The influence of mite predation on the efficacy of the gall midge, Dasineura sp., as a biocontrol agent of Australian myrtle, Leptospermum laevigatum (Myrtaceae) in South Africa.
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Date
2010
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University of Fort Hare
Abstract
Dasineura sp. is a gall forming midge that was introduced into South Africa for the
biocontrol of the Australian myrtle, Leptospermum laevigatum. It causes galls on both
the vegetative and reproductive buds of the plant. Although Dasineura sp. was initially
regarded as a potentially successful agent, galling up to 99% of the buds of the host
plant, it has been preyed on by native opportunistic mites, which caused a decline in the
performance of the midge as a biocontrol agent of L. laevigatum. This raised a concern
about whether this fly will be able to perform effectively in the presence of its new
natural enemies. Therefore, the objectives of this study were to:
1) ascertain whether mite abundance has seasonal variations;
2) determine if plant density and plant size have an effect on midge predation by the
mites; and
3) determine if midge predation varies in different locations.
The study was conducted at three sites in the Hermanus area, Western Cape Province.
Every three weeks for thirteen months, galls were collected and dissected so as to
count and record the numbers of midge larvae, pupae, adults and mites that were
found. Data collected showed that predation varied with season, and the mites were
scarce during the flowering season. Predation also varied among the study sites and
plant density had an effect on midge predation. Midges in smaller plants (saplings) were
more vulnerable to predation than those in the bigger plants (plants from isolates and
thickets). It was concluded that although mites have an effect on midge populations,
they do not prevent their establishment on the plant. Therefore, a survey should be
done in two to three years time to check if the midges are still persisting on the plant, and recommendations are that a new agent should be released to supplement the
midges.
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Keywords
Biocontrol agent, Dasineura sp., Density, Leptospermum laevigatum, Mites, Predation.