The ability to utilize plant-derived food sources and suboptimal prey when the main prey is scarce may enhance competitiveness and invasiveness of entomophagous species such as Harmonia axyridis. Alternative food sources are particularly abundant in flower strips and other agri-environment schemes to promote biodiversity and may thus also benefit the invasive species. We investigated the effects of alternative food sources on the development and reproduction of H. axyridis. Laboratory experiments demonstrated that larvae of H. axyridis developed into adults and produced offspring when reared solely on aphids, lepidopteran eggs or maize pollen but not when they were fed only lepidopteran caterpillars or buckwheat flowers. When fed a combination of the latter two suboptimal food sources, however, some H. axyridis larvae developed into fertile adults. Flowering plant species differed in their food quality to sustain ladybird survival and development when fed alone or in combination with suboptimal prey. Differences in food quality of flower species were confirmed in field-cage studies where newly emerged adults were exposed for six days to different plant species and their energetic compartments were analyzed subsequently. Overall Fagopyrum esculentum and Centaurea cyanus provided a higher food quality than Calendula arvensis in those experiments and mixing flower species did not provide an additional benefit. The results show that the harlequin ladybird can sustain itself not only on optimal prey, but also utilize alternative, animal- and plant-derived diets. This could provide H. axyridis a competitive advantage over those native ladybird species that depend on aphids for their reproduction.