Investigation of arthropods associated with agroforestry in Machakos, Kenya
A survey of arthropods associated with exotic multipurpose trees and food crops used in agroforestry and indigenous tree species (Tamarindus indicus) grown in the wild was conducted in 1990 and 1991. The exotic multipurpose tree species (MPTs) used were: Gliricidia septum, Leucaena leucocephala and Cassia siamea growing in an alley cropping system and the food crops were Vigna unguiculata, Zea mays, and Cajanus cajan. The consequences of growing taxonomically related plant species in agroforestry systems were considered. Insecticidal knockdown technique was used to sample arthropods from the foliage. 92 species of arthropods belonging to 48 Families were found to be associated with tree species and the crops. Of these, 27 species were on MPTs, 3 species on crops, 12 species on the chosen wild tree, 13 species were shared amongst MPTs and crops while 14 species were shared between MPTs and the wild tree. A further 19 species were shared between all the groups of plant species (MPTs, crops and the wild tree) with 4 species being shared between the wild and the crop. The effect of tree pruning on arthropod communities on trees was also investigated and the results showed that pruned trees supported a richer arthropod fauna than unpruned trees. Through simulated defoliation, the impact of insect defoliators on four MPTs was also investigated. There was a considerable reduction in overall growth rate, height and diameter increment depending on the level of defoliation. Several stress factors often predispose plants to insect attack. Pruning, as a stressing agent was investigated in relation to infestation of Cassia siamea by the stem borer Xyleutes capensis. Out of the 200 pruned trees surveyed, 90 % showed the signs of borer attack, compared with only 31.5 % of unpruned trees. The role MPTs can play in aggravating infestation of stored pulses by a bruchid beetle, Callosobruchus chinensis was investigated by comparing its survival on seeds of three MPTs and Cajanus cajan. Whereas the bruchid was able to oviposit on both the MPT and crop seeds, no larval emergence was observed in any of the MPTs showing that the MPTs may not support reproduction and development of C. chinensis in the field.