Sowing Date and Plant Density Effects on Pest and predator Numbers in Haricot Bean Fields
1992
Tsedeke Abate
The effects of sowing dates and plant densities on African bollworm, Helicoverpa armigera; pod weevils, Piezotrachelus various and Apion spp.; leafhoppers, Empoasca spp.; and the predators Orius spp. and Tropiconabis capsiformis were tested in haricot bean fields at Awasa and Melkasa during the 1987 and 1988 main seasons. The experiments were laid out in a split-plot design with sowing dates as main plots and plant densities (100,000 to 500,000 seeds ha-1) as subplots, in three replications; each subplot was 7 m x 10 m. Combined ANOVA, over seasons and locations, showed significant effects of sowing date, plant density, season, location and some of their interactions. H. armigera numbers and pod damage by same declined with late sowing in 1987 but increased in 1988. Percent pod damage in 1988 was greater than in 1987; it is suggested that these changes were associated with the amount of rainfall. Empoasca sp. numbers tended to increase with late sowing, particularly at Melkasa, where this pest seemed to be important. Orius numbers were also greater in late sown plots than early. Sowing date did not have a significant effect on Piezotrachelus and Tropiconabis numbers. Percent pod damage, and Piezotrachelus and Empoasca numbers increased with increasing plant density; by contrast, plant density did not influence Orius spp. and T. capsiformis numbers. Piezotrechilus and T. capsiformis were more abundant at Awasa than at Melkasa Whereas Empoasca and Orius spp. were more prevalent at Melkasa than at Awasa. Results of these experiments suggest that manipulation of sowing date and plant density provide opportunities for manipulating pest and natural enemy numbers to develop low cost and environment friendly pest management strategies.
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