Investigation on the diebact of A. nilotica in the Blue Nile area associated with the bark beetle Chrysobothris dorsata [Sudan]

This study was concerned with the investigation of the phenomenon of diebacck, affecting Acacia nilotica regenerated in the Blue Nile area, which extends from Senar Reservoir to Er-Roseires Dam, caused by the bark beetle Chrysobothris dorsata. The study particularly concentrated in four forests; Sero and El-Rumela on the western bank of the Blue Nile besides El Gazair and Dangada on the eastern bank. A total of 46 study plots were randomly selected from the four forests. These plots comprised 16 plots in El Gazair forest, 12 plots in both Sero and El-Rumela forests and six plots in Dangada forest. Each selected plot had a square shape, with an area of one hectare and located in one of the three site types which were persent in the four forests. These site types are "Gerif" which is immediatly adjacent to the bank, followed by the "Maya" which is the lower lands where the flooding water aggregates forming the basins, then"Karab" which is the higher land adjacent to the "Maya". In June 1992 in each selected plot, the total number of trees, the healthy and infested, were counted. Identification of the infested trees was based on the outer symptoms and dissection of the branches to ensure the presence of the dieback beetle in one of its stages. The symptoms included dry yellow to brown foliage and dry dark/black branches with many exit holes of the beetle all over its surface and tendency to break easily. By counting the infested trees in the different selected plots of the four forests, the frequency of infestation in each of them was determined. The counting of the infested trees was repeated twice in December 1992 and June. The morphology features of the adults of C. dorsata, its larra and pupa were presented. The results revealed that the intensity of larvae in the infested branch was higher in the stressed wood (one larva/5.4 cm to power3) than in the natural wood (one larva/23.4 cm to power3) and the moisture content in the branch was higher in the healthy portion (11.350) that in the infested portion (4.866) 1993 for the evaluation of the rate of infestation. The repeated counting of the infested trees ensured the progress of the dieback in the different plots.No significant differences in the frequency and rate of infestation were found between the different site types in most of the selected plots. However, when the different site types were pooled in the four selected forests, higher infestation was found in "Maya" site type (66.65) followed by "Karab" (65.55) and then "Gerif" (49.30). Comparison between diameter at breast height, (DBH), tree height, volume and mean annual increment, (M.A.I.) of 20 infested, randomly selected trees and 20 healthy ones was carried out in the selected plots to evaluuate the impact of the infestation on the above mentioned growth parameters. Significant reductions in these parameters were recorded in the infested trees. The mean reduction in DBH, tree height, volume and M.A.I. of the infested trees, pooled in the different plots of the four selected forests, in reference to healthy ones was 26.16, 18.05 and 53.95 respectively. The total loss in volume of A. nilotica wood as a result of the infestation by C. dorsata was extrapolated for the period from December 1983 to June 1993 and was estimated at 52m to power3 with a loss in the mean annual increment of 26 m to power3 per year. This total loss in valume was estimated to be equivalent to approximately 107 million Sudanese pounds. The natural factore and the management practices influencing the selected forests were studied. the natural factors included silt deposition, water height and soil moisture content while management practices included .