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Spider mites (Tetranychidae) are a major group of sucking pests of many
agricultural and horticultural crops worldwide. Tetranychus truncatus Ehara is the
predominant mite species infesting economically important crops of Kerala.
Novel acaricides are being extensively used for managing mite infestation, which
has led to development of significant level of resistance in mite populations to
these acaricides. This, along with other adverse effects of synthetic acaricides to
the environment, calls for alternative strategies in mite pest management.
The entomopathogenic bacteria Photorhabdus luminescens (Thomas and
Poinar), a natural symbiont of the entomopathogenic nematode, Heterorhabditis
spp. has potential virulent properties and the ability to infect a wide range of
insect pests and a few mite pests. An array of toxins produced by the bacteria are
responsible for this appreciable insecticidal activity. Most of the orally active
toxins are encoded by tca and tcd loci of TC genes. These toxins might also be
responsible for the acaricidal activity of the bacterium. In this context, the present
study was undertaken to identify the virulent strain of P. luminescens against T.
truncatus and to elucidate the molecular mechanism of virulence against T.
truncatus.
Eleven bacterial isolates associated with the entomopathogenic
nematodes, Heterorhabditis spp. were isolated in the study. Cultural
characterization of the bacterial isolates showed that the bacterial colonies were
circular to irregular, entire, opaque, smooth colonies on NBTA medium. All the
isolates were found to be Gram negative and rod shaped. For molecular
characterization, the 16S rRNA gene of the isolates was amplified, sequenced and
subjected to BLASTn for homology search. One species of EPN symbiotic
bacteria, P. luminescens and eight nonsymbiotic/ associated bacteria namely, two
isolates of Pseudomonas aeruginosa, five isolates of Ochrobactrum and one
isolate of Stenotrophomonas maltophila were identified. Phylogenetic tree was
constructed based on the gene sequences of 16S rRNA to validate the bacterial
identification.Laboratory bioassays were carried out to evaluate the efficacy of the
both cell suspension and cell free supernatant (CFS) of five bacterial isolates (1
isolate of P. luminescens, 2 isolates of P. aeruginosa, and 2 isolates of
Ochrobactrum sp.) separately against the eggs and adults of T. truncatus. Though
the isolates did not show any significant ovicidal action, they recorded significant
adulticidal action against the mite. The P. luminescens isolate was superior over
other isolates recording 64.67 and 60.67 per cent mortality for CFS and cell
suspension, respectively at 96 h of treatment
The virulent P. luminescens isolate was subjected to characterization of
the toxin complex (TcdA gene). The full length gene (7.5 kb) was split into eight
regions and the primers were designed to amplify each regions separately. Among
the eight regions, only six got amplified and were sequenced. Toxin gene
sequence of the isolated P. luminescens bacteria involved in the virulence against
T. truncatus was partially sequenced and the variations in the sequence were
analyzed.
The study identified a potential isolate of P. luminescens with
significant adulticidal action against T. truncatus and the major gene TcdA
encoding the toxin complexes which possess appreciable insecticidal activity was
sequenced and characterized. The P. luminescens isolate obtained in the study can
be evaluated for efficacy against major insect pests in the region in order to
identify the potential in pest management.