Study

Xylem feeding by periodical cicada nymphs on tree roots

White, Jean; Strehl, Carol E.

Ecological Entomology, 1978

Peer-reviewedfield observationwild

What they did

Cicada nymphs were excavated from soil and their feeding sites on tree roots examined, with mouthpart position and the tissue penetrated identified to establish what they actually consume underground.

Studied in: Magicicada
Sample size: excavated nymphs and feeding site examination

What they found

Nymphs feed on xylem fluid from tree roots — the dilute upward stream of water and minerals — rather than on the sugar-rich phloem sap most sap-feeding insects target. Feeding sites are on woody roots, and nymphs move between them as they grow through successive instars.

What it means

The authors' reading, and ours. Where they differ, that difference is the point.

How the authors put it

The extremely long nymphal period is consistent with a diet of xylem fluid, which is nutritionally very poor.

How NatureHQ reads it

This supplies the missing explanation for the seventeen years, and it is the piece most accounts leave out. The cycle is usually presented as a pure anti-predator strategy, and predator satiation explains why emergences are synchronised — but not why they are so far apart. Xylem fluid is almost entirely water with traces of minerals: it is the worst diet available in a tree, and an insect living on it grows extremely slowly. The animal is not waiting for seventeen years. It is taking seventeen years, because that is how long it takes to build an insect out of nearly nothing.

Limitations

Excavation disturbs the feeding relationship being studied.
Direct measurement of ingestion rate underground is not possible with this method.
Whether xylem specialisation is universal across cicadas is not addressed.