Agroforestry holds promise, as a sustainable land management system that balances ecosystem demands to sustain diversity and productivity, while meeting multiple use and sustained yield needs of agriculture. To meet this premise however, pests that are major causes of losses in agricultural systems should be considered. Unfortunately, this discipline is little understood in agroforestry although its importance has recently raised considerable concerns. This has become more apparent with the appearance of serious pest problems on some promising agroforestry woody species and the recent emphasis on tree domestication, use of high value trees and improved tree germplasms in traditional agroforestry systems. The present study provides information on pests in agroforestry with particular attention to the diversity and impact of insects and pathogens on A/nus species in Uganda.
Overall, the study was aimed at assessing the role of herbivorous insects and pathogens on the cultivation of A/nus species in Uganda. The specific objectives of the study were to; (i) characterise the community structure of pests and arthropod natural enemies on A/nus species, (ii) monitor the seasonal abundance of arthropods and pathogens as well as some of their damage on A/nus species, (iii) evaluate A/nus species and provenances against insect and pathogen attack (iv) determine the effect of defoliation on Alnus and an associated crop under monoculture and biculture systems, (v) investigate farmers' knowledge and perceptions of pest problems in agroforestry, and (vi) evaluate selected control measures against common pests on Alnus species. In order to accomplish these objectives, a series of nursery and field experiments as well as laboratory analyses of samples were performed.
Results of this study indicate that Alnus acuminata and A. nepalensis are hosts to several injurious insects, pathogens and vertebrates, some of which are capable of causing considerable damage on the A/nus species. The incidence and abundance of herbivorous insects and pathogens varied markedly over spatial and temporal scales, as were their damage levels. In addition, significant differences were evident among
provenances of A. acuminata and A. nepalensis in the growth rates and levels of pest infestation, indicating the scope for selecting and breeding for pest resistance. Generally, the most severe pest infestations on the A/nus species occurred in the nursery and such damage can significantly reduce the survival and growth of nursery and outplanted seedlings. This implies that pest management measures are necessary in nurseries to ensure successful field establishment of Alnus seedlings. Under field conditions, prevalent insect and pathogen infestations on the Alnus species were leaf perforations and brown leaf spot. Established outplanted Alnus seedlings can compensate for continuous mild defoliation, but high defoliation levels can severely reduce the growth rate, nutrient content, and seemingly the nitrogen fixing ability of the species. In contrast, defoliation of Alnus increased the growth rate and nutrient content of associated maize plants compared to those grown with non-defoliated Alnus, but these parameters were lower in maize grown in mixed than in sole systems. These results indicate that pest infestations on Alnus may not only influence the performance of the species, but may also have significant effects on the performance of associated plants with important bearing in nutrient cycling in agroforestry.
Most farmers were aware of some damage symptoms on Alnus species, but their ability to control observed infestations was hampered mainly by lack of knowledge of the causal agents and appropriate control measures. Similarly, farmers'
ability to manage pest problems seemed generally weak on both trees and crops although they were more knowledgeable of pest infestations on crops than on trees. It is therefore important to improve farmers' pest management ability by providing them with field diagnostic tools and relevant educational materials.