From a physiological perspective, honeydew is dominated by first, egesta, the components of ingested phloem sap that have not been assimilated by the insect (some phloem sap compounds may be enzymatically altered by gut enzymes prior to egestion), and second, excreta, waste products of insect metabolism that are eliminated via the gut after transfer from the body tissues to the gut lumen.
In quantitative terms, honeydew is dominated by sugars. In the best studied group, the aphids, the principal sugars in honeydew are typically different from the sugar ingested by the insect and usually of higher molecular weight. This reflects the vital osmoregulatory function of the gut in phloem-feeding insects. The osmotic pressure of phloem sap is generally considerably higher than the osmotic pressure of the insect body fluids, creating a tendency for the insect to lose tissue water to the gut.
Amino acids are the principal nitrogenous compound in phloem sap. Not all of the ingested amino acids are absorbed across the insect gut and assimilated (estimates of assimilation efficiency vary from 60 to 99%), and amino acids are routinely recovered from insect honeydew.
Honeydew also contains nitrogenous excretory compounds but generally at very low concentrations. This is because the symbiotic microorganisms in homopterans act as an internal sink for waste nitrogen compounds. For example, uric acid, the principal nitrogen waste compound of the planthopper Niloparvata lugens, is not voided in the honeydew, but retained within the insect body and metabolized by the insect's symbiotic yeasts. Similarly, ammonia, the dominant waste nitrogen compound of aphids, is in low concentration in their honeydew because their symbiotic bacteria Buchnera consume much of the ammonia synthesized by these insects.
Honeydew may contain microorganisms and viruses derived either from the ingested phloem sap (and passed directly through the gut) or from the resident insect microbiota. For example, aphids feeding on plants infected with the luteovirus barley yellow dwarf virus will ingest viral particles from the phloem sap; those particles that are not transported into the insect hemocoel are expelled in the aphid honeydew. Plant viruses multiplying in the insects may also pass into the gut and occur in honeydew.
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