Agriculture Reference
In-Depth Information
strikingly blackened or burnt appearance, hence the colloquial term 'sooty
baobab'. The phenomenon is now regarded as episodic and related to lengthy
periods of below-average rainfall, aggravated lately by increasingly intensive
land use in arid areas (Piearce et al ., 1994).
Parasitic higher plants
In natural stands and on farms, branches of Uapaca kirkiana are sometimes
attacked by the parasitic flowering plants of the family Loranthaceae. The species
commonly seen are Agelanthus subulatus , Phragmanthera cornetii , Tapinanthus
dependens and Viscum congdonii (Ngulube et al ., 1995). Sclerocarya birrea is
similarly attacked by Agelanthus crassifolius , A. prunifolius , Erianthemum dregei ,
Helixanthera garciana , Pedistylis galpinii and Tapinanthus globiferus (Hall et al .,
2002). Many of these parasitic plants cause the death of twigs and branches, but
without any counter-measure the entire tree can also be killed in a few years. The
effect of these parasites on the fruit load of miombo trees is still not quantified.
Mites and insects
Mites are ubiquitous pests of agricultural crop plants in southern Africa (Meyer,
1981). Several species of mites are known to attack crop plants, some of
which are among the most serious pests. Some mites, such as Calacarus
citrifolii , Brevipalpus spp., Oligonychus coffeae , Tetranychus lombardinii and T.
cinnabarinus , are known to attack hundreds of different plant species each
(Meyer, 1981), and miombo fruit trees are likely to be affected by some of
these species. However, little is known about the mite pests of indigenous
miombo fruit trees. The only reported cases concern unidentified mites
damaging young shoots of Sclerocarya birrea , causing malformation of leaves
and stems (Hall et al ., 2002), and Tetranychus mites on Ziziphus mauritiana (G.
Sileshi, unpublished results).
Insects are responsible for perhaps the most noticeable and extensive
damage to trees in miombo woodlands, such as malformations of shoots, stems
and leaves. For instance, insect-induced shoot galls can stimulate the formation
of a witch's broom of adventitious shoots or foliage, which can also be a result
of fungal infection. For instance, leaf and shoot galls are frequent defects of
Parinari spp. (Parker, 1978), although the causal agents have not been
identified. Gall midges (Diptera: Cecidiomyiidae), gall wasps (Hymenoptera:
Cynipidae) and eriophyid mites (Acarina) can induce gall formation.
Among the most destructive of foliage-feeding arthropods are insects of the
orders Orthoptera (grasshoppers, locusts, etc.), Lepidoptera (butterflies and
moths) and Coleoptera (beetles). Grasshoppers and locusts are highly mobile and
their characteristic damage is seen in the much-eaten leaves that remain after the
pests have gone. Caterpillars (lepidopteran larvae) are most easily located during
their feeding, and large colonies on a single host tree have been observed
to rapidly cause a high degree of defoliation (Parker, 1978). Among the
Lepidoptera, perhaps the most notable defoliators are caterpillars of emperor
moths (Saturniidae). For instance, caterpillars of Bunaea alcinoe completely
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