The Role of Predatory Insects in Regulating the Population of Wheat Aphids at Ambo

Abstract

This study was conducted for three seasons (1993 to 1995) to determine the role of predatory insects in regulating the population of wheat aphid at Ambo. The change in the number of aphids in uncaged, open-sided and closed cage plots was determined and related
to changes in the density of predators. In 1993, nonsignificant differences in aphid population development was observed between all the treatments; significant differences were observed in 1994 and 1995. Aphid populations in closed cage plots increased at a higher rate and reached a peak in October (106 and 48 aphids/ shoot in 1994 and 1995, respectively). Nonsignificant differences are detected between open sided caged and uncaged plots in mean number of aphids recorded per plant. The aphid populations in open
sided and uncaged plots increased at slower rate and ranged from 5 to 37 aphids/shoot in open sided cages and 5 to 26 aphids/shoots in uncaged plots. In 1994 four to five, while in 1995 eight to twelve fold differences are observed in aphid numbers in closed cages and the other two treatments. Four phases of aphid population development were observed. In both seasons the divergence phase of the aphid population in open-sided cages coincided with an increase in the number of predators and a negative association between predator numbers and aphid abundance was apparent. Aphid predators Sphaerophoria ruppelli (Wiedman) ( Diptera: Syrphidae:) and Adonia variegata (Goeze) spp. tredecinsignata Muls and Cheilomenes lunata (F) (Coleoptera: Coccinellidae), comprised the dominant species recorded at Ambo and has a significant role in reducing the population of wheat aphids at Ambo condition. In both seasons a substantial number of aphids were also found parasitized by Aphidius hortensis (Marshal) and Aphidius setiger (Mackauer) (Braconidae).