Downy mildew ( Peronospora aestivalis ) infection of alfalfa alters the feeding behavior of the aphid ( Therioaphis trifolii ) and the chemical characteristics of alfalfa

Tripartite plant–pathogen–phytophagous insect interactions have been widely studied. However, the effect of pathogens on phytophagous insects remains uncertain. Although alfalfa is a widely cultivated forage legume, Peronospora aestivalis and Therioaphis trifolii often co-occur in alfalfa, reducing quality and yields. To explore the relationships between alfalfa Zhongcao No. 3, P. aestivalis , and T. trifolii , an electrical penetration graph and four-arm olfactometer were used to determine T. trifolii behavior on infected and uninfected alfalfa. The physicochemical properties of infected alfalfa leaves were determined. The effects of volatiles on the selection behavior of T. trifolii were analyzed, and the effect of substance content on host selection behavior was determined. Peronospora aestivalis altered the nutrient composition of alfalfa, and T. trifolii preferentially fed on moderately infected alfalfa (P2) rather than severely infected alfalfa (P3). The E1 and E2 waveform durations were the shortest (11.87 min) and the longest in P2 (122.82 min), respectively. Therioaphis trifolii mostly chose moderately (25.71%) and severely (37.14%) infected alfalfa plants via olfaction. The analysis of volatile components revealed 53 substances in 8 categories. Among them, decanal (1×) and naphthalene (1000× and 10,000×) had a strong repellent effect on T. trifolii , whereas naphthalene (1×), cedrol (100×), 2,6,10,14,18,22-tetracosahexaene, 2,6,10,15,19,23- (1000× and 10,000×) attracted T. trifolii . Our results provide new insights into plant disease and insect pest control and prevention.