Insect Control: Biological and Synthetic Agents - Index of

40 2: Indoxacarb and the Sodium Channel Blocker Insecticides
Higher temperatures increase desiccation and speed
of kill. Unlike the pyrethroids, indoxacarb exhibits
a positive temperature coefficient. Uncoordinated
insects may fall off the plant and desiccate, drown,
or become subject to predation. Affected insects can
stay alive for 4–96 h, depending on the dose of
indoxacarb and susceptibility of the insect. Insects
exposed to subparalytic doses eat much less than
untreated larvae, develop more slowly, gain less
weight, and pupate and emerge later than untreated
insects.
2.4.2. Block of Spontaneous Activity in
the Nervous System
RH-3421 and RH-1211, two highly active pyrazolines,
were used as model compounds in an electrophysiological
analysis of poisoned P. americana,
together with RH-5529, which was useful for
some experiments because of its ready reversibility
(Figure 1). The upper panel in Figure 5 compares
extracellular recordings from three parts of the
nervous system before and shortly after paralysis
with 5 mgg 1 RH-3421. The connectives between
the second and third thoracic ganglia are major
pathways for interneurons within the central nervous
system (CNS); the crural nerve is the major
leg nerve, with both sensory and motor traffic; and
the cercal nerve is exclusively sensory. In all cases,
the complete absence of neural activity upon dihydropyrazole
poisoning is striking. It has likewise
been shown that activity in the CNS connectives of
M. sexta larvae is completely blocked after paralysis
with indoxacarb or DCJW via injection (Wing et al.,
1998). The lower panel in Figure 5 shows recordings
from S. frugiperda larvae before and shortly after
paralysis with both DCJW and indoxacarb. In the
CNS connectives and also in the ventral nerve roots
of the abdominal ganglia, which carry motor and
sensory axons, there is no nerve activity in the paralyzed
insects. The assessment of the state of the
nervous system during poisoning is completed with
Figure 6, which shows that even in a cockroach
paralyzed for 24 h, tactile stimulation of the trochanter
could elicit sensory spikes in the crural
nerve that in turn initiate reflex motor activity in
the same nerve. This is a clear demonstration that
axonal conduction and synaptic transmission function
more or less normally in paralyzed insects; the
evident defect is that the nerves no longer generate
action potentials spontaneously.
Normally, even in a quiescent insect, there is
background or spontaneous action potential activity
in the nervous system arising from pacemaker
cells in the CNS and from tonic sensory receptors.
Mechanoreceptors exist in many places on the
cuticle to detect cuticular deformations or hair
Figure 5 SCBI poisoning inhibits all spike activity in the nervous system. Representative extracellular recordings from the central
nervous system (CNS) and peripheral nerves of paralyzed insects and untreated DMSO-injected controls. Adult male cockroaches
were treated by injection of 5 mgg 1 RH-3421 and fifth instar Spodoptera larvae weighing 500 mg were treated with 5 mgg 1 DCJW or
indoxacarb, 3–4 h before dissection of and recording from the paralyzed insects. (Cockroach data: reproduced with permission from
Salgado, V.L., 1990. Mode of action of insecticidal dihydropyrazoles: selective block of impulse generation in sensory nerves.
Pestic. Sci. 28, 389–411; ß Society of Chemical Industry, permission is granted by John Wiley & Sons Ltd. on behalf of the SCI.
Spodoptera data previously unpublished.)