Biology Reference
In-Depth Information
initiation of moulting (the apolysis step),
without completion of morphogenesis.
During a moult, ecdysteroid levels first
rise to stimulate onset of apolysis and cuti-
cle synthesis but then must fall to facilitate
the release of eclosion hormone (EH)
(Truman
et al.
, 2002) and the ecdysis-
triggering hormone (ETH) (Zitnan
et al.
,
1999). These last substances act in concert
to trigger insect ecdysis during the final
stages of the moult. Methanol and ethyl ace-
tate extracts and the major component of
the ethyl acetate extract verbascoside may
have the effect of disrupting ecdysteroid
metabolism to result in an inhibition of
emergence behaviour, or may, alternatively,
act directly to inhibit the release of ETH
(Hesterlee and Morton, 1996), as was
observed.
(10 mg/ml, 12.5%) extracts showed signifi-
cant delay of pupation (Table 15.5). Above
35 mg/ml, no larvae emerged to pupation
with ethyl acetate and methanol extracts.
Delays in time to pupation (>24 days) for
hexane (>35.0 mg/ml), ethyl acetate (>10 mg/
ml) and methanol (>10.0 mg/ml) were
observed. Furthermore, concentrations of
ethyl acetate and methanol between 10.0
and 20 mg/ml significantly reduced pupal
weights. Ethyl acetate was the extract that
produced the greatest effect on pupal
weights above 1.0 ppm (data not shown),
whereas Me-Ced, Me-Myrt and Me-Yuc
extracts produced the greatest effect on
pupal weights at 10.0 ppm, as previously
reported (Céspedes
et al.
, 2000, 2006; Torres
et al.
, 2003).
The percentage of emergence of adults
from the pupae was also drastically affected
by these extracts. The greatest reductions
were shown with ethyl acetate (35.0 and
50 mg/ml, 100%), and methanol (20-50 mg/ml,
100%), and in almost all could be observed
deformities. Thus, at higher concentrations
the most active extracts from
Calceolaria
talcana
completely blocked the percentage of
adult emergence, because no viable adults
emerged from the pupae at this step.
During insect development the shed-
ding of the cuticle, known as moulting, or
ecdysis, occurs. Moulting affects the entire
body wall and all internal parts that are
formed as invaginations of the wall.
Collectively, all changes that involve
growth, moulting and maturation are known
as morphogenesis. The moulting process
begins when epidermal cells respond to
hormonal changes by increasing their rate
of protein synthesis. The first step of moult-
ing is apolysis: the separation of epidermal
cells from the inner surface of the old endo-
cuticle and the formation of the subcuticu-
lar space. A moulting gel (including
enzymes) is secreted into this space. An
insect larva that is actively constructing
new exoskeleton is said to be in a pharate
condition (Marks, 1980). In this study, anal-
ysis of the test insect fed with
C. talcana
extracts revealed a developmental disrup-
tion in which the insects died (between 10
to 50 ppm) during pharate conditions after
15.5.7
Growth index and relative growth
index for
S. frugiperda
Larvae that reached pupal stage belonged to
the lowest concentration groups, and the
pupae that emerged showed many deformi-
ties. Thus, in all treatments the average
time to reach the mean weight of the adult
stage relative to the time needed for control
larvae to reach the adult stage was signifi-
cantly delayed. The growth index (GI or
number of surviving larvae/total larvae
used) and relative growth index (RGI or
GI treated/GI control; Zhang
et al.
, 1993)
showed (Table 15.6) that the strongest effects
were at 35.0 mg/ml by ethyl acetate and ver-
bascoside (GI 0.28 and 0.09, respectively),
and at 50 mg/ml by methanol extract and
gedunin (GI 0.28 and 0.10, respectively).
These parameters, together with the LD
95
(the lethal dose producing 95% of death)
values, established that the greatest effect
was shown at 25.0 mg/ml by verbascoside
(100% mortality), and at 35.0 mg/ml by
ethyl acetate and methanol, respectively
(100% mortality). Compared with the effect
of a phytoecdysteroid such as ajugasterone
C, which showed a GI of 0.88 at 100 mg/ml
(Céspedes
et al.
, 2005), these extracts
produced the highest insecticidal effects.
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