During insect feeding, a complex interaction takes place at the feeding site, with plants deciphering molecular information
associated with the feeding herbivore, resulting in the upregulation of the appropriate defenses, and the herbivore avoiding
or preventing these defenses from taking effect. Whiteflies can feed on plants without causing significant damage to mesophyll
cells, making their detection extra challenging for the plant. However, whiteflies secrete honeydew that ends up on the plant
surface at the feeding site and on distal plant parts below the feeding site. We reasoned that this honeydew, since it is
largely of plant origin, may contain molecular information that alerts the plant, and we focused on the defense hormone salicylic
acid (SA). First, we analyzed phloem sap from tomato plants, on which the whiteflies are feeding, and found that it contained
salicylic acid (SA). Subsequently, we determined that in honeydew more than 80% of SA was converted to its glycoside (SAG).
When whiteflies were allowed to feed from an artificial diet spiked with labeled SA, labeled SAG also was produced. However,
manually depositing honeydew on undamaged plants resulted still in a significant increase in endogenous free SA. Accordingly,
transcript levels of PR1a, an SA marker gene, increased whereas those of PI-II, a jasmonate marker gene, decreased. Our results
indicate that whiteflies manipulate the SA levels within their secretions, thus influencing the defense responses in those
plant parts that come into contact with honeydew.