Insect Control: Biological and Synthetic Agents - Index of

220 6: The Spinosyns: Chemistry, Biochemistry, Mode of Action, and Resistance
Table 6 Dependence of various effects in Periplaneta americana on aqueous concentration of spinosyn A
Aqueous
concentration (nM) Symptoms a
Increased CNS
efferent activity a (%)
The earliest symptoms are due to prolonged involuntary
muscle contractions that subtly alter the posture
of the insect. This is evident in cockroaches
and houseflies, where the first symptom is a lowering
of the head and elevation of the tail, caused by
involuntary straightening of the hindlegs. In the
second phase of poisoning, the posture changes
become so severe that the insects topple over and
cannot right themselves, becoming prostrate. At
this point, there are widespread fine tremors in all
muscles, which sometimes can only be seen under a
microscope. In hard-bodied insects, all appendages
tremble constantly, and in soft-bodied insects such
as caterpillars, the skin appears to crawl. In the last
phase, the movements cease and the insects are
paralyzed. The symptoms will be described in more
detail for cockroaches, fruit flies, and tobacco
budworm larvae.
The 24 h injection LD50 (median lethal dose) of
spinosyn A was 0.74 (0.41–1.34) mgg 1 for adult
male P. americana. The first noticeable symptom
was elevation of the body, due to depression of
the coxae, extension of the legs, and flexion of the
tarsae (Figure 6b). In this first poisoning phase, the
cockroaches still walked around, and, if disturbed
by touching, would resume a normal posture temporarily.
These symptoms even persisted after the
insect was decapitated (Figure 6c). As poisoning
progressed to phase 2, the cockroaches became
more uncoordinated and fell on their backs, as a
result of asymmetric extension of the legs. The tarsae
of the prostrate insects remained strongly flexed,
and the insects showed tremors, which were not
seen before prostration (Figure 6d). At the lowest
effective dose, 0.625 mgg 1 , prostration occurred
approximately 24 h after injection, whereas at
5 mgg 1 it occurred within 4 h. The insects also
Activation of nicotinic
acetylcholine receptor
(nAChN) b (%)
1 None 0 4 >50
5 None 0 28 100
10 None 6 35 100
21 Difficulty righting 38 45
30 54 52
60 Prostration 68 65
100 80 73
1000 100 94
a Symptoms and increased CNS afferent activity from Salgado et al. (1998).
b Activation of nAChN from Salgado and Saar (2004).
c Inhibition of small-neuron GABA-R from Watson (2001).
Inhibition of small-neuron g-aminobutynic
acid receptor (GABA-R) c (%)
showed other symptoms of excitation, such as
wing beating and abdominal bloating resulting
from their swallowing large amounts of air. The
prostrate cockroaches eventually ceased trembling
as they entered the third or quiescent poisoning
phase, during which movement could still be elicited
by a disturbance, but grew gradually weaker. The
legs gradually relaxed as the insects became paralyzed,
and no further movement could be elicited.
No recovery was observed.
Adult male fruit flies, D. melanogaster, exposed
to spinosyn A in sugar water, exhibited a characteristic
set of sublethal symptoms. The 24 h LC50
(median lethal concentration) was 8.0 (5.7–12.1)
ppm Initially, flies were able to remain upright and
even walk around, but most had difficulty maintaining
normal posture. When standing still, the tibiae
slowly flexed and the tarsae extended, causing the
legs to pull together and the body to rise. Approximately
every 5 s, the fly compensated by spreading
its legs into a normal stance, after which the process
was repeated, with alternate slow pulling together,
followed by rapid spreading, of the legs. While this
effect occurred in nearly all flies at this concentration,
some flies also held the wings abnormally.
Normally, the wings are folded over the back, but
in many poisoned flies they were either open straight
out or folded down.
Heliothis virescens larvae prostrated by spinosyn A
typically curled up due to abdominal flexion and
lay on their sides, exhibiting widespread fine tremors.
The true legs were extended and trembling,
as were the mouthparts and even the surface of
the soft cuticle. These excitatory effects were not
obvious to the naked eye, but were easily seen with
a stereomicroscope. Diuresis was also a common
symptom of spinosyn A poisoning in H. virescens