Investigation on the Dieback of A. nilotica in the Blue Nile Area Associated with the Bark Beetle Chrysobothris dorsata.

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El Siddig Mohamed Ali, Ahmed
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This study was concerned with the investigation of the phenomenon of cliebacck, affecting Acacia nilotica regenerated in the Blue Nile area, which extends from Senar reservoir to Er-Roseires Dam, caused by the bark beetle Chrysohothris 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 has a square shape, with an area of one hectare and located in one of the three site types which were present in the four forests. These site types are " Gerif"' which is immediately adjacent to the river 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 I 993 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. 1.) of 20 infested, randomly selected trees and 20 healthy ones was carried out in the selected plots to evaluated 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%, 53.94% 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 528 m3 with a loss in the mean annual increment of 26 m3 per year. This total loss in volume was estimated to be equivalent to approximately 107 million Sudanese pounds. The natural factors 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 thinning quantity and quality in relation to tree age and the ideal number of trees per hectare. No significant correlation was found between silt height and the degree of infestation. However, the highest frequencies and rates of infestation were in the plots of high deposition of silt. Also, no significant correlation was found between silt height and the reduction recorded in DBH, volume and M.A.I., although the silt height in different selected plots was significantly inversely proportional to tree height. Correlation was also not significant between water height and the degree of infestation. The results revealed no significant correlation between soil moisture content and the degree of infestation, though the reduction recorded in DBH, tree height and volume were significantly in direct proportion with soil moisture content at the depths of 30 and 90 cms. The results showed significant impact of thinning quantity and quality on the degree of infestation and the losses in the different measured parameters. The highest infestation frequencies and rates were recorded in heavily thinned plots. In Sero forest in "'Maya" site type, where the ideal number of trees is 338 per hectare, the actual number of trees was 59. In this plot the highest degree of infestation was recorded (77.97%). The correlation between thinning quantity and quality and the losses recorded in the different measured parameters were significantly in inverse proportion. The probabilities of the correlation between them and the M.A.I. were 0.0176 and 0.0178 respectively. The morphological features of the adults of C. dorsata, its larva and pupa were provided. The results revealed that the intensity of larvae in the infested branch was higher in the stressed wood (one larva/5.4 cm3) than in the natural wood (one larva/23.4 cm3) and the moisture content in the branch was higher in the healthy portion (11.350%) than in the infested portion (4.866%).
Investigation on the Dieback of A. nilotica in the Blue Nile Area Associated with the Bark Beetle Chrysobothris dorsata.