Morphologic and metabolic data of Philaenus spumarius froghoppers

The xylem sap of vascular plants is an unlikely source of nutrition, being
both nutrient poor and held under tensions (negative pressures) that can
exceed 1 MPa. But some insects feed on xylem sap exclusively, extracting
copious quantities using a muscular cibarial pump. However, neither the
strength of the insect’s suction, nor the direct energetic cost of xylem
ingestion, have ever been quantified. Philaenus spumarius froghoppers were
used to address these gaps in our knowledge. Micro-CT scans of its
cibarium and measurements of cibarial muscle sarcomere length revealed
that P. spumarius can generate a maximum tension of 1.3 ± 0.2 MPa within
its cibarium. The energetic cost of xylem extraction was quantified using
respirometry to measure the metabolic rate (MR) of P. spumarius while they
fed on hydroponically-grown legumes, while xylem sap excretion rate and
cibarial pumping frequency were simultaneously recorded. Increasing the
plants’ xylem tensions up to 1.1 MPa by exposing their roots to
polyethylene glycol did not reduce the insects’ rate of xylem excretion,
but significantly increased both MR and pumping frequency. We conclude
that P. spumarius can gain energy feeding on xylem sap containing
previously reported energy densities and at xylem tensions up to their
maximum suction capacity.