Page 33 - Volume 6, Issue 1, January 2013
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© Benaki Phytopathological Institute
Toxicity of essential oils against
Tribolium confusum
31
adult mosaics) being counted as dead. Also
the larvae that survived the tests were kept
until pupation to examine possible delayed
mortality or morphological deformations.
Four to seven doses were tested for each
essential oil and developmental stage com-
bination. Each dose was repeated three or
four times. Due to differential toxicity, dos-
es of the compounds tested ranged from
0.27 to 165 μl/l of air depending on the es-
sential oil, insect developmental stage, age
and sex (based on preliminary tests). All ex-
periments were carried out in an incubator
at 26
±
1°C, 65
±
5% r.h. and 12 h light/12 h
dark photoperiod.
Statistical analysis
Data obtained from each dose-response
bioassay were subjected to probit analy-
sis and LC
50
values and 95% confidence lim-
its were estimated. The comparisons among
LC
50
values were based on the overlap of
confidence limits (Finney, 1971). All analyses
were conducted using the statistical pack-
age SPSS 14.0 (SPSS, 2004)
Results
Fumigant toxicity of essential oils
The fumigant toxicity (LC
50
values) of the
essential oils of
L. hybrida,
L. nobilis
,
T. ori-
entalis,
C. sinensis
,
C. limon
and
O. vulgare
against immature and adult
T. confusum
is
given in Tables 1, 2, 3, 4, 5 and 6, respective-
ly. All but
O. vulgare
essential oils exhibited
strong fumigant toxicity to all developmen-
tal stages of
T. confusum
. LC
50
values ranged
between 1.8 and 109 μl/l air depending on
the essential oil and insect developmental
stage, age and sex.
The most susceptible stage of
T. confu-
sum
to the vapours of
L. hybrida
essential oil
was the 10-day-old larvae (LC
50
, 1.8 μl/l air),
while the most tolerant was the 31-day-old
larvae (LC
50
109.9 μl/l air) (Table 1). The esti-
mated LC
50
values for female and male pu-
pae were very close (37.3 and 38.7 μl/l air,
respectively). An increase in the adults’ sus-
ceptibility to the essential oil vapours with
increasing age was recorded, with females
being more tolerant than males, especially
for 60-day-old adults, where the observed
differences were statistically significant.
The most susceptible stage to the va-
pours of
L. nobilis
oil was that of the 10-day-
old larvae, and the most tolerant that of the
25-day-old larvae (Table 2). In the case of pu-
pae, males were more susceptible than fe-
males to the essential oil vapours. Also, adult
susceptibility to the vapours decreased with
increasing age, but no significant differenc-
es between the sexes were observed.
The10-day-old larvae were the most vul-
nerable to the vapours of
T. orientalis
oil, fol-
lowed by pupae and adults (Table 3). Nev-
ertheless, the calculated LC
50
values for all
stages (except those for the young larvae)
could not provide a clear picture of the de-
gree of susceptibility because in most cases
the statistical analysis did not reveal signifi-
cant differences.
A similar case to the
T. orientalis
oil was
recorded for
C. sinensis
and
C. limon,
with
young larvae being more susceptible than
older ones (Tables 4 and 5). A slight (but not
statistically significant) decrease in suscepti-
bility of adults to the citrus essential oil va-
pours with increasing age was observed.
The essential oil of
O. vulgare
showed a
completely different effect from all the oth-
er essential oils tested. In fact, with the ex-
ception of 10-day-old larvae and pupae, its
vapours did not provoke mortality even at
doses up to 165 μl/l air (Table 6).
Effect on surviving larvae
The larvae that survived exposure to es-
sential oils did not exhibit any noticeable
delayed mortality or any kind of morpho-
logical abnormalities (data not shown).
Effect on pupae
All the essential oils tested caused a
greater or lesser proportion of deformations
in
T. confusum
pupae. These morphological
deformations were noted in both males and
females, with pupae trapped in the pupari-
um, individuals with an appearance interme-
diate between pupae and adults (adultoids),
