Fitness components of small and large males (emerging respectively from small and large hosts) of Trichogramma evanescens Westwood (Hymenoptera: Trichogrammatidae) were compared in laboratory experiments. These components were the longevity, courtship, mate competition, and daily and lifetime fertilization. Small males were less fit than large males: they had a reduced longevity, took longer to induce female mating behavior, rarely succeeded in mating females when in competition with large males and had lower fertilization capacity; female fertilized by small males oviposited in the first two days of their life only one third of the progeny of daughters mated by large males. The impact of males fitness on optimal sex allocation by females parasitoids was evaluated by measuring the primary and tertiary sex ratios (proportion of males in the progeny) produced by a female T. evanescens when ovipositing in small and large hosts (low and high quality patches). Females of T. evanescens, an arrhenotokous species, have control on the sex ratio of their offspring by regulation of the sperm's access to the egg. Certain combinations of male and female eggs deposited in a host patch result in greater fitness than others. As predicted, females produced a significantly higher proportion of males on the low quality patches. T. evanescens females adjusted the sex ratio allocated to a patch of homogeneous quality and under complete local mate competition to the expected fitness of their sons, as smaller males have a lower fitness and are expected to have low fertilization capacity and therefore fertilize less females. To optimize her fitness gain (the capacity of the individual to transmit its genes), the sex ratio is increased in order to have all daughters mated.

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Differences in plant structure and host abundance, distribution and density within and between plants may affect host finding success of searching parasitoids. The main objective of this research consisted in developing and validating a model that can predict parasitism by Trichogramma evanescens on various plant structures. Also, we evaluated the effect of both artificial and natural plant structures on host encountering success and on searching behavior of two species of Trichogramma. / Size (S), heterogeneity (H) and connectivity (C) define plant structure. The development of the model of parasitism was based on laboratory experiments using three dimensional artificial plants of different combinations of S, H and C. The model was then validated with experiments, within greenhouse, using natural cruciferous plants of different structures. Significant regressions of observed values of parasitism as a function of those predicted by the model were obtained. / The effect of plant structure on two species of Trichogramma was studied by using three species of Lepidoptera and three structurally different but closely related crucifers. Also the distribution pattern of parasitized eggs according to leaf side and plant height was characterized. A repeated measure ANOVA in time where plant ages defined the repeated measures showed that all three main effects (plant, host, parasitoid) had significant effect on parasitism and only the parasitoid x plant structure interaction was significant. Mean percent of parasitism was higher on cabbage, intermediate on broccoli and lower on Brussels sprouts whereas cabbage appeared to be intermediate in plant structure, broccoli appeared to be the most simple and Brussels sprouts the most complex plant structure. On simple plant structure, both wasp species performed well on the three varieties of plant but T. evanescens outperformed T. pretiosum more often on the various plant structures. A doubly repeated measures ANOVA in space revealed significant effects of leaf side and plant height on parasitism, being greatest under leaf surface and at the base of the plant. The interaction between those two variables was not significant. / The effect of plant structure on searching behavior of T. evanescens was determined by direct observation of individual females searching on simple and complex plant structures during 1 hour. Time, frequence and sequence associated to activities and plant parts explored were obtained and analyzed. Plant structure had significant effect on time budget associated to activities (walking, resting, flying) and plant parts explored. / This study demonstrates that plant structure mediates ecological interactions by affecting host finding success and this may have important implications in population dynamics, evolution of hosts and parasitoids but also in biological control programs.