Biological control of
invasive tree species
in South Africa
FAC Impson and CL Lyons
A
cross South Africa and indeed
globally, a number of tree species
are considered highly invasive and
problematic. Typically, invasive
trees and shrubs share several similar
characteristics. Most are fast growers that
reach reproductive maturity within a few
years, many are adaptable to varying climates
and soils and, perhaps most importantly, they
generally produce large quantities of long-
lived seed. In turn, it is the adaptability and
vigorous growth of many tree species that
historically led to widespread introductions of
such plants, and this undoubtedly exacerbated
the problems. Many tree species are well
known for their uses in agroforestry, the
ornamental market, stabilisation of drift sands
and for animal fodder as well as providing a
source of human food. Whilst not all “useful”
trees are invasive, the realities of managing
those that are, is not always easy and without
controversy.
Mechanical and chemical control of invasive
trees is generally difficult and expensive, and
often temporary due to constant regeneration
of seedlings, and biological control integrated
with one or other of these methods should
be promoted as a more long-term and
sustainable method where possible. South
Africa has a long history in weed biocontrol
and is considered a world leader in terms
“Before” (heavy pod loads) and “after” (heavy gall loads) introduction of the black wattle
gall midge to reduce seed production.
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