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Bamboo-leaf responsible for cytotoxic, molluscicidal, anti-sickling, anesthetic, antibacterial,

Bamboo-leaf prickly ash, Z. armatum (Family:
Rutaceae) is a versatile and edible plant species in nature. About 250 species of
the genus Zanthoxylum found in
temperate and tropical areas across the World (Pirani, 1993).  Many of these species are frequently used in
daily life as condiments and for therapeutic remedies (Epifano et al., 2011).
Several species of genus Zanthoxylum are
reported to have significant repellent and larvicidal properties (Kumar et al.,
2016a; Kaleeswaran et al. 2018 and references therein).  Zanthoxylum species are known to
contain several alkaloids, aliphatic and aromatic amides, lignans, coumarins,
sterols, carbohydrate residues etc, which are responsible for cytotoxic,
molluscicidal, anti-sickling, anesthetic, antibacterial, insecticidal,
anti-fungal and anti-inflammatory properties in this plant  (Singh and Singh, 2011; Firake et al., 2014
and references therein).  Volatile
compounds of many essential oils of these plants contain alcohols, aldehydes,
alkanes, and terpenoids, particularly mono-terpenoids and these are also responsible
for its biological activity (Lawless, 2002).

Significant larvicidal properties were observed in
extract of pericarp and leaf of Z.
armatum against brassica caterpillar pest, P. brassicae; however this
effect was very less in case of seed extract. Based on literature, we used only
n-hexane fractions of seeds, leaf and pericarp of Z. armatum for the
bioassay experiment; since hexane fraction of leaf and fruits contain large
proportion of two major carbonyl compounds (viz.
‘2-undecanone’ and ‘2-tridecanone’) and the monoterpene ketone ‘Piperitone’
(Bisht et al., 2014; Kayat et al., 2016; Kumar et al., 2016a).  These three compounds are reported to have larvicidal
activities against insect-pests viz., Keiferia
lycopersicella (Walsingham), S.
exigua (Hubner), Plutella xylostella, S. litura and mosquito
larvae (Lin et al., 1987;   Koliopoulos et al. 2010, Marques et al., 2011;
Kumar et al., 2016a; Kim and Ahn 2017; Kaleeswaran et al 2018).  However, there was no literature yet available
on the larvicidal activity Z. armatum against
P. brassicae.

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Based on bioassay experiments, the pericarp
extract was found more effective against the larvae followed by leaf extract.
Larvicidal activity could be due to presence of a large proportion of carbonyl
compounds and monoterpene ketones in the pericarp and leaves of Z. armatum (Lin et al., 1987; Koliopoulos
et al., 2010; Marques et al., 2011; Kumar et al., 2016a).  Recently, Kayat et al. (2016) also reported that,
the hexane extract of Z. armatum fruit
contains the maximum proportion of carbonyl compounds (27.3%) and Piperitone
(6.71%). The presence of strong aroma in rutaceous crops are mainly due to
these compounds and the aromatic compounds are usually stored in the pericarps of
Zanthoxylum species (Melo and Zickel,
2004; Yamazaki et al., 2007) and other rutaceous shrubs e.g. citrus or orange
peel etc. Our results are in corroboration with several reports, who
demonstrated that the pericarp has more insecticidal activities than other
plant parts (Liu et al., 2009; Supabphol and Tangjitjareonkun, 2014).

We found more contact toxicity (topical
application) of extract to the larvae than oral feeding (leaf dip method).  Based on LC50 values, the toxicity
of n-hexane pericarp extract was higher even than commercially available
bio-pesticide, Azadirachtin 0.15 EC, which is recommended against P. brassicae. Contact toxicity of the
hexane fractions of leaf and pericarp has been demonstrated for the first time
by Kaleeswaran et al (2018) against S. litura caterpillars. Hieu et al.
(2012) reported that, Zanthoxylum has
acetyl cholinesterase (AChE) inhibitory activities. However, Kumar et al.
(2016a) revealed the oral toxicity of the n-hexane fraction of Z. armatum leaf extract to the larvae of
P. xylostella. This variation could
be possible due to variation in pest species, host plant and other factors,
etc., since different pest species have different mechanisms of detoxification
of xenobiotics (Hameed et al., 2011).

An inverse relationship was found in the leaf area
consumption and concentration of the extract.  Larvicidal action and antifeedent effect of Z.
armatum could be due to the presence of large proportion insecticidal
compounds like ‘2-undecanone’, ‘2-tridecanone’ and ‘piperitone’.  Currently, the exact modes of action and
molecular targets of these compounds are not known in insects.  Nevertheless, the compound ‘2-undecanone’ might
be responsible for antifeedent effect, since it inhibit the odorant receptors
(ORs) in insect (Bohbot and Dickens, 2010).  The ORs are located in the dendritic membrane
of the olfactory sensory neurons (OSNs) of the insects; which is  responsible for triggering olfactory
transduction by changing the action potentials as a message sent to the brain
(Kaupp, 2010). Another compound, ‘2-tridecanone’ is known to increase
cytochrome P450 enzyme level in treated insects (Rose et al., 1992; Li et al.,
2014).  Furthermore, the lipophilic compound
‘piperitone’ have fast penetration properties into insects which consequently
interfere with biochemical and physiological functions (Ahn et al., 1998) and
also justified the contact toxicity in the present study. All three compounds
are reported to be toxic against several insects (Massotti et al., 2003; Zuzarte
and Salgueiro, 2015).  In fact, they act synergistically
in terms of insecticidal activity (Mossa, 2016). Thus combination of the action
of these compounds perhaps ultimately caused larval mortality in this study. Our
findings are supported by many reports, which have demonstrated the antifeedent
properties of Zanthoxylum species
against insects, including P. brassicae (Liu et al., 2009; Ge and Weston,
1995; Paul and Sohkhlet, 2012; Wang et
al., 2015).

In this study, the developmental time (mainly
larval and pupal duration) was significantly extended; while percentage of
pupation and adult emergence was considerably reduced, when the larvae were
exposed to sub-lethal doses of pericarp extract. The compounds present in the
plants may individually or collectively contribute to make larvicidal,
pupicidal, adult emergence inhibition and other bioactivities in insects (Eliman et al. 2009).   Alteration in physiological and biochemical
functions, leading to starvation in larvae, might be responsible for this. Since
feeding on limited diet can increase the larval and pupal period of insect
(Metspalu, 2003).  Extended developmental
time and reduced pupation is reported in other lepidopteran larvae, S. litura fed with Z. limonella extract (Arivoli and Tennyson 2013). Paul and Sohkhlet
(2012) also found extended pupal period and larval mortality of 4.67% in P. brassicae due to Z. armatum extract.

In the present investigation, the egg hatchability
was found to be inversely related to the concentration of the extract.  The exact mechanism of ovicidal action has not
yet been understood. Additional studies on action of individual compounds could
give more insights on this aspect. Nevertheless, the ovicidal action of the
pericarp of Zanthoxylum species
has also been reported in several insect-pests including oriental leaf
worm, S. litura (Kaleeswaran et al 2018), fruit fly, Bactrocera dorsalis
(Marr and Tang, 1992), mosquito, Culex
spp. (Tennyson et al., 2011), Colorado potato beetle, Leptinotarsa decemlineata (Ginesta et al. 1994) and other organisms
(Qi et al., 2015).  


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