comparer leurs réactions biologiques et leur comportement vis-à-vis des haricots et des pois sauvages. Les deux variétés, indépendamment de leurs passés géographique ou alimentaires, ont pondu des oeufs sur les deux types de graines et al répartition dans le temps de la ponte des oeufs s'est révélée être similaire pour les deux variétés sur chaque denrée. Les deux variétés se sont nourries et développées sur les deux types de graines, et aucune différence significative entre les deux variétés n'a été constatée dans l'importance de la progéniture se développant sur chacun des deux types de graines.Une concurrence entre le Z. subfasciatus et le Callosobruchus maculatus (un parasite cosmopolite courant des pois sauvages) a été étudiée en effectuant des expériences en série de substitutions. Des mélanges d'adultes des deux espèces ont été mis sur des pois sauvages, les proportions des deux espèces étant différentes dans les divers traitements. Les coefficients de concurrence, basés sur l'importance de la progéniture de chaque espèce provenant des divers mélanges, ont été calculés et il a été constaté qu'aucune des espèces n'avait un advantage de concurrence évident par rapport à l'autre.On en a conclu que le Z. subfasciatus devait être considéré comme un parasite cosmopolite éventuel des pois sauvages stockés.

     

Partial characterization of a major gut thiol proteinase from larvae of Callosobruchus maculatus F.

Much of the proteolytic activity in the digestive tract of Callosobruchus maculatus larvae can be attributed to a thiol proteinase(s) that hydrolyzes [3H]methemoglobin optimally at pH 5.0. Maximal hydrolysis of [3H]methemoglobin, [3H]alpha-casein, and N-benzoyl-DL-arginine napthylamide-(BANA) required the presence of thiol reducing agents. Larval gut proteinase activity was strongly inhibited by p-hydroxymercuribenzoic acid (pHMB), Nethylmaleimide (NEM), and iodoacetic acid (IAA) but was unaffected by the Bowman-Birk and Kunitz proteinase inhibitors from soybeans or by lima bean trypsin inhibitor. L-Trans-epoxysuccinyl-leucylamido-(4-guanidino)-butane (E-64), a specific inhibitor of thiol proteinases, potently inhibited proteolysis of [3H]methemoglobin by larval gut homogenates. Proteolytic activity in the larval gut was located in the lumen contents and thus appears to play a major role in extracellular digestion. The pH of the larval midgut is slightly acidic, and midgut contents exhibit a negative redox potential, conditions supporting the activity of a thiol proteinase. The significance of these findings is discussed with reference to the vulnerability of this digestive proteinase as a target for existing or genetically engineered plant chemical defenses.     

Why do host‐deprived seed beetles ‘dump’ their eggs?

Abstract.  If host seeds are absent, females of the seed beetle Callosobruchus maculatus sometimes 'dump' eggs on unsuitable substrates, which causes complete larval mortality and decreases female lifespan. To understand the possible function of this behaviour, the present study examines genetic variation in dumping behaviour between and within populations. When deprived of hosts, females from an African population are much more prone to dump eggs than Asian females, most of which dump no eggs over their lifetimes. Egg dumping therefore cannot be explained as a simple, species-wide constraint imposed by the accumulation of mature oocytes. A transfer experiment tested the hypothesis that dumping eggs improves a female's ability to exploit the subsequent availability of seeds, perhaps by preventing a disruption of oocyte maturation. Contrary to prediction, the number of eggs laid after females are transferred to seeds is inversely related to the number dumped during deprivation. Two bidirectional selection experiments revealed heritable variation in egg-dumping behaviour within the African population. Deprived females from the dumper line dump more than twice as many eggs as do females from the nondumper line. Pre-adult development time is significantly longer in the nondumper line, which suggests that trade-offs with other fitness components could maintain variation in egg-laying behaviour within populations. The divergent responses to host availability by African and Asian females may represent a pleiotropic effect of similarly divergent responses to host quality.     

Consequences of inbreeding for the cowpea seed beetle, Callosobruchus maculatus (F.)(Coleoptera: Bruchidae)

Inbreeding is said to reduce vigour and fitness. It may also determine how a population responds to selection. Local populations of Callosobruchus maculatus , the cowpea seed beetle, are established annually from small numbers of founders and the species has been distributed to many parts of the world where isolated populations may have been founded by very small numbers of individuals. After more than 20 generations of inbreeding, inbred lines have been shown to diverge from a common ancestral stock in similar directions with respect of some variables such as developmental speed, but haphazardly in respect of other parameters such as male weight. The respective roles of drift and of selection as effective evolutionary forces in inbred lines are discussed in the light of these results. It is argued that some intraspecific differences in C. maculatus may be explained as a product of periodic inbreeding, but that the process does not impair the ability to adapt to local conditions so contributing to the status of the species as a pest of international importance.     

