Effect of alfalfa downy mildew (Peronospora aestivalis) infection on the growth and development of Therioaphis trifolii (Homoptera: Aphidiidae)

Downy mildew disease, which is caused by Peronospora aestivalis (Mastigomycotina: Peronosporaceae) and the spotted alfalfa aphid Therioaphis trifolii (Hemiptera: Aphididae), affects Medicago sativa (alfalfa) production. P. aestivalis and T. trifolii frequently occur together under natural conditions. In this study, alfalfa-P. aestivalis-T. trifolii were used as the research objects to explore the effects of different degrees of P. aestivalis infection on the growth and reproduction of T. trifolii, and the life table of T. trifolii population was constructed based on the fecundity parameters. The results revealed that alfalfa downy mildew infection had a significant effect on T. trifolii population. T. trifolii fed on mildly infected alfalfa plants exhibited the longest life span (18.9 d) and the highest number of nymphs (30.6) compared to those fed on alfalfa in other treatments. In terms of population life table parameters, the intrinsic rate of increase (0.626) and finite rate of increase (1.869) of T. trifolii fed on moderately infected alfalfa were the highest, and the generation time (8.310) and population doubling time (1.108) were the shortest, indicating that moderately infected alfalfa are conducive to T. trifolii population development. This study provides insights into the effects of alfalfa downy mildew on the main reproductive parameters and population of T. trifolii on M. sativa.