The leaf nitrogen content is generally accepted as an indicator of food quality and as a factor affecting host selection by phytophagous insects. The alate pea aphids, Acyrthosiphon pisum Harris, Aphididae) were given a choice among non-nodulated pea plants (Pisum sativum L.) supplied with one of four nitrate-N levels (0, 3, 15 and 30 mM). When whole plants were exposed to aphids for 7 days, the results indicated that the settling response of alatae, and subsequently the reproduction of alighted aphids, increased as the level of N supply or the concentration of total soluble nitrogen of the leaves increased, with the exception of the highest N supply (30 mM N). However, the density of settled alatae (in terms of number per unit leaf area) increased as the level of N supply decreased, being greatest on N-deficient plants (0 mM N treatment) and lowest density on N-sufficient (15 mM N) and N-excess (30 mM N) plants. In a free-choice experiment, equal-sized leaf discs taken from the different N treatments were exposed to alate adults for 24 h. The settling response of aphids was positively affected by leaf colour (yellowing), with the greatest number settled on yellow leaf discs (N-deficient plants) and fewest settled on green or dark-green discs (N-sufficient and N-excess plants). Relationships between level of N supply, total soluble nitrogen concentration, total chlorophyll concentration, plant growth parameters and aphid abundance (number of alatae per plant) or density (number of alatae per unit leaf area, or per leaf disc) were established. The implications of results for integrated aphid management were discussed.

Keywords