The effect on Aphis fabae of diel changes in their food quality

Abstract

ABSTRACT. Marked diurnal changes in total soluble carbohydrates were found in broad bean leaves, with a maximum of c. 11% of dry weight at 18.00 hours to a minimum of c. 4% at 06.00 hours. If reflected in the phloem sap, this would mean that aphids receive different food from plants by night and by day. The growth rate of Aphis fabae was therefore compared on broad bean plants under light and darkness. Although the aphids showed the same mean gross growth rate under both conditions, those in the dark had a higher final percentage moisture content, their dry weight increase being negligible. Counts of honeydew droplets on a ‘honeydew clock’ indicated a reduction in excretion during the dark. Measurements of haemolymph concentration of Acyrthosiphon pisum were obtained by comparison of haemolymph vapour pressure with that of known sucrose solutions, using capillary tubes. Haemolymph samples collected at the end of the day showed a higher (mean 11.53 atm) and more variable osmotic pressure than those obtained at the end of the night (mean 8.62 atm). The more dilute sap in the plant at night may benefit the aphid by allowing it to compensate for osmotic stress incurred during daytime feeding. These results suggest that the diurnal change in total soluble carbohydrates in leaves of the broad bean is reflected in the phloem sap.