Page 39 - Volume 6, Issue 1, January 2013

© Benaki Phytopathological Institute
Toxicity of essential oils against Tribolium confusum 37
genesis could be attributed to the action of
some of their monoterpnenes components.
Such potent action on pupal morphogene-
sis has been demonstrated after exposure of
T. confusum pupae to some monoterpenes
(terpinen-4-ol, 1,8-cineole, linalool, R-(+)-li-
monene and geraniol) (Stamopoulos et al.,
2007). Moreover, some citrus essential oils
(lemon, sweet orange and grape fruit) ap-
plied against the late third- and early fourth-
instar larvae of Aedes (Stegomyia) albopic-
tus (Skuse 1894), the so-called “Asian tiger
mosquito”, produced insect growth regula-
tor (IGR)-like properties (Giatropoulos et al.,
2012). Such effects were also observed by
Amos et al. (1974) after incorporating various
terpenoids into the diet of T. castaneum and
T. confusum, and by other authors working
with hydroprene (Bell and Edwards, 1999;
Arthur, 2003; Arthur and Dowdy, 2003)
Contrary to what observed for pupae,
the larvae that survived the toxicity tests
and were kept until pupation to examine
possible delayed mortality or morphologi-
cal deformations did not exhibit such abnor-
malities. It is known that some terpenoids
display activity similar to that exerted by the
Juvenile Hormone analogues, when larvae
of insects are subjected to their vapours for
a long time, performing extra moults and fi-
nally developing into either larval-pupal in-
termediates or normal pupae which pro-
duce both morphologically normal adults
and adultoids (Amos et al ., 1974; Semple,
1992). Two likely explanations can be adopt-
ed here: either the toxicity tests’ limited ex-
posure time (48h) was insufficient for the
expression of the aforementioned phenom-
ena or, as Semple (1992) reports, ‘larvae of
most holometabolous insects such as Lepi-
doptera and Coleoptera are susceptible only
at the end of the last larval instar, while the
pupae are susceptible for several hours or at
most a few days after the last larval ecdysis’.
Although our findings are preliminary,
they could form a basis for further investiga-
tion of the questions raised in this work. In
particular, additional research is needed to
improve our understanding of how essential
oil vapours act during morphogenesis, giv-
en that the mechanisms underlying the ap-
pearance of adultoids and mutilated adults
have been, to the best of our knowledge, in-
adequately studied.
Overall, essential oils tested with the ex-
ception of O. vulgare essential oil were high-
ly toxic against larvae, pupae and adults of
T. confusum indicating the potential of their
possible utilization as fumigants in protec-
tion of stored products in storehouses by
reducing the risks associated with the use
of synthetic insecticides. However, many
aspects of their release kinetics after ap-
plication and the effect of factors such as
temperature, relative humidity and stored
product commodities, must be studied to
determine whether these substances can
be realistically applied as fumigants against
stored-product insects in practice.
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