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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. • Therioaphis trifolii and Peronospora aestivalis often occur on alfalfa leaves at the same time. • The occurrence of Peronospora aestivalis can effectively affect the growth and development of Therioaphis trifolii. • There were differences in the growth and development of Therioaphis trifolii under different degrees of infection. • Feeding on moderately infected alfalfa is more beneficial to the reproduction of Therioaphis trifolii population. [ABSTRACT FROM AUTHOR]

Therioaphis trifolii (Monell) is a well‐known pest in alfalfa cultivation in China. Individuals of T. trifolii were allowed to feed on 10 alfalfa cultivars and their growth, reproduction, and population life table parameters were estimated. Nymphs that fed on Hunter River had the longest development time, whereas nymphs that fed on Defu had the shortest development time. Adults that fed on Debao, Zhongmu No. 2, and Defu had the shortest development time, whereas adults that fed on Golden Empress and Hunter River had the longest development times. Individuals that fed on Zhungeer produced the greatest number of offspring (13.950), and those that fed on Hunter River produced the fewest offspring (6.000). The survival rate of T. trifolii was the highest on Algonquin and lowest on Hunter River, whereas the survival time was the longest on Golden Empress and shortest on Debao. The population that fed on Algonquin had the highest intrinsic growth rate (0.560) and shortest population doubling time (1.237 days), whereas those that fed on Hunter River had the lowest intrinsic growth rate (0.217) and longest population doubling time (3.194 days). The life table parameters of the experimental T. trifolii population were used to quantify the resistance strengths of the cultivars. Results indicate that the Hunter River cultivar may effectively resist damage from T. trifolii. This study provides a theoretical basis for breeding aphid‐resistant alfalfa cultivars, enabling further research into the prevention and control of aphid infestation. [ABSTRACT FROM AUTHOR]