Effect of Soy Leaf Flavonoids on Pea Aphid Probing Behavior

Simple Summary Flavonoids are plant phenolic compounds whose biological activities include participation in plant responses to various stresses of biological and environmental origins, including protection against insect herbivore attack. We were interested in whether the specific flavonoids detected in soybean leaves have the potential to discourage the pea aphid from infesting other leguminous plants, peas in particular. We immersed the pea leaves in ethanolic solutions of the flavonoids apigenin, daidzein, genistein and kaempferol, offered them to the pea aphids and observed their behavior when they probed plant tissues with their piercing-sucking mouthparts. We discovered that aphids readily probed the pea leaves whether they were treated with flavonoids or not. However, later on, the behavior of the aphids changed depending on the flavonoid applied. Apigenin, daidzein and kaempferol caused a decrease in the intensity of plant sap ingestion. In addition, daidzein and kaempferol made the finding of sap-transporting vessels more difficult for aphids. In contrast, genistein did not affect the pea aphids' feeding activity. Our findings provide the plant breeders and plant protection services with information on what direction their efforts should take to protect leguminous plants against aphids in a sustainable and environmentally friendly way. Flavonoids detected in soybean Glycine max (L.) Merr. (Fabaceae) cause various alterations in the metabolism, behavior, and development of insect herbivores. The pea aphid Acyrthosiphon pisum (Harris) (Hemiptera: Aphididae) poses potential threat to soybeans, but the effect of individual flavonoids on its feeding-associated behavior is relatively unknown. We monitored probing behavior (stylet penetration activities) of A. pisum on its preferred host plant, Pisum sativum L. untreated (control) and treated with 0.1% ethanolic solutions of flavonoids apigenin, daidzein, genistein, and kaempferol. We applied the electrical penetration graph (electropenetrography, EPG) technique, which visualizes the movements of aphid stylets within plant tissues. None of the applied flavonoids affected the propensity to probe the plants by A. pisum. However, apigenin enhanced the duration of probes in non-phloem tissues, which caused an increase in the frequency and duration of stylet mechanics derailment and xylem sap ingestion but limited the ingestion of phloem sap. Daidzein caused a delay in reaching phloem vessels and limited sap ingestion. Kaempferol caused a reduction in the frequency and duration of the phloem phase. Genistein did not affect aphid probing behavior. Our findings provide information for selective breeding programs of resistant plant cultivars to A. pisum.