Nitrogen fertiliser input alters the prey consumption rate of a widespread aphid predator, <i>Harmonia axyridis</i>

Abstract The sharp rise in synthetic inorganic fertiliser use (notably nitrogen) over decades has caused unknown effects on ecosystems and the services they provide, such as biocontrol. The present study aimed to test the hypothesis that host plant nitrogen input alters the interactions between pests and natural enemies, specifically investigating whether aphid consumption by Harmonia axyridis differed depending on nitrogen fertiliser input. It was hypothesised that H. axyridis would consume fewer aphid individuals reared under nitrogen conditions that are optimal for aphid fecundity. Observed effects of different nitrogen inputs in Chinese cabbage on the aphid Myzus persicae and the predator H. axyridis support our hypothesis: At lower nitrogen levels, the fecundity and longevity of M. persicae increased in the absence of predators, and the numbers of adult aphids H. axyridis consumed to reach temporary satiation and to finish the 3rd and 4th instars decreased. In addition, the fitness of H. axyridis , indicated by the pupal weight, also reduced at the nitrogen treatment favourable for aphid fecundity. Nitrogen level had no significant effect on the developmental duration of H. axyridis from the 3rd instar to pupation or on its pupation rate. These findings revealed the mechanisms underlying the long observed amplified changes in biocontrol efficacy with different nitrogen inputs, to which the herbivore population growth and consumption rate by their predators show contrasting responses.