Sweet problems: insect traits defining the limits to dietary sugar utilisation by the pea aphid, Acyrthosiphon pisum

Plant phloem sap is an extreme diet for animals, partly because of its high and variable sugar content. The physiological and feeding traits of the pea aphid Acyrthosiphon pisum that define the upper and lower limits to the range of dietary sucrose concentrations utilised by this insect were determined principally using chemically defined diets containing 0.125–1.5 mol l–1 sucrose. On the diets with 0.125 mol l–1 and 1.5 mol l–1 sucrose, the aphids died as larvae within 8 and 14 days of birth, respectively. On the other diets, 60–96% of aphids developed to adulthood, and the 0.5 mol l–1 and 0.75 mol l–1 diets supported the highest fecundity. The diet with 0.125 mol l–1 sucrose was ingested at 36% of the rate of the 0.25 mol l–1 sucrose diet, but >90% of ingested sucrose-carbon was assimilated on both diets. This suggests that the lower limit is dictated by the aphid feeding response, specifically, a requirement for a minimal concentration of sucrose for sustained feeding. The haemolymph osmotic pressure of aphids on diets with 0.125–1.5 mol l–1 sucrose was up to 68% higher than on 0.125–1.0 mol l–1 sucrose diets, but diet consumption and sucrose-carbon assimilation was not reduced on the very high sucrose diets relative to 1.0 mol l–1 sucrose. This suggests that failure of the osmoregulatory capacity of the insects on high sucrose diets may define the upper limit to the range of dietary sucrose utilised by the aphids. The mean haemolymph osmotic pressure of aphids on plants with phloem sap containing 0.37–0.97 mol l–1 sucrose was 1.61±0.063 MPa (mean ± s.e.m.), not significantly different from that (1.47±0.059 MPa) on diets with 0.25–1.0 mol l–1 sucrose. It is concluded that the osmoregulatory response of aphids to diets and plants are comparable, and, more generally, that the feeding and osmoregulatory capabilities of the aphids are compatible with the phloem sugar levels commonly encountered by aphids feeding on plants.