Evaluation of exotic and South African isolates of Beauveria bassiana as potential mycoacaricides of Tetranychus urticae Koch
Date
2008-06-30T08:36:20Z
Authors
Bhana, Nainisha Morar
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Abstract
Economic losses caused by the two spotted spider mite (Tetranychus urticae
Koch) in the agricultural industry prompted the initiation of this project to
evaluate the potential of native and exotic Beauveria bassiana isolates as
biocontrol agents against this pest. Before the selection of a potential
mycopesticide for use at a commercial scale, several crucial criteria must be taken
into consideration. The application of some of these criteria have been the focus
of this study and include: production of virulent B. bassiana inoculum, an
evaluation of the virulence of the fungus against T. urticae, monitoring of the
fungal infection cycle in T. urticae and establishing a phylogenetic evaluation of
B. bassiana isolates using rDNA sequence analysis.
A two-stage in vitro diphasic fermentation process produced B. bassiana
inoculum. In the first submerged phase the impact of nutrient treatments (carbon
and nitrogen) at different concentrations (3% and 4%) in a 1:1 ratio was
investigated for mycelium dry mass production and spore yield. The 4% nutrient
concentration yielded a higher mycelium dry mass yield compared to the 3% and
was therefore used in the second semi-solid phase to stimulate aerial conidia
formation in response to low nutrient stress. The fungal structures produced
during the first phase of the diphasic fermentation process were submerged
conidia recognized as small, spherical structures with a smooth form. In
comparison, inoculum of the second semi-solid phase produced aerial conidia
with small, spherical, rough surfaces and a brittle appearance assumed to be
related to nutrient deprivation. Nutritional parameters exploited in this study
favoured conidia production for use as a potential mycopesticide.
An in vitro bioassay compared the infectivity of exotic and native isolates of B.
bassiana against T. urticae adults. All the isolates were pathogenic with mite
mortality increasing over time. Differences in the virulence of the B. bassiana
isolates were demonstrated suggesting host-specificity. With respect to the native
isolates B. bassiana (PPRI 04305) was more virulent than the B. bassiana sensu
latu isolates (PPRI 04304 and PPRI 04306). The differences in the virulence of
the native isolates are reflections of genetic differences demonstrated in the
phylogenetic analyses in this study. The results of the preliminary bioassay study
suggest that B. bassiana has the potential as a biocontrol agent of T. urticae.
Microscopy was used to morphologically visualise the post infection cycle of a
native B. bassiana isolate (PPRI 04305) in the two-spotted spider mite. The
infection cycle observed in the current study is in agreement with those described
in a number of agricultural pests. However, aspects not observed before with T.
urticae infection included limited hyphal growth on the cuticle surface before
penetration, per os mode of entry, cuticular melanization, lateral hyphal
development under the cuticle and aerial hyphal emergence through the setal
annulum on the dorsal surface of the cadaver. These observations will stimulate
further research in the development of B. bassiana as a mycoacaricide.
rDNA analysis of the ITS1-5.8S-ITS2 regions of different Beauveria species was
assessed for strain genotyping and population studies. Fitch parsimony and
neighbour joining analyses displayed species differentiation and confirmed that B.
bassiana was not a monophyletic group but a species complex. Distinct clades in
the phylogenetic analyses in the current study were matched to four species of
Beauveria: B. bassiana, B. cf. bassiana, Beauveria brongniartii and Beauveria
caledonica species. Two South African isolates PPRI 04304 and PPRI 04306
morphologically assigned to B. bassiana, are assumed to be either B. caledonica
or a close relative of B. caledonica based on the rDNA analysis. However, due to
the lack of confirmation of the change of species identification of these native
isolates, they are regarded as B. bassiana sensu latu. Results from this study
demonstrated the importance of rDNA analysis in biocontrol studies for
population studies and species differentiation.
The material in this dissertation highlighted some important characteristics
relevant for the biocontrol of T. urticae by B. bassiana. Aerial conidia produced
by the cost-effective diphasic fermentation process were virulent against T.
urticae and demonstrated high percentage mortalities. B. bassiana was shown to
be a generalist pathogen with strain-dependent differences in nutrient preferences
and virulence against the mite. Differences in the infectivity of the native isolates
B. bassiana (PPRI 04305) and B. bassiana sensu latu (PPRI 04304 and PPRI
04306) were reflections of the genotypic separation of the isolates demonstrated
by rDNA analysis. The results obtained from this research project are promising
for the ongoing research and development of Beauveria isolates as efficient
mycoacaricides against T. urticae for the South African agricultural market.
Description
Keywords
Beauveria bassiana, Tetranychus urticae, biocontrol