Samplings: October 2021

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Meadow froghopper (P. spumarius)

Philip G. D. Matthews

The thousands of species of sucking insects in the suborder Auchenorrhyncha are all plant eaters, with some feeding exclusively on sap extracted from the xylem, one of two conduits (the phloem being the other) that transport water and nutrients from a plant’s roots to its limbs and leaves. However, xylem sap is nutrient poor and under negative pressure, and neither the strength of the suction needed nor the energy cost of extracting it have ever been quantified. Now, Elisabeth Bergman, Emma Green, and Philip Matthews, of the University of British Columbia, Canada, have completed a study of the tiny Hemipteran, the froghopper (Philaenus spumarius), known in juvenile form as the common meadow spittlebug.

Xylem-feeding insects insert their stylet, a straw-like appendage, into the plant’s xylem vessels. To suck the sap out, they must generate a tension exceeding that of the negative-pressure xylem. The researchers used micro-CT scans of froghopper heads to create 3-D renderings revealing the morphology and muscle architecture of the cibarium—the piston-like area inside the mouth. By examining the structure of the muscles pulling the ‘piston’ to generate the sucking force, the team calculated the force exerted per unit area.

The energetic cost of sucking xylem sap was unknown for froghoppers. But unlike an athlete, “you can’t stick a tiny insect on a treadmill to measure its metabolic rate,” explained Matthews. The researchers used flow-through respirometry, clipping a minute sealed chamber to a bean plant stem with the insect inside. By flushing air through the chamber while the insect was sucking, they could measure how much carbon dioxide it produced and thereby its metabolic rate. They experimented with placing xylem sap under increased tension by putting the bean’s roots in polyethylene glycol instead of water, making the insects work harder to suck liquid out. From video analysis, the researchers measured pumping frequency and xylem excretion rate—the liquid output of insect urine—to see how much sap the insects had processed.

The researchers found that froghoppers suck with xylem-extracting tensions greater than one megapascal—a force about three times more than previously assumed. Increasing xylem tension did not slow the bugs down. Said Matthews, “they just ramped their metabolic rate up and up and up,” while significantly increasing pumping rate. Matthews finds it fascinating that while plants generate negative pressure passively using transpiration, these insects use brute force to obtain sap, sucking harder than any creature recorded to date. (Proc. B)