Habitat shape, metapopulation processes and the dynamics of multispecies predator-prey interactions

1. The effects of habitat shape, connectivity and the metapopulation processes of persistence and extinction are explored in a multispecies resource-consumer interaction. 2. The spatial dynamics of the indirect interaction between two prey species (Callosobruchus chinensis, Callosobruchus maculatus) and a predator (Anisopteromalus calandrae) are investigated and we show how the persistence time of this interaction is altered in different habitat configurations by the presence of an apparent competitor. 3. Habitat structure has differential effects on the dynamics of the resource-consumer interaction. Across all habitat types, the pairwise interaction between C. chinensis and A. calandrae is highly prone to extinction, while the interaction between C. maculatus and A. calandrae shows sustained long-term fluctuations. Contrary to expectations from theory, habitat shape has no significant effect on persistence time of the full, three-species resource-consumer assemblage. 4. A stochastic metapopulation model for a range of habitat configurations, incorporating different forms of regulatory processes, highlights that it is the spatially explicit population dynamics rather than the shape of the metapopulation that is the principal determinant of interaction persistence time.     

Discrimination of the age of conspecific eggs by an ovipositing ectoparasitic wasp

The recognition and avoidance of already parasitized hosts is a major issue in parasitoid behavioural ecology. A key factor affecting the fitness reward expected from superparasitism is the probability that the second or subsequent egg laid on a host will win the contest with the first-laid egg. The present study investigated the ability of females of the solitary ecto parasitoid Anisopteromalus calandrae Howard (Hymenoptera: Pteromalidae) to (i) discriminate between unparasitized Callosobruchus maculatus (Fabricius) (Coleoptera: Bruchidae) hosts and those parasitized by a conspecific, and (ii) discriminate between a host parasitized by an egg just laid (2 h) and one parasitized by an egg about to hatch (28 h). However, they did not adjust their offspring sex ratio on already parasitized hosts compared to unparasitized ones. Our results show that A. calandrae females can discriminate between parasitized and unparasitized hosts, as they lay more eggs on the latter. The probability of the second or subsequent egg laid on a host (superparasitism) winning the contest with a conspecific increases as the time between the two ovipositions decreases. Consequently, parasitoid females should lay more eggs on recently parasitized hosts than on those that have been parasitized for a long time (i.e., when the first eggs are about to hatch), and that is indeed what they were found to do. To increase their fitness in spite of the presence of already parasitized hosts, A. calandrae females have developed highly discriminative capacities regarding the parasitism status of hosts.     

Natural selection hampers divergence of reproductive traits in a seed beetle

Speciation is thought to often result from indirect selection for reproductive isolation. This will occur when reproductive traits that cause reproductive isolation evolve (i) as a by‐product of natural selection on traits with which they are genetically correlated or (ii) as an indirect result of diversifying sexual selection. Here, we use experimental evolution to study the degree of divergent evolution of reproductive traits by manipulating the intensity of natural and sexual selection in replicated selection lines of seed beetles. Following 40 generations of selection, we assayed the degree of divergent evolution of reproductive traits between replicate selection lines experiencing the same selection regime. The evolution of reproductive traits was significantly divergent across selection lines within treatments. The evolution of reproductive traits was both slower and, more importantly, significantly less divergent among lines experiencing stronger directional natural selection. This suggests that reproductive traits did not evolve as an indirect by‐product of adaptation. We discuss several ways in which natural selection may hamper divergent evolution among allopatric populations.     

Predictable modification of body size and competitive ability following a host shift by a seed beetle

Interfertile populations of the seed beetle Callosobruchus maculatus differ genetically in several behavioral, morphological, and life-history traits, including traits that affect the intensity of larval competition within seeds. Previous studies have suggested that this variation depends on differences in host size. I performed a selection experiment in which replicate beetle lines were either maintained on a small, ancestral host (mung bean) or switched to a larger, novel host (cowpea). After 40 generations, I estimated survival, development time, and adult mass on each host, both in the presence and absence of larval competition. The shift to cowpea substantially reduced body size; irrespective of rearing host, adults from the cowpea lines were more than 10% lighter than those from the mung bean lines. Switching to cowpea also improved survival and reduced development time on this host, but without decreasing performance on the ancestral host. The most striking effect of the shift to a larger host was a reduction in larval competitiveness. When two even-aged larvae co-existed within a seed, the probability that both survived to adult emergence was > or = 65% if larvae were from the cowpea lines but < or = 12% if they were from the mung bean lines. The adverse effects of competition on development time and adult mass were also less severe in the cowpea lines than in the mung bean lines. By rapidly evolving smaller size and reduced competitiveness, the cowpea lines converged toward populations chronically associated with cowpea. These results suggest that evolutionary trajectories can be predictable, and that host-specific selection can play a major role in the diversification of insect life histories. Because host shifts by small, endophagous insects are comparable to the colonization of new habitats, adaptive responses may often include traits (such as larval competitiveness) that are not directly related to host use.