PhD

The rice bug, Leptocorisaspp. is one of the most important sap sucking
insect pests of rice in the tropics. Both nymphs and adults suck sap from
developing rice grains during the milky grain stage, leading to discoloured, empty
or half-filled grains. Yield loss due to the bug infestation ranges from 10 to 35 per
cent.
Management of the bug essentially involves spraying with insecticides, for
want of safer options. Biological control of rice bug remains hampered by absence
of ideal bioagents. The entomopathogenic fungus (EPF), Beauveria bassiana
(Bals.) has been reported as infecting rice bug, from several parts of the world,
including India. In this context, the present study was undertaken with the
objectives of collection, isolation and molecular characterization of local strains
of B. bassiana, evaluation against Leptocorisa spp., identification of a cost
effective mass production technology for the selected strain of B. bassiana, and
formulation of the most effective strain of B. bassiana.
Surveys were conducted over two seasons during 2016-17 at major rice
growing tracts of Alappuzha, Ernakulam, Thrissur and Palakkad districts of
Kerala to collect EPF infecting rice bug. However, EPF infected rice bug cadavers
were obtained only from one location i.e., Pattambi in Palghat district.As only one
isolate was obtained, a potential fungal pathogen earlier obtained from rice bug at
Vellanikkara, Thrissur and maintained at AICRP on BCCP was also included in
further studies.
The above two isolates were identified through study of colony characters
as well as through ITS sequencingand Basic Local Alignment Search Tool
(BLAST). The isolate from Vellanikkara was identified as Beauveria bassiana,
while the isolate from Pattambi was identified as Choanephora cucurbitarum.
The pathogenicity of the two isolates was assessed through laboratory as
well as pot culture studies. In the laboratory, B. bassianaisolate recorded more
than 90 per cent reduction in population of rice bug nymphs and adults five andseven days after treatment respectively at all concentrations evaluated while C.
cucurbitarum recorded only 18 to 53 per cent reduction in bug population.
Pot culture studies confirmed the superiority of B. bassianaisolate over C.
cucurbitarum with B. bassiana treated pots registering over 99 per cent mortality
by 20 th day after treatment as against 27 to 31 per cent mortality in case of the
latter.B. bassianaisolate was further evaluated at its most effective concentration,
under field conditions in both Thrissur and Palakkad districts.B. bassiana proved
to be on par with the insecticide malathion from 10 th day onwards at both
locations.
The compatibility of B. bassiana isolate with selected insecticides and
fungicides was assessed using poisoned food technique. The results showed that
the insecticide acephate was the most compatible with 31.29 per cent growth
inhibition while the fungicide propiconazole was the least compatible with 85.71
per cent inhibition.
Methods for talc based, oil based and aqueous formulations of B.
bassianawere standardized.Palm oil was identified as the most suited carrier for
oil based formulation through inhibition zone technique as well as through
assessment of viability of B. bassiana spores. Glycerol(3%), carboxy methyl
cellulose (0.5%) and chitin (5%) formed the best adjuvants for oil based,
aqueousand talc based formulations respectively, based on viable cfu count 30
days after storage.
All the three formulations were as effective as malathion in reducing rice
bug populations in field trials. However, talc formulation was significantly
superior to the remaining formulations in terms of yield as well as grain damage.
Talc, oil and aqueous formulations had shelf life durations of six, five and four
months respectively.