<p>Bed bugs (<i>Cimex lectularius</i>
L.) are globally important human ectoparasites. Their management necessitates the use of multiple control
techniques. Plant-derived essential oils are extracts from aromatic plants that
represent one of the alternative control measures for bed bug control, in
addition to mechanical options and synthetic pesticides. However, there is
limited information available on the efficacy and toxicology of plant essential
oils against bed bugs. This project was designed with the aim to provide
in-depth information on efficacy, toxicology and mode-of-action of essential
oils and their insecticidal constituents in bed bugs. Initially, I evaluated topical
and fumigant toxicity of fifteen essential oil components against adult male
bed bugs of the Harlan strain (an insecticide susceptible strain). Neurological
effects of the six most toxicologically active compounds were also determined.
In both topical and fumigant bioassays, carvacrol and thymol were the most
active compounds. Spontaneous electrical activity measurements of the bed bug
nervous system demonstrated neuroinhibitory effects of carvacrol, thymol and
eugenol, whereas linalool and bifenthrin (a pyrethroid class insecticide) produced
excitatory effects. Further, I evaluated the efficacy and neurological impacts of
a mixture of three neuroinhibitory compounds; carvacrol, eugenol and thymol in
1:1:1 ratio against adult male bed bugs of the Harlan strain. This mixture of
monoterpenoids as well as the mixture of synthetic insecticides exhibited a
synergistic affect in topical bioassays. In electrophysiology experiments, the
monoterpenoid mixture led to higher neuroinhibitory effects, whereas a mixture
of synthetic insecticides caused higher neuroexcitatory effects in comparison
to single compounds. </p>
<p>In the next objective of my
dissertation, I compared the efficacy of five plant essential oils (thyme,
oregano, clove, geranium and coriander), their major components (thymol,
carvacrol, eugenol, geraniol and linalool) and EcoRaider<sup>®</sup>
(commercial product) between pyrethroid susceptible (Harlan) and field
collected (Knoxville) bed bug populations. Initially, I found that the
Knoxville strain was 72,893 and 291,626 fold resistant to topically applied
deltamethrin (a pyrethroid class insecticide) compared to the susceptible
Harlan strain at the LD<sub>25</sub> and LD<sub>50</sub> lethal dose levels,
respectively. Synergist bioassays and detoxification enzyme assays showed that
the Knoxville strain possesses significantly higher activity of cytochrome P450
and esterase enzymes. Further, Sanger sequencing revealed the presence of the
L925I mutation in the voltage gated sodium channel gene. The Knoxville strain,
however, did not show any resistance to plant essential oils, their major components
or EcoRaider<sup>®</sup> in topical bioassays (resistance ratios of ~ 1). In the
final objective, I evaluated the efficacy of binary mixtures of above-mentioned
essential oils or their major components or EcoRaider<sup>®</sup> with deltamethrin
in susceptible and resistant bed bugs. In topical application bioassays, binary
mixtures of essential oils or their major components or EcoRaider<sup>®</sup> and
deltamethrin at the LD<sub>25</sub> dose caused a synergistic increase in
toxicity in resistant bed bugs. Further, I studied the inhibitory effects of
major essential oil components on detoxification enzyme activities (cytochrome P450s,
esterases and glutathione transferases). Detoxification enzyme assays conducted
using protein extracts from bed bugs pre-treated with essential oil constituents
showed that these compounds significantly inhibited cytochrome P450 activity in
the resistant strain, but esterase and glutathione transferase activity were
unaffected. No inhibition of detoxification enzyme activities was observed in
the Harlan strain bed bugs pre-treated with essential oil constituents.</p>
<p>In conclusion, my dissertation
research has created the foundation for utilization of natural products for bed
bug management by (i) describing the efficacy of plant essential oils and their
components against bed bugs, (ii) discovering synergistic interactions between essential
oil components at the nervous system level, (iii) determining susceptibility of
deltamethrin-resistant bed bugs to plant essential oils and their constituents and
(iv) identifying synergistic effects of essential oils or their components on
toxicity of pyrethroid insecticides and underlying mechanisms of this synergistic interaction. </p>
<br>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/12126897 |
| Date | 17 April 2020 |
| Creators | Sudip Gaire (8703072) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/TOXICOLOGY_OF_PLANT_ESSENTIAL_OILS_IN_BED_BUGS/12126897 |
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