An Ecological Study of Coccinellid Predators Associated with Aphid Pests on some Important Field and Vegetable Crops grown in Khartoum State

Abstract

Field and laboratory studies were carried out for two seasons (1999/ 2000 and 2000/2001). Field studies were aimed mainly at investigating incidence of predators and aphids on various field crops e.g. sorghum, cotton, wheat, berseem and some vegetables. Two wild plants (Usher, Calotropis procera Ait and Liweis, Leptadenia heterophylla Decne) were also included in the survey. Laboratory studies were confined on a single coccinellid species (Hippodamia variegata Goeze). For this species, studies were directed at its feeding preference, biology and life tables. In the field, H. variegata was found to be the most prevalent predator on many crops at Shambat, especially on berseem, wheat and maize. Of the two cotton varieties, more predators were recorded on Barakat 90 (Gossypium barbadense L.) than on Acala 67 (G. hirsutum Linn.). Wheat variety Debeira harboured more predators than the other two (Condor and Nelein). Records of Coccinellid predators were generally higher than those of non-coccinellids, non-coccinellid in this study were Chrysoperla carnea Steph., the green lacewings; Xanthogramma aegyptium (Wied) syrphid larvae and spiders. H. variegata was observed feeding and breeding on Aphis nerii on Usher plant, which is considered toxic and avoided by many predators. Also Scymnus sp. was frequently found feeding on A. nerii (Boy) on Liweis plants. More predators were observed on an unsprayed eggplants field than on a sprayed one. Laboratory studies revealed that H. variegata may withstand relatively long periods of starvation (average duration for starvation to death was 17.4 days with range of 7-25 days). Rate of cannibalism was high in both adult and 4th. instar larva (percent cannibalized eggs was 94% and 93.6% for adult and 4th. instar larva respectively). The beetle was also found to relatively prefer feeding on Melanaphis sacchari Zhnt. and Schizaphis graminum Rond than Aphis craccivora and Aphis gossypii, and Aphis nerii was the least fed on. H. variegata was also found to produce slightly higher number of eggs when fed on M. sacchari than on A. gossypii, A. craccivora and Schizaphis graminum. Life tables studies indicated that H. variegata general performance was better in the first generation than in the second one. The innate capacity for increase (rm) of H. variegata was found to reflect the productivity of this beetle, hence, the beetle can be regarded as a predator with good potentials as a biocontrol agent