© Benaki Phytopathological Institute
Theou
et al.
36
i.e.
L. hybrida
showed the highest toxic effect
against 10 and 25 day-old larvae,
L. nobilis
was the most toxic for 31 day-old larvae and
C. sinensis
and
C. limon
were the most toxic
for adults. Lemon and orange essential oils
are particularly rich in limonene (Giatropou-
los
et al
., 2012) which was found to be one
of the most toxic compounds among other
monoterpenes against adults of
T. confusum
(Stamopoulos
et al
., 2007).
Origanum vulgare
essential oil vapours,
with the exception of 10-day-old larvae and
pupae, failed to result in mortality even at
doses up to 165 μl/l air. Our results concern-
ing
O. vulgare
essential oil, are in correlation
with those obtained by Demirel
et al.
(2009)
which demonstrated that
O. vulgare
,
O. on-
ites
L. and
O. minutiflorum
L. essential oils
possessed the weakest toxicity among eight
different essential oils evaluated against
T.
confusum
.
According to our findings, tolerance to
the essential oils increases as the immature
stages grow older, which coincides with ex-
posure data of
T. confusum
larvae to some
monoterpenes (Stamopoulos
et al
. 2007) as
well as data of
T. castaneum
and
Acanthos-
celides obtectus
(Say) larvae to various essen-
tial oil vapours (Huang
et al
., 1997; Liu and
Ho, 1999; Papachristos and Stamopoulos,
2002). A possible explanation is that the ex-
isting difference in body size may be respon-
sible for the variations in susceptibility.
The
L. hybrida, L. nobilis, C. sinensis
and
C.
limon
essential oils, in their vapour form, are
known to be effective against other stored
product insect pests. Essential oils from
L.
hybrida, L. nobilis
and
C. sinensis
are effective
against the bean weevil
A. obtectus
(Papach-
ristos and Stamopoulos, 2002; Papachristos
et al
., 2004)
and essential oils of
C. sinensis
and
C. limon
are effective against adults and
larvae of
Callosobruchus maculates F., Sitophi-
lus zeamais Motsch.
and
Dermestes maculatus
Deg.
(Don-Pedro, 1996).
In all these cases, the
LC
50
values of essential oils were much lower
from the one calculated for
T. confusum
in the
present work indicating lower susceptibility
of
T. confusum
to essential oil vapours com-
pared to other stored product insect pests.
Essential oils of
L. nobilis
from various or-
igins were found to be also effective against
adults of
T. castaneum
, but the LC
50
values
reported (ranged from 172 to 217 μl/l air)
were much higher than the present one cal-
culated for the adults of
T. confusum
(Jemâa
et al
., 2012). Nevertheless, the
O. vulgare
es-
sential oil vapours have been found to be a
highly effective fumigant against the adults
of
T. castaneum
with LC
50
value of 55 mg/l air
(Kim
et al
., 2010). These results are indicative
of the highly idiosyncratic toxicity of individ-
ual oils even between closely related species
such as
T. confusum
and
T. castaneum.
Susceptibility of adults to the essential
oils seems to increase with the adult’s age,
even though this is not statistically proved
in most cases. Older insects tend to be more
vulnerable to the essential oil vapours, per-
haps because they are less able to metabo-
lise and/or detoxify these substances.
In all but one case (
L. hybrida
), male pu-
pae seem to be relatively more susceptible
than female pupae although, in most cases,
the recorded differences were not statisti-
cally significant. Similar slight sex differenc-
es in susceptibility were also observed in the
adult stage. These differences could be con-
sistent with an innate difference in mode of
action rather than with size, especially as
the difference in size between males and
females of
T. confusum
is almost negligible
and cannot justify the divergence in suscep-
tibility observed in other stored product in-
sects (Papachristos and Stamopoulos, 2002;
Papachristos
et al
., 2004).
The derailing of morphogenesis at the
pupal stage and the appearance of adul-
toids and mutilated adults could be ex-
plained by assuming a direct effect on the
insect hormonal system similar to that of
the insect growth regulators (IGRs). Accord-
ing to the literature,
L. nobilis
essential oil
contains high amounts of 1,8 cineole (Me-
diouni
et al
., 2012) and that of
L. hybrida
con-
tains 1,8 cineole and linalool (Papachristos
et al.
, 2004). Lemon and orange essential
oils consist almost of limonene (Giatropou-
los
et al
., 2012) and the effects of these es-
sential oils on
T. confusum
pupae morpho-
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