Insect Control: Biological and Synthetic Agents - Index of

134 4: Insect Growth- and Development-Disrupting Insecticides
Figure 10 Control (left) and tebufenozide intoxicated by ingestion
(right) larvae of the white tussock moth. While the
control larva continues normal growth and development, the
tebufenozide intoxicated larva undergoes a precocious lethal
molt. In this case the intoxicated larva is in the slipped head
capsule stage.
Figure 11 Photomicrograph showing the severe growth inhibitory
effects of ingested tebufenozide in spruce budworm
larvae. While both larvae show slipped head capsules, one
larva (bottom) shows an additional effect, extrusion of the gut.
decemlineata (Smagghe et al.,1999c; Dhadialla and
Antrium, unpublished observations), and in
cultured abdominal sternites of the mealworm,
Tenebrio molitor (Soltani et al., 2002). Some general
conclusions can be drawn from these studies.
Examination of the cuticle following intoxication
with any of the three bisacylhydrazines revealed
that the larvae synthesize a new cuticle that is
malformed (Figure 12; Dhadialla and Antrim,
unpublished data). Unlike during normal cuticle
synthesis, the lamellate endocuticule deposition in
bisacylhydrazine intoxicated larvae is disrupted and
incomplete. The epidermal cells in intoxicated larvae
have fewer microvilli, show hypertrophied
Golgi complex and an increased number of vesicles
compared to normal epidermal cells active in cuticle
synthesis. The visual observations of precocious
production of new cuticle have also been demonstrated
by in vivo and in vitro experiments to demonstrate
the inductive effects of tebufenozide and
20E on the amount of chitin in S. exigua larval
cuticle and chitin synthesis in cultured claspers of
O. nubilalis, respectively (Smagghe et al., 1997).
At the physiological level the state of ‘‘hyperecdysonism,’’
coined by Williams (1967), manifested by
bisacylhydrazines in intoxicated susceptible larvae
is achieved by various mechanisms. Blackford and
Dinan (1997) demonstrated that while larvae of
the tomato moth, Lacnobia oleracea, detoxified
ingested 20E as expected, it remained susceptible
to ecdysteroid agonists RH-5849 and RH-5992.
This suggested that the metabolic stability of the
ecdysone agonists induced the ‘‘hyperecdysonism’’
state in the tomato moth larvae. In another study,
RH-5849 was shown to repress steroidogenesis in
the larvae of the blowfly, Caliphora vicina, asa
result of its action on the ring gland (Jiang and
Koolman, 1999). However, the production of ecdysteroids
in abdominal sterintes of T. molitor, cultured
in vitro in the presence of RH-0345, increased compared
to that in control cultured abdominal sternites
(Soltani et al., 2002). Ecdysteroid production,
measured by an ecdysteroid enzyme immunoassay,
by sternites cultured in vitro in the presence of
1–10 mM RH-0345 increased with increasing incubation
times and concentrations of the bisacylhydrazine.
Topical application of 10 mg RH-0345
to newly ecdysed pupae also caused a significant
increase in hemolymph ecdysteroid amount as compared
to control treated pupal hemolymph ecdysteroid
levels. However, there was no effect
in the timing of the normal ecdysteroid release in
the hemolymph. Contrary to the in vitro and in vivo
effects observed in the above Tenebrio study,
Williams et al. (1997) observed that injection of
RH-5849 into larvae of the tobacco hornworm,
Manduca sexta, induced production of midgut
cytosolic ecdysone oxidase and ecdysteroid phosphotransferase
activities, which are involved in
the inactivation of 20E. In addition, both 20E
and RH-5849 caused induction of ecdysteroid
26-hydroxylase activity in the midgut mitochondria