Action of a novel nonsteroidal ecdysteroid mimic ... - Insects.ugent.be

90 Guy Smagghe, Danny Degheele
P
‘3
350
-m-
5 150
5
0 100
04 I
0 5 10 15 20
Time (days after first oviposition)
Fig. 4. Effect of tebufenozide on the cumulative fecundity per
female of L. decemlineata adults, at 20 pg per adult by topical
treatment and at 30 or 100 mg litre-’ by continuous feeding
on treated potato leaves. The arrow indicates the moment when
topical treatment took place or oral application started. The
symbol * indicates that all adults were dead.
73( k 8) and 69( k 8)% for the groups treated topically
with 0 and 20 pg per adult, respectively.
3.3 Toxicity and effect of tebufenozide on Heteroptera
No effect on growth, weight gain or ecdysis was observed
on third- (N,) and last- (N5) instar nymphs of P. sagitta
at doses ranging from 1 to 40 pg per nymph, topically
applied either on the dorsal or on the ventral side.
Likewise, no toxicity and no effects on the development
of N, and N, bugs were noted when last-instar S. exigua
larvae previously treated with 20 pg per larva were
provided as prey.
In a preliminary assay with 0. insidiosus, no effects on
the development of any nymphal stage (N,-N,) or adult
formation were noted when fresh eggs of S. exigua
previously dipped in 1 g litre-’ tebufenozide were
supplied ad libitum.
3.4 Toxicity and effects of tebufenozide on Orthoptera
First- and second-instar L. migratoria migratorioides
larvae showed a pattern of growth and ecdysis into the
following instar after a continuous treatment with 30 or
100 mg litre-’ tebufenozide which was the same as that
of the control insects.
4 DISCUSSION
Tebufenozide acts primarily by inducing a premature and
lethal larval moult, especially in larval Lepidoptera. Our
results strengthen the concept that the compound
stimulates epidermal cells to undergo apolysis prematurely
and to synthesize a new larval cuticle by imitating the
activity of ecdysteroids. In addition, LC50 values of
tebufenozide against the different stages of S. exempta
and S. exigua illustrate the uniformity of response with
respect to the larval stage, which is consistent with the
pattern of RH-5849 against the different stages of several
Lepidoptera obtained under comparable conditions.2*’2
The minimum concentration of tebufenozide (by means
of a continuous oral application) needed to induce a
premature moult in the different larval instars was similar
for each species: in third- to sixth-instar larvae (L3-Ls)
of S. exempta at a concentration of 20.02 mg litre-’, and
in L1-L5 of S. exigua at 21 mg litre-’. LC50 values for
last instars reached lower levels, since abnormal and
lethal pupation are included in the mortality percentages
for last-intar larvae.
The inability of treated larvae to ecdyse out of the old
cuticle might be provoked by an interference with
eclosion hormone (EH) production/release, since it is
known that this hormone is released by a drop in
ecdysteroid titre prior to ecyd~is.~~ Residual amounts of
ecdysteroid agonist in the insect body may cause an
inhibition of EH-release. Similar results of death during
a prematurely promoted moult have been reported
previously, following application of natural ecdysteroids
to different insect species.31
The decrease in weight gain and feeding in larval
Lepidoptera treated with tebufenozide may result from
an ecdysonergic activity of the compound. It is well
known that cessation of feeding and weight gain occurs
prior to ecdysis as a result of ecdysteroid ~ecretion.~’ In
contrast, no such effects were observed in larvae of L.
decemlineata, D. virggera, P. sagitta and L. migratoria at
similar doses/concentrations of tebenufenozide.
The different toxicities of ecdysteroid agonists on
last-instar larvae of S. exempta, S. exigua and L.
decemlineata applied orally cannot be explained by
differences in retention, distribution and metabolic
detoxification of the compound in the insect body.”,33
Consequently, we consider that the wide range of
susceptibilities to the ecdysteroid-mimicking compounds
in the different insect species may be explained by
differences in the structure of the EcR and their binding
affinity for the ecdysteroid agonist ligand molecules. The
latter hypothesis is consistent with the concept that
structure and biochemical properties of EcR may differ
among insect species.34 In general, although the hypothesis
seems reasonable that the different toxicities of
ecdysteroid agonists are induced by differences in
EcR-binding affinity, further research is required.
In this study, it was noted that tebufenozide possessed
little or no insecticidal activity against larvae of two
coleopterans, L. decemlineata and D. virgqera virgqera,
against various nymphal stages of the predatory heteropterans,
P. sagitta and 0. insidiosus, or against larvae
of the orthopteran, L. migratoria migratorioides, as