Genistein and Aphid Probing Behavior: Case Studies on Polyphagous Aphid Species

(1) Background: Genistein is a naturally occurring flavonoid with a rich spectrum of biological activities, including plant-herbivore interactions. The aim of the study was to evaluate the effect of exogenous application of genistein on aphid behavior during probing in plant tissues. (2) Methods: <i>Vicia faba</i>, <i>Brassica rapa</i> ssp. <i>pekinensis</i>, and <i>Avena sativa</i> were treated transepidermally with a 0.1% ethanolic solution of genistein, and the probing behavior of generalist aphid species <i>Aphis fabae</i>, <i>Myzus persicae</i>, and <i>Rhopalosiphum padi</i> was monitored on their respective treated and untreated host plants using electropenetrography (=electrical penetration graph technique, EPG); (3) Results: Genistein did not deter aphid probing activities in non-phloem tissues. In <i>A. fabae</i> and <i>R. padi</i>, a trend towards reduction and in <i>M. persicae</i> a trend towards increase in phloem sap ingestion occurred on genistein-treated plants, but these trends were not statistically significant. (4) Conclusions: Genistein is not a deterrent chemical against generalist aphid species studied; therefore, it is not recommended for practical application.