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The biomechanics and energetics of xylem feeding in the meadow spittlebug (Philaenus spumarius) Bergman, Elisabeth Anne

Abstract

The watery sap within the xylem vessels of vascular plants is thought to exist under high tensions (i.e., negative pressures) that routinely exceed -1 MPa, as well as being very nutrient poor. While this should make xylem sap an energetically unfavourable source of nutrition, some Hemipteran insects within the suborder Auchenorrhyncha feed on it exclusively, extracting copious quantities of this liquid using a muscular cibarial pump. However, neither the strength of the insect’s suction, and thus the maximum xylem tensions that the insect can feed at, nor the energetic cost of xylem feeding, have been determined. Here I used adult meadow spittlebugs (Philaenus spumarius) to address this gap in knowledge. First, the maximum suction they could generate was calculated from biomechanical principles using morphological data obtained from micro-CT scans of their cibarial pump. Second, the metabolic rates (MR) of adult P. spumarius were measured while feeding on hydroponically-grown plants (Vicia faba, Pisum sativum, and Medicago sativa) with known xylem tensions, while their rate of xylem sap extraction and cibarial pumping frequency (fpump) were obtained from simultaneously recorded video footage. Furthermore, during these feeding experiments, the plants were exposed to the osmolyte polyethylene glycol (PEG), revealing how the insects changed their feeding behaviour in response to increasing xylem tensions. These findings indicate that the cibarial pump is capable of generating an average maximum tension of -1.3 MPa. This is higher than any xylem tension recorded from their food plants using the Scholander-Hammel pressure bomb method. In addition, while it was calculated that the xylem sap likely contains sufficient sugars to sustain the energetic requirements of pumping, the total MR of the feeding insect could be satisfied only by assuming contributions from amino acids.

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