Plant-insect-predator interactions: Reference to Aphis nerii Boyr. (Homoptera: Aphidae) and Menochilus sexmaculatus Fabr. (Coleoptera: Coccinellidae) on the milkweed, Calotropis gigantea L. (Asclepiadaceae)

Effect of host plants phytochemistry such as protein, carbohydrate, lipid and cardenolides on the population build-up of the yellow milkweed apid, Aphis nerii Boyr. and its coccinellid predator, Menochilus sexmaculatus Fabr. were studied with reference to meteorological factors, viz, temperature, humidity and rainfall. Nutritional value of Calotropis gigantea L. served as the intrinsic factor, while meteorological factors were considered as the extrinsic factors. The population of A. nerii remained too high on ecotype A than the ecotype B. But, the predator did not vary much on either ecotype in larval and adult stages. There was significantly positive correlation between A. nerii and M. sexmaculatus larvae (0.809) and adults (0.949). Also, corelation in-between the M. sexmaculatus adults and larvae was significantly positive (0.817). Both A. nerii and M. sexmaculatus did not show significant correlation with any one of the phytochemical parameters. However, the population of aphids and its predator showed significant negative correlation with rainfall (A. nerii, 0.490; M. sexmaculatus (larvae), 0.480; M. sexmaculatus (adults), 0.594). Wide variation in the concentration of cardenolides in the mature leaves of Calotropis was recorded. Ecotype A had higher concentration than ecotype B. A. nerii did not prefer to feed on bushes having higher concentration of their secondary metabolic substances.