Closeup of Aphids (image) Penn State Share Print E-Mail Caption The flavonoids are not present in the phloem -- vascular tissue in plants that conducts the sugars aphids seek -- but are in the epidermal cells that form the outermost layer of defense. When aphids repeatedly probe and puncture the epidermal cells with their stylets, or beaks, they take up the flavonoids that lead to their demise. Credit US Department of Agriculture Usage Restrictions None Share Print E-Mail Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.