Dispersal to predator-free space counterweighs fecundity costs in alate aphid morphs

1. Specialisation in dispersal by alate aphids often imposes constraints on other functions, particularly a reduction in fecundity due to wing development. Short-distance flight from patches of high population density to uninfested plants may provide temporary predator-free space, compensating for low fecundity. However, this theoretical prediction has not been explored experimentally. 2. To test this hypothesis, a field cage experiment was conducted in which Aphis glycines populations initiated with controlled proportions of apterous and alate individuals were exposed to predation, while predator-free space was accessible only through flight. It was predicted that an investment in alate individuals would benefit a population under predation, regardless of associated costs to fecundity. 3. As expected, a strong trade-off was observed between fecundity and wing development. However, populations initiated with a fixed proportion of alate and apterous individuals showed no reductions in final population size compared with populations initiated with apterous individuals exclusively. Moreover, the initial presence of alate individuals in the populations increased aphid prevalence (i.e. proportion of plants colonised). Similarly, both increased population size and prevalence were observed when predator-free space was accessible through flight, as opposed to when it was inaccessible. 4. These results show that despite high costs to fecundity, an investment in alate individuals is neither beneficial nor detrimental to population size when predator-free space is accessible, but increases aphid prevalence. It is concluded that prevalence might provide an ecological advantage important enough to warrant the production of alate morphs under intense predation.