PEA APHID (HOMOPTERA, APHIDIDAE) AND WATER-DEFICIT INTERACTION ON ALFALFA STEM GROWTH

The action of the pea aphid, Acyrthosiphon pisum (Harris), on alfalfa plants subjected to water deficit was investigated in a growth chamber . The main variable evaluated was the stem elongation rate (SER). The experimental conditions applied were suited to control and to measure plant water deficit and to evaluate aphid dynamics. The experiment was conducted over a 12-d period, beginning with the aphid infestation da y (D-0). At the end of the experiment, leaf water potential (LWP) was -1 MPa and aphid population per stem was 225 aphids in the nonirrigate d treatment and 300 aphids in the irrigated treatment. A significant d ecrease in SER was first caused by the aphid effect at 63 h after D-0, when populations reached on average 9.8 aphids per stem. Water defici t effect became significant at 188 h after D-0. The interaction betwee n aphid effect and water deficit became significant 215 h after D-0. T he possibility to relate SER to an optimum-rate SER(0) (SER of the non infested irrigated stems), LWP and aphid infestation is discussed.