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If males and females affect reproduction differentially, understanding and predicting sexual reproduction requires specification of response surfaces, that is, two-dimensional functions that relate reproduction to the (numeric) densities of both sexes. Aiming at rigorous measurement of female per capita fertilization response surfaces, we conducted a multifactorial experiment and reanalyzed an extensive data set. In our experiment, we varied the density of male and female Leptinotarsa decemlineata (Colorado potato beetles) by placing different numbers of the two sexes on enclosed Solanum tuberosum (potato plants) to determine the proportion of females fertilized after 3 or 22 hours. In the reanalysis, we investigated how the short-term fertilization probability of three Drosophila strains (melanogaster ebony, m. sepia, and simulans) depended on adult sex ratio (proportion of males) and total density. The fertilization probability of female Leptinotarsa decemlineata increased logistically with male density, but not with female density. These effects were robust to trial duration. The fertilization probability of female Drosophila increased logistically with both sex ratio and total density. Treatment effects interacted in m. sepia, and simulans. These findings highlight the importance of well-designed, multifactorial experiments and strengthen previous experimental evidence for the relevance of sex-specific densities to understanding and prediction of female fertilization probability.

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1 Although the successful management of the Colorado potato beetle Leptinotarsa decemlineata (Say) depends on the prevention of its dispersal, its walking pattern in the landscape remains poorly understood. In the present study, post-diapause, early summer, late summer and colony adult beetles, both fed and unfed before release, were tracked with a harmonic radar to establish their walking movement pattern in a bare-ground field. 2 The random walk model successfully described the dispersal of all beetle types, whether fed or unfed. 3 The diverse life history of this species was manifested by an increased distance travelled and deviations of individual paths from the random model. Starved postdiapause beetles travelled furthest and individual paths deviating from random were both local and directed, probably aiming to maximize opportunities for host colonization. Starved early summer beetles also travelled further than fed beetles but relied more on random movement to disperse in the habitat. Starving had little impact on the distance travelled or the path deviations of late summer beetles that are searching for overwintering site rather than hosts. 4 The increased displacement of starving beetles over fed beetles corresponded with an increased walking step and index of straightness. 5 The impact of starvation on travel distance was greater than expected from laboratory tests. 6 In conclusion, the results obtained in the present study suggest a random walking pattern to search arable land until host volatile or visual impulses trigger a more directed walk or flight.