Insect Control: Biological and Synthetic Agents - Index of
Insect Control: Biological and Synthetic Agents - Index of
Insect Control: Biological and Synthetic Agents - Index of
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observed that both indoxacarb <strong>and</strong> its activated<br />
metabolite were similarly potent in intoxicating<br />
Lepidoptera (Wing et al., 1998; Tsurubuchi et al.,<br />
2001). This again argues for efficient bioactivation<br />
in this order <strong>of</strong> insects.<br />
Indoxacarb bioactivation was also studied in<br />
the hemipteran cotton pest, the tarnished plantbug<br />
(Lygus lineolaris), compared to its hemipteran predator,<br />
big-eyed bug (Geochoris punctipes) (Tillman<br />
et al., 2001). These studies again demonstrate that<br />
the onset <strong>of</strong> neurotoxic symptoms in both bugs was<br />
well correlated with the appearance <strong>of</strong> the activated<br />
metabolite DCMP (albeit at slower rates than observed<br />
in Lepidoptera). The importance <strong>of</strong> oral uptake<br />
as a major route <strong>of</strong> bioactivation/intoxication,<br />
compared to tarsal absorption <strong>of</strong> residues or<br />
through dermal uptake, was clearly evident. Interestingly,<br />
laboratory studies showed both pest <strong>and</strong><br />
predator to be similarly sensitive to indoxacarb,<br />
although Geochoris could tolerate higher levels <strong>of</strong><br />
indoxacarb before showing signs <strong>of</strong> intoxication.<br />
However, in treated cotton fields, Lygus has been<br />
found to be more sensitive to indoxacarb treatment<br />
while Geochoris has been relatively refractory; further<br />
studies showed that this was largely due to<br />
infield behavior <strong>and</strong> the preference <strong>of</strong> Geochoris<br />
to feed on pest insects <strong>and</strong> their eggs as opposed to<br />
foliage, which significantly slowed oral indoxacarb<br />
uptake (Tillman et al., 2001).<br />
In addition, all current evidence points to a SCBI<br />
structural requirement for an underivatized urea linkage<br />
to be present to exert strong, quasi-irreversible<br />
insect Na þ channel blocking effects, as opposed to<br />
derivatized ureas.<br />
2.3.2. Catabolism <strong>of</strong> Indoxacarb <strong>and</strong><br />
Other Na + Channel Blocker <strong>Insect</strong>icides<br />
Relatively little is known about the metabolic breakdown<br />
<strong>of</strong> indoxacarb or other SCBIs in insects. On<br />
balance, it is clear that bioactivation is occurring<br />
much more rapidly than degradation after field<br />
application, when satisfactory insect control is<br />
observed. However, like any other novel insecticide,<br />
resistance management will be a key to maintaining<br />
the agricultural utility <strong>of</strong> this insecticide class.<br />
Indeed, several field populations <strong>of</strong> North American<br />
oblique-b<strong>and</strong>ed leafroller (Choristoneura rosaceana)<br />
have shown a degree <strong>of</strong> insensitivity to indoxacarb,<br />
which is unique for Lepidoptera (Ahmad et al., 2002).<br />
Though this insect has never been exposed to indoxacarb<br />
for commercial control in the field, it has been<br />
treated with several other insecticide classes, including<br />
a number <strong>of</strong> organophosphates. Current studies<br />
indicate that resistant strains <strong>of</strong> this leafroller<br />
maybeabletocatabolizeindoxacarbmorerapidly<br />
2: Indoxacarb <strong>and</strong> the Sodium Channel Blocker <strong>Insect</strong>icides 39<br />
than susceptible strains; the specific mechanisms<br />
<strong>and</strong> metabolites are currently being characterized<br />
(Hollingworth et al., unpublished data).<br />
Interestingly, indoxacarb is apparently degraded<br />
primarily by higher animals via routes including<br />
cytochrome p450 mediated attack <strong>of</strong> the indanone<br />
<strong>and</strong> oxadiazine rings, while N-decarbomethoxylation<br />
is a relatively minor pathway (EPA, 2000; Scott,<br />
2000). The rapid metabolic degradation is a critical<br />
factor responsible for the high nontarget animal safety<br />
<strong>of</strong> indoxacarb.<br />
2.4. Physiology <strong>and</strong> Biochemistry <strong>of</strong><br />
the Na + Channel Blockers<br />
2.4.1. Symptoms <strong>of</strong> SCBI Poisoning<br />
in <strong>Insect</strong>s: Pseudoparalysis<br />
Pyrazolines <strong>and</strong> indoxacarb produce identical acute<br />
neurotoxic symptoms in the American cockroach<br />
Periplaneta americana, progressing through initial<br />
incoordination (5–20 min after 1–10 mg injection),<br />
then tremors <strong>and</strong> prostration, <strong>and</strong> finally a distinctive<br />
pseudoparalysis, so named because the apparently<br />
paralyzed insects produce violent convulsions<br />
when disturbed. This pseudoparalysis was maintained<br />
for 3–4 days, after which the ability to<br />
move when disturbed waned. By 4–6 days, the<br />
insects could be considered dead. Lower doses<br />
caused a similar progression <strong>of</strong> symptoms, but the<br />
time before appearance <strong>of</strong> symptoms was delayed.<br />
This unusual pseudoparalysis was key to unraveling<br />
the complex mode <strong>of</strong> action <strong>of</strong> this family in insects.<br />
Irreversible pseudoparalysis was also observed in<br />
lepidopterous larvae, with disturbance during the<br />
pseudoparalyzed state leading to squirming, <strong>and</strong><br />
tremors <strong>of</strong> legs <strong>and</strong> m<strong>and</strong>ibles evident upon microscopic<br />
examination. Higher doses <strong>of</strong> pyrazolines,<br />
especially in lepidopterous larvae, led within a few<br />
hours to flaccid paralysis (Salgado, 1990). Injection<br />
<strong>of</strong> 10 mgg 1 indoxacarb into fifth instar M<strong>and</strong>uca<br />
sexta larvae produced excitatory neurotoxic symptoms<br />
leading to convulsions <strong>and</strong> then relatively<br />
rapidly to flaccid paralysis (Wing et al., 1998). As<br />
discussed above, indoxacarb itself is converted<br />
in vivo to DCMP by esterase-like enzymes. The pseudoparalytic<br />
symptoms caused by indoxacarb have<br />
also been clearly observed in Spodoptera frugiperda<br />
larvae treated by injection <strong>of</strong> 20 mgg 1 indoxacarb or<br />
DCJW, <strong>and</strong> also in male P. americana injected with 1,<br />
3, or 10 mg g 1 DCJW (Salgado, unpublished data).<br />
Under field conditions, after ingesting or being<br />
directly sprayed with indoxacarb, insects will<br />
irreversibly stop feeding within a few minutes to 4 h;<br />
higher doses cause more rapid onset <strong>of</strong> symptoms.