A considerable amount of work has been recently devoted to the study of progeny allocation strategies by insect parasitoids under different environmental constraints (see Waage, 1986, for a review). All these studies present each of the oviposition strategies (e.g. number of eggs laid per host, superparasitism) as a phenomenon which is progressively settled by natural selection during the course of generations. However, this optimal progeny allocation theory is conceivable only if the variation in wasp's biological traits involved are under a genetic control on which natural selection could act. We have decided therefore to look for a genetic variability in these traits in a Trichogramma maidis Pintureau & Voegel6 (Hym.; Trichogrammatidae ) population.Based on histological staining which allows to count the number of Trichogramma eggs deposited in each host (Ephestia kuehniella Zeller), we compared different wasp isofemale lines (i.e. families) each of which was submitted to different conditions (i.e. parasite/host ratios).The results show that the control of superparasitism seems to be genetically determined. This appears true both for the average number of eggs deposited in each host, and for the frequency distribution of wasp eggs among hosts.The evolutionary consequences of such results and their possible applications in order to improve the wasp's mass-rearing conditions are discussed.
Studies were conducted between 1993 and 1996 using 3 natural grape vine populations, 1 susceptible laboratory strain, and 1 resistant selected strain of Drosophila melanogaster L. In vitro monooxygenase activity (ethoxycoumarine-O-deethylation) (ECOD) was recorded from microsomal fractions of all strains. Results varied over a 6-fold range between susceptible laboratory Canton and resistant selected RDDT strains and over a 2-fold range between the Canton strain and natural populations of flies. Few significant variations of ECOD activity were detected among the natural populations despite many insecticide treatments, but activities were significantly correlated with toxicological tolerance to 5 of the 15 insecticides (deltamethrin, fipronil, chlorpyriphos ethyl, DDT, and diazinon). Moreover, immunoblotting responses of microsomal protein encoded by Cyp6A2 showed that the levels of expression were quantitatively correlated with toxicological tolerance to almost the same group of insecticides (deltamethrin, fipronil, chlorpyriphos ethyl, DDT, fenvalerate, and fenthion). However, the level of CYP6A2 expression in some natural strains (still weakly resistant) was almost comparable with one of the resistant strains. In vivo monooxygenase activity recorded in individual abdomens of flies showed that frequency distributions of ECOD activity in natural populations overlapped those of the resistant and laboratory strains, which were much narrower. Substantial and fast frequency changes (of the narrowness) that obtained in laboratory were related to either the time of rearing of 1 of the natural populations or selecting this population with an insecticide that has a toxicology correlated with both of the monooxygenase signs measured. Perspectives on using the CYP6A2 expression and ECOD activity for detecting a resistance mechanism by cytochrome P450 in field populations are discussed.
biologique, F06602 Antibes RÉSUMÉ L'étude de l'effet de masse dans des groupes de taille croissante et de taux sexuel différent, lorsque le superparasitisme est réduit, permet de mettre en évidence les interactions comportementales entre individus mâles et femelles. La descendance et la longévité, après une diminution plus ou moins importante et rapide en fonction de l'importance du groupe et de la proportion des mâles, montrent une relative stabilisation. Simultanément, la baisse du taux sexuel de la progéniture serait essentiellement due à la réponse de la femelle à des stimuli provenant du groupe d'individus (fécondation moins fréquente des oeufs). Mots clés additionnels : Longévité, importance et taux sexuel de la descendance. SUMMARY Study of the effect of density in Trichogramma maidis (Hym., Trichogrammatidae) when superparasitism is restricted. Study of the effect of density on groups of different sizes and sex-ratios, when superparasitism is restricted, allowed us to demonstrate behavioral interactions between males and females. The number of descendants, after decreasing to a variable extent and at a variable speed depending on the size of the group and on the proportion of males, became relatively stable. The cause of the phenomenon lays in decreased fecundity in keeping with lower laying duration and disturbed behaviour. Longevity of females, estimated from the mortality rate before 8 days, was also subject to this effect of density, particularly if the male ratio was important. The sex-ratio (! rate) of the offspring decreased slightly between restricted groups and larger groups, the phenomenon being in keeping with a modification of the ovocyte fertilization rhythm and with a bad compensation of the beginning of laying, which supplies many males.
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