Insect Control: Biological and Synthetic Agents - Index of

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7 Bacillus thuringiensis: Mechanisms and Use A Bravo and M Soberón, Instituto de Biotechnologı´a, Cuernavaca Morelos, Mexico S S Gill, University of California, Riverside, CA, USA ß 2010, 2005 Elsevier B.V. All Rights Reserved 7.1. General Characteristics 247 7.2. Virulence Factors and the PlcR Regulon 248 7.3. Insecticidal Toxins 248 7.3.1. Classification and Nomenclature 248 7.3.2. Structure of Toxins 250 7.3.3. Evolution of Three-Domain Toxins 254 7.4. Mode of Action of Three-Domain Cry Toxins 255 7.4.1. Intoxication Syndrome of Cry Toxins 255 7.4.2. Solubilization and Proteolytic Activation 256 7.4.3. Receptor Identification 256 7.4.4. Toxin Binding Epitopes 258 7.4.5. Receptor Binding Epitopes 260 7.4.6. Cry Toxin–Receptor Binding Function in Toxicity 261 7.4.7. Toxin Insertion 262 7.4.8. Pore Formation 262 7.5. Synergism of Mosquitocidal Toxins 264 7.6. Genomics 265 7.6.1. Sequence of Plasmid pBtoxis 265 7.7. Mechanism of Insect Resistance 266 7.7.1. Proteolytic Activation 266 7.7.2. Receptor Binding 266 7.7.3. Oligosaccharide Synthesis 268 7.8. Applications of Cry Toxins 268 7.8.1. Forestry 268 7.8.2. Control of Mosquitoes and Blackflies 268 7.8.3. Transgenic Crops 269 7.9. Public Concerns on the Use of B. thuringiensis Crops 269 7.1. General Characteristics Bacillus thuringiensis is a member of the Bacillus cereus group that also includes B. cereus, B. anthracis, and B. mycoides (Helgason et al., 2000). The feature that distinguishes B. thuringiensis from the other members of the B. cereus group is its entomopathogenic properties. This bacterial species produces insecticidal proteins (d-endotoxins) during sporulation phase as parasporal inclusions, which predominantly comprise one or more proteins, called Cry and Cyt toxins. These protein toxins are highly selective to their target insect, are innocuous to humans, vertebrates, and plants, and are completely biodegradable. Therefore, B. thuringiensis is a viable alternative for the control of insect pests in agriculture and disease vectors of importance in human public health. Numerous B. thuringiensis strains have been isolated that show activity towards lepidopteran, dipteran, or coleopteran insects (Schnepf et al., 1998). In recent years B. thuringiensis strains active against Hymenoptera, Homoptera, Orthoptera, and Mallophaga insect orders and to other noninsect organisms like nematodes, mites, and protozoa have been isolated (Crickmore et al., 1998; Wei et al., 2003). The entomopathogenic activity of B. thuringiensis is mainly due to Cry toxins. One feature that distinguishes these Cry proteins is their high selectivity for their target insect. More than 200 different cry genes have been isolated, and this constitutes an important arsenal for the control of a wide variety of insect pests. In this chapter we will summarize the present knowledge of the pathogenic properties of B. thuringiensis,
248 7: Bacillus thuringiensis: Mechanisms and Use the mode of action of Cry toxins, and their application in agriculture and disease-vector control. 7.2. Virulence Factors and the PlcR Regulon The B. cereus group is characterized by its pathogenic capabilities to a diverse group of organism, such as mammals in the case of B. anthracis and B. cereus, and insects in the case of B. thuringiensis. The pathological characteristics of the different members of the B. cereus group are mainly due to the presence of specific toxin genes that for the most part encoded by large, self-transmissible extrachromosomal plasmids (Schnepf et al., 1998). However, other chromosomal encoded virulence factors contribute to the pathogenic effects of different members of the B. cereus group (Dubois and Dean, 1995; Lereclus et al., 1996; Agaisse et al., 1999; Guttman and Ellar, 2000). In the case of B. thuringiensis, it has been known for some time that the spores synergize the effects of Cry proteins (Dubois and Dean, 1995). This synergistic effect was shown to be due to the production of several virulence factors, such as a-exotoxins, b-exotoxins, chitinases, phospholipases, and enterotoxins (Lereclus et al., 1996; Agaisse et al., 1999; Guttman and Ellar, 2000). Mutation of genes encoding some of these virulence factors showed that they contribute to the pathogenicity of B. thuringiensis to insects (Lereclus et al., 1996; Fedhila et al., 2002). In the case of phosphatidylinositol specific phospholipase C (PI-PLC), disruption of the structural gene plcA diminished the synergistic effects of spores indicating that at least this virulence factor was necessary for an efficient virulence of the bacterium (Lereclus et al., 1996). This is also the case for the inhA2 genethatcodes for a zinc-requiring metalloprotease (Fedhila et al., 2002). Characterization of the mechanism controlling plcA gene expression led to the discovery of a pleiotropic transcriptional regulator, PlcR. Mutations in the plcR gene abolished plcA gene expression and the virulence of the bacterium (Lereclus et al., 1996; Agaisse et al., 1999). A genetic screening of PlcR regulated genes identified several extra cellular virulence factors including a secreted RNase, an Slayer protein, phospholipase C, and enterotoxins, indicating that the PlcR regulon includes a diverse set of virulence factors (Agaisse et al., 1999). Also, the inhA2 gene was shown to be under the control of PlcR (Fedhila et al., 2003). A proteome analysis of secreted proteins from B. cereus strain ATCC 14579 showed that disruption of the plcR gene reduced the levels at least 56 exported proteins (Gohar et al., 2002). Alignment of the promoter regions of the PlcR regulated genes identified a 20-nucleotide palindromic sequence (‘‘PlcR box’’) that is required for PlcR activation (Agaisse et al., 1999). Although, several PlcR regulated genes are also present in B. cereus and B. anthracis (Guttman and Ellar, 2000), only the B. cereus PlcR protein is functionally equivalent to that of B. thuringiensis, since the B. anthracis plcR gene is disrupted by a transposon-like sequence indicating that production of virulence factors in B. anthracis is blocked or may be different from that of B. thuringiensis and B. cereus (Agaisse et al., 1999). In fact, expression of a functional PlcR in B. anthracis leads to the activation of a large set of genes encoding enzymes and toxins that have protease, phospholipase, and hemolysis activities (Mignot et al., 2001). PlcR regulated genes are expressed at the end of the vegetative growth phase during the stationary phase of growth (Lereclus et al., 1996). PlcR expression is negatively regulated by the sporulation key factor Spo0A, indicating that this gene is only temporally expressed in vegetative cells before the onset of sporulation (Lereclus et al., 1996). The PlcR protein senses cell density and is, therefore, part of a quorum sensing mechanism (Slamti and Lereclus, 2002). The papR gene, which encodes a 48 amino acid peptide and is located downstream of the plcR gene, is activated by PlcR and the gene product is secreted. Outside the cell, PapR is degraded by extra cellular proteases and the C-terminus end pentapeptide is internalized to the bacterium cytoplasm by the oligopeptide permease Opp (Slamti and Lereclus, 2002). Inside the cell, the pentapeptide binds PlcR activating the capacity of PlcR to bind to the PlcR box sequence. This binding then induces the activation of PlcR specific gene expression (Slamti and Lereclus, 2002). It was also shown that the PlcR activating peptide is strain specific for the activation of the PlcR regulon. Single amino acid changes on the activating PapR peptide were enough for strainspecific induction of the PlcR regulon (Slamti and Lereclus, 2002). These results suggest that quorum sensing of related B. thuringiensis strains is restricted by the sequence homology of the C-terminal pentapeptide of PapR. Figure 1 shows the PlcR regulon network including the gene targets and the regulation of PlcR activation by PapR protein. 7.3. Insecticidal Toxins 7.3.1. Classification and Nomenclature The major determinants of B. thuringiensis insecticidal properties are the d-endotoxins (Schnepf et al., 1998). These endotoxins form two multigenic families, cry and cyt. Cry proteins are specifically toxic to orders of insects, Lepidoptera, Coleoptera,
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INSECT CONTROL BIOLOGICAL AND SYNTH
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INSECT CONTROL BIOLOGICAL AND SYNTH
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CONTENTS Preface vii Contributors i
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PREFACE When Elsevier published the
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J T Andaloro E. I. Du Pont de Nemou
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1 Pyrethroids B P S Khambay and P J
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activity. In contrast to most other
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Figure 