Insect Control: Biological and Synthetic Agents - Index of

larvae. The injection LD50 of spinosyn A against
50–70 mg H. virescens larvae was 0.23 (0.1–0.52)
mg g 1 (Salgado, 1998).
In summary, one of the first symptoms of spinosyn
poisoning in all insects is leg extension, due to
involuntary muscle contractions. Later in poisoning,
all muscles appear to be affected.
6.4.2. Gross Electrophysiology
6: The Spinosyns: Chemistry, Biochemistry, Mode of Action, and Resistance 221
Figure 6 Typical symptoms shown by American cockroaches (Periplaneta americana) after injection with spinosyn A, in this case at
2 mg. (a) Control. (b) Depression of the legs elevates the body; note how all leg joints are extended. (c) The symptoms persist even
after decapitation, and even worsen, as the whole body flexes. (d) Even after prostration, the tarsae remain strongly flexed, as seen
in this decapitated insect. (Reprinted with permission from Salgado, V.L., 1998. Studies on the mode of action of spinosad: insect
symptoms and physiological correlates. Pestic. Biochem. Physiol. 60, 91–102; ß Elsevier.)
6.4.2.1. Spinosyns excite the central nervous system
in vivo and in vitro Although many muscles are
activated during spinosyn poisoning, tarsal flexion
was an excellent model because it is one of the first
symptoms to appear and persists throughout poisoning,
even when the poisoned insect is decapitated
and pinned on its back, allowing its physiological
basis to be easily studied. This symptom is caused by
contraction of the tarsal flexor muscle, located in
the tibia (Snodgrass, 1952). The onset of spinosyn-
A-induced tarsal flexion was accompanied by continual
firing of the tarsal flexor muscle (Figure 7a).
In control insects, the muscle was relaxed except for
activity during sporadic movements (not shown).
Cutting all contralateral nerve roots did not diminish
flexion, and when all ipsilateral nerves except
nerve 5, which innervates the tarsal flexor muscle,
were cut, the tarsus still remained flexed, and the
firing of the flexor muscle continued (Figure 7b).
However, immediately upon severing nerve 5, the
tarsus relaxed and the muscle recording became
silent (not shown). Therefore, the smallest unit exhibiting
the flexion was the leg and its ganglion attached
through nerve 5, and it was concluded that
spinosyn-induced tarsal flexion was caused by activation
of the flexor motor neuron, driven by events
occurring either within that neuron or elsewhere
within that ganglion (Salgado, 1998).
Nerve recordings made from various nerves
showed that central nervous system (CNS) and
motor activity was generally increased in spinosyn A
poisoned cockroaches, concomitant with appearance
of excitatory symptoms and persisting even after
paralysis. Alternatively, it could be demonstrated
that the excitatory effects of spinosyn A were specifically
on the CNS; there were no direct effects on
axonal conduction, sensory receptor function, or
neuromuscular function (Salgado, 1998).
The next step in analyzing the etiology of spinosyninduced
hyperactivity was to determine whether spinosyns
could directly increase nerve activity when
applied to isolated ganglia. Figure 8 shows a ratemeter
recording of the activity in a segmental nerve
of an isolated larval housefly synganglion. In the
control and before application of spinosyn A, the
activity varied widely. Following treatment with
100 nM spinosyn A, the swings in activity became