HONEYDEW SUGARS AND OSMOREGULATION IN THE PEA APHID ACYRTHOSIPHON-PISUM

Pea aphids, Acyrthosiphon pisum, containing their symbiotic bacteria ( untreated aphids) and experimentally deprived of their bacteria by tre atment with the antibiotic rifampicin (antibiotic-treated aphids) were reared on the plant Vicia faba, The sugars in the honeydew produced b y untreated aphids comprised predominantly the monosaccharides glucose and fructose, while the honeydew of antibiotic-treated aphids contain ed considerable amounts of oligosaccharides of up to 16 hexose units, The honeydew and haemolymph of the aphids were iso-osmotic, and their osmotic pressure was significantly lower in untreated aphids (0.91-0.9 5 MPa) than in antibiotic-treated aphids (1.01-1.05 MPa) (P<0.05). For insects reared on chemically defined diets containing 0.15-1.0 mol l( -1) sucrose (osmotic pressure 1.1-4.0 MPa), the osmotic pressure of th e aphid haemolymph did not vary with dietary osmotic pressure, but was regulated to approximately 1.0 MPa in untreated and 1.3 MPa in antibi otic-treated aphids, The sugars in the aphid honeydew varied with diet ary sucrose concentration; with monosaccharides dominant at low concen trations and oligosaccharides dominant at high concentrations of dieta ry sucrose, The lowest dietary sucrose concentration at which honeydew oligosaccharides were detected was 0.2 mol l(-1) for the antibiotic-t reated aphids and 0.3 mol l(-1) for untreated aphids, These data indic ate that the aphid, and not its associated microbiota, mediates the sy nthesis of oligosaccharides when the osmotic pressure of the ingesta i s high.