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Changes in population abundance of the African root and tuber scale stictococcus vayssierei Richard (hornoptera; stictococcidae) on cassava in the bas-fleuve district in the democratic Republic of Congo
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A two year follow up of the African root and tuber scale (ARTS) population dynamics was conducted on two cassava cultivars, a sweet variety Lueki and a bitter variety Mvuama, in a highly infested site of secondary agro-ecological forest zone of Bas-Flueve District, in D.R. Congo. Both cultivars were infested throughout the period of the observations, but significantly higher populations (P< 0.01) occurred on the sweet variety Lueki while the bitter variety Mvuama harboured less scales. Three planting dates (early, intermediate and late planting) were considered to follow the changes in the populations of ARTS on these two varieties. ARTS population development on the plant showed the same trend for the three planting dates on both varieties. The peaks in the population occurred at the onset of the main dry season, in May in the plots of the intermediate planting and, in June in the early and late planted plots, indicating seasonal effects. The late planting date showed the highest peaks and higher infestations throughout (P< 0.05). Dramatic drops in ARTS populations occurred just after the peaks, from 700 individuals to < 100 individuals for the highest peak. Instrinsic mortality increased with the increase in population densities. No evidence of predation was observed. In addition to natural mortality, larvae migration/dispersal occurred. Such high mortalities coupled with migration/dispersal were attributed to the over-population of the ARTS on the plants, indicating density effects. These observations show that cassava cultivars, cultivation season, dry season effects, natural mortality and migration are major factors influencing ARTS population abundance on cassava.