Influence of hedgerow species on the relative abundance of orthropods in agroforestry systems
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The potential of agroforestry systems such as hedgerow intercropping is well studied in terms of maintaining soil fertility and conserving soils. However, information is scarce on how hedgerow systems affect pests of crops and their natural enemies. The objective of this study was therefore to (1) monitor and assess the effects of hedgerow systems affect pests of crops and their natural enemies in comparison with sole cropping and (2) evaluate the effect of different hedgerow species on arthropod diversity. The experiment had 10 treatments, comprised of 9 tree species (Gliricidia sepium, Grevillea robusta, Senna siamea, S. spectabilis, Flemingia congesta, Croton megalocarpus, morus alba, calliandra calothyrsus, and Lantana camara) and a sole crop control. The size of the plot was 10m x 10m and treatments replicated four times in a randomized complete block design (RCBD). This study was conducted from October 1995 to July 1996, covering two crop seasons and the intervening dry period. Bean (cv. Rosecoco) in the short rains (Nov 1995 to Jan 1996) and maize (cv. Katumani) in the long rains (Apr to July 1996) were grown in association with the above hedgerows an sole crops. To monitor air borne and crawling arthropods, two yellow pan and pitfall traps were installed in every plot, with one set of each type of traps located adjacent to tree rows an the other set at center of the plot. Sampling and scoring of arthropods was done on weekly basis. To evaluate the level bean to maize infestation by different pests, three sites were selected in every plot; (1) close to hedge upslope, (2) close to hedge down slope and (3) away from hedges at the center of the plots. The results of insect infestation/damage studies indicated that bean fly infestation was significantly higher overall in intercropped bean than in sole system. However, plots intercropped with G. robusta, S. siamea, S. spectabilis or C. megalocarpus were less damaged than with G. sepium or F. congesta. The hedgerows did not seem to influence aphid infestation on beans. Intercropped maize suffered less stalk borer damage and less aphid infestation than sole maize. The hedgerow species did not show appreciable differences among themselves in the overall effect on both these maize pests. Infestation by these pests however, was significantly less in the crop rows close to than away from hedges. Results from the trap catches indicated significantly lower number of winged aphids in intercropped bean plots than in sole crop plots during the short rains. However, no significant catch difference was noted between the two systems during the long rains. However, no significant catch difference was noted between the two systems during the long rains. Within hedgerow intercropping, aphid catches were significantly lower close to hedges than away from hedges. The effect of hedgerows on pollen beetles during the three seasons; the hedgerows significantly reduced the catch of beetles during the short rains, they increased the beetle catch during the dry period and exercised no marked difference during the long rains. The relative abundance of crickets during both the cropping seasons and grasshoppers during the long rains were not affected by intercropping and distance from hedgerows. Ladybird beetle catch was not significantly different between the sole and the intercropped plots in both crops. Interestingly, the catch of beetles from bare plots was higher than from plots different tree hedges during the dry period. During short rains and the subsequent dry period, plots with G. sepium hedges recorded significantly higher catch of wasps than sole cropped plots. Although hedgerow species did not show significant differences among them, significantly greater number of wasps were trapped closer to hedges than away from hedges during the long rains and the preceding dry period. While intercropping did not influence the movement of spider populations on bean crop in the short rains, the pitfall trap catches were greater by 80% on intercropped maize than sole-cropped maize during the long rains. Species differences among hedgerows was not significant but the spider catch close to hedges was significantly higher than that away from the hedges. Specimens of arthropods collected during the experiment period were preserved for identification. Some of the arthropod specimens were identified at the National Museums of Kenya and a list of identified specimens is presented. Unidentified specimens at the time of preparing this thesis were sent to South Africa for identification. Some of the methodological issues and limitations experienced during the present studies in evaluating agroforestry systems on arthropod dynamics are discussed. The major challenge is integration of the effects of agroforestry systems on different categories of arthropods.