MSc

The research work entitled “Bioactivity of essential oil from the tea tree
Melaleuca bracteata F. Muell. against stored product pests” was carried out at the
Department of Agricultural Entomology, College of Agriculture, Vellayani during
2020 to 2022 with an objective to isolate and characterize and evaluate the bioactivity
and persistence of toxicity of M. bracteata essential oil against stored product pests viz.
red flour beetle, Tribolium castaneum (Herbst) and pulse beetle, Callosobruchus
chinensis (Linn.).
The essential oil (EO) from M. bracteata was extracted by hydro distillation in
clevenger type apparatus. Essential oil yield per 100g leaves was observed as 1.08 mL.
The efficacy of tea tree essential oil was tested against the laboratorymaintained culture of T. castaneum and C. chinensis by contact, fumigant and repellent
bioassays and the persistence of toxic activity of the essential oil on the test insects were
also evaluated under laboratory conditions.
In contact toxicity bioassay, the LC50 and LC90 values of the EO against T.
castaneum were 25.16 % and 69.94 %, respectively at 24 hours after treatment (HAT).
The LC50 and LC90 values of EO against C. chinensis were 0.05 % and 2.73 %,
respectively at 24 HAT. In fumigant toxicity bioassay of T. castaneum, the LC50 and
LC90 values of the EO were 7.7mLL-1
and 22.95mLL-1
at 24 HAT. The LC50 and LC90
values of EO against C. chinensis were 4.30 μLL-1
and 29.96 μLL-1
, respectively at 24
HAT.
In the case of T. castaneum, the maximum repellency observed was 93.75 % at
5% concentration of EO. Cent per cent repellency was not observed here because of the
fumigant effect at higher doses which scattered the released insects into both treated
and untreated filter paper halves in Petri plates. In case of C. chinensis, the insects get
repelled at a very low concentration. 100 per cent repellency was observed at 0.01%
and 0.05% concentration of EO after 2 h and 45 min, respectively.
The toxic effect of M. bracteata EO persisted up to 34 days on C. chinensis
whereas upto 10 days for T. castaneum when exposed to LC90 values obtained from
136
fumigant bioassay. The GC-FID and GC-MS profiling of the EO showed 46
components where, methyl isoeugenol (69.64%) a phenylpropanoid was found to be
major component followed by methyl cinnamate (8.73%) and methyl eugenol (7.40%),
which may alone or synergistically with other constituents caused the insecticidal
effects.
Experiments on the effect of the EO on the detoxifying enzymes of T.
castaneum and C. chinensis showed that the enzymes like cytochrome P 450, carboxyl
esterase were significantly lower in essential oil treated insects, whereas the activity of
glutathione S- transferase was found to be higher when compared to untreated insects.
Similarly, the total protein content was also found lower in essential oil treated insects
compared to the untreated insects.
In the present investigation, M. bracteata was found to have a good essential oil
yield of 1.08 per cent. The M. bracteata EO showed strong contact, fumigant and
repellent toxicity against C. chinensis when compared to T. castaneum. Against C.
chinensis, the toxic activity of the essential oil persisted up to 34 days. Methyl
isoeugenol (69.53%) was found to be the major component in the essential oil, followed
by methyl cinnamate (8.73%) and methyl eugenol (7.41%), which may alone or
synergistically with other constituents caused the insecticidal effects. The essential oil
in the present investigation has effects on the detoxification enzymes of the test insects.