PhD

The study on “Mealybugs of vegetable ecosystems and tritrophic
interactions of brinjal mealybugs” was conducted at College of Agriculture,
Vellayani during 2017 to 2020 with the objectives to identify mealybugs and their
natural enemy fauna in solanaceous and cucurbitaceous vegetables, to carry out
the molecular characterization of mealy bugs in solanaceous and cucurbitaceous
vegetables and to find out the tritrophic interactions of mealybugs infesting
brinjal.
An investigation on mealybug diversity in solanaceous and cucurbitaceous
crops of Kerala revealed a total of six mealybug species viz., Coccidohystrix
insolita, Ferrisia virgata, Paracoccus marginatus, Phenacoccus solenopsis,
Planococcus citri and Pseudococcus jackbeardsleyi. Besides, four mealybugs viz.,
Crisicoccus hirsutus, Maconellicoccus hirsutus, Planococcus lilacinus and
Rastrococcus iceryoides infesting other vegetable crops were also recorded from
Kerala. The study also revealed that the population of mealybug, C. insolita
collected from different regions of Kerala exhibited significant morphological
variation. The molecular characterization studies proved that the population
belongs to C. insolita and the variations may be environmental induced.
An exploration of the host range of mealybugs infesting solanaceous and
cucurbitaceous vegetables in Kerala revealed a total of 113 plants under 73 genera
belonging to 31 families, out of which 14 plants were recorded as new host
reports. A rich natural enemy fauna on mealybugs belongs to five orders viz.,
Coleoptera, Lepidoptera, Diptera, Neuroptera and Hymenoptera were documented
from Kerala. Twenty species of mealybug predators from five different families
were recorded, among which the predominant family was Coccinellidae with 16
species under six genera. Among the various predators collected, Cacoxenus sp.
was recorded for the first time as a predator of P. jackbeardsleyi.
Eleven hymenopteran parasitoids belonging to five families were
documented from mealybugs, of which the majority belongs to the family
Encyrtidae. Four new host-parasitoid associations were also recorded for the first
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time. The study also identified six hyperparasitoids under the family Encyrtidae,
of which two species were recorded as new report which includes Cheiloneurus
sp. and Prochilonerus sp. associated with the mealybug P. solenopsis.
The ants associated with mealybugs in different agroecosystems were also
investigated and a total of 14 species of ants belonging to nine genera under three
subfamilies were recorded, of which the most dominant subfamily was
Formicinae followed by Myrmicinae and Dolichoderinae. The present study also
revealed 15 new ant-mealybug associations for the first time.
The molecular characterization of eight mealybug species was carried out
that complemented the morphological identification of species. A total of eight
nucleotide sequences were submitted to NCBI GenBank and accession numbers
were generated. The sequences were also submitted to BOLD and illustrative
barcodes were generated. The barcodes of C. insolita, C. hirsutus and
P. jackbeardsleyi were generated for the first time. The diversity of
endosymbionts of the mealybug C. insolita was carried out for the first time. The
study revealed a total of 15 phyla of endosymbionts on C. insolita, of which
Proteobacteria was the predominant one.
An experiment was conducted at Instructional Farm, Vellayani to identify the
tritrophic interaction in brinjal, C. insolita and its natural enemies. The study
revealed that out of the ten brinjal cultivars evaluated, the lowest mean population
of mealybugs was observed in Pusa Uttam whereas the hybrid cultivar, Udit
recorded the highest number of mealybugs. The lowest mean per cent leaf
infestation was recorded in Pusa Purple Long which was statistically on par with
Pusa Uttam whereas the hybrid cultivar, Udit was severely damaged by the
mealybugs. Among the tested cultivars, Pusa Purple Long was recorded under the
group resistant whereas Udit and Haritha were included under the highly
susceptible group based on the mean per cent leaf infestation caused by
C. insolita. The cultivar Haritha attracted the highest number of predators whereas
the maximum mean population of spiders were observed in Pusa Purple Long.
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The correlation analysis on the mean population of mealy bugs with
biophysical parameters revealed that trichome density, length width ratio of leaf
and number of branches exhibited a significant positive correlation with mean
population of mealybugs. Correlation studies on the mean population of natural
enemies with biophysical parameters revealed that plant height exhibited a
significant negative correlation whereas leaf thickness and number of branches
revealed a non-significant negative correlation with the mean population of
natural enemies. Among the biochemical parameters of brinjal cultivars, total
phenol content exhibited a significant negative correlation with the mean
population of mealybugs whereas total chlorophyll content showed a significant
positive correlation. The correlation between biochemical parameters of brinjal
cultivars with the mean population of natural enemies revealed that total phenol
and carotenoid content exhibited a non-significant negative correlation whereas
total protein content, reducing sugar and total chlorophyll content showed a nonsignificant positive correlation.
The info-chemical mediated interactions in brinjal cultivars, mealybug and its
natural enemies were studied using a multi-armed olfactometer and Y-shaped
olfactometer assay. The results revealed that the cultivar Udit attracted the highest
number of natural enemy Chrysoperla zastrowii sillemi while the cultivar Pusa
Uttam attracted the lowest number. The variation in preference shown by the
natural enemies may be related to the difference in volatile compounds emanating
from the host cultivar. The GC-MS analysis of the synomonal compounds of the
cultivar, Udit revealed a total of eleven compounds whereas the cultivar Pusa
Uttam was recorded with a total of five compounds. The Y shaped olfactometer
studies revealed that C. zastrowi sillemi adults showed more preference towards
the synomonal compounds of mealybug infested plants than that of healthy plant
synomonal extracts. On comparing the volatile profile of synomones of mealybug
infested and healthy plants revealed that the synomonal extracts of the mealybug
infested cultivar Udit comprised of 11 compounds whereas healthy plant
synomonal extracts contained nine compounds.
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The relative response of C. zastrowi sillemi towards the kairomonal
compounds of C. insolita was evaluated in a Y tube olfactometer and the results
revealed that the highest mean number of adult lacewings were attracted to the
kairomonal compounds of mealybug compared to control. The kairomonal
extracts of the mealybug comprised of seven compounds and the composition of
these hydrocarbons may determine the selectivity of natural enemies towards the
preferred host.
The present study recorded a total of six mealybug species infesting
solanaceous and cucurbitaceous vegetables and its host range, natural enemies and
associated ants in different agro ecological regions of Kerala. The molecular
characterization studies supported the morphological taxonomy and the sequences
were submitted to NCBI Genbank and BOLD. The study also elucidated the
interactions mediated by plant traits and info-chemicals in brinjal-mealybugnatural enemy tritrophic systems. The study recorded Pusa Purple Long as
resistant cultivar and Udit and Haritha as highly susceptible cultivar to C. insolita.
The plant traits viz., trichome density, length width ratio of leaf, number of
branches, total phenol content and total chlorophyll content exhibited significant
correlation with mean population of mealybugs whereas plant height exhibited
significant correlation with mean population of natural enemies. The study on
info-chemical mediated tritrophic interactions revealed that the concentration and
composition of volatile compounds determine the differential selectivity of natural
enemies. The volatile compounds identified in this study can also be used as a cue
in moderating the behavior of natural enemies in the ecosystems. So a thorough
knowledge on the tritrophic relations in the ecosystem will aid in manipulating
these interactions to devise a better pest management strategy.