Plants transport sugary phloem through cells called sieve tubes. Like an oil pipeline, the sieve tubes will automatically block the flow if the sieve tube is punctured. This limits the amount of sap that leaks from a stem when it is cut.
The signal for sieve elements to block the phloem flow is the release of calcium ions. When an aphid punctures the sieve tube, calcium is released. How can an aphid prevent the flow from stopping?
Aphids have a salivary secretion technically called “watery saliva” that is secreted when their mouthparts enter plant phloem. Among other substances, watery saliva contains proteins with numerous calcium binding sites. These proteins bind to the calcium that is released. Thus, aphids intercept and terminate the signal for the sieve tube to stop the flow of phloem.
Other salivary secretions seal the puncture wound around the mouthparts to prevent fluids from leaking into or out of the phloem. This allows aphids to feed continuously without moving or expending much energy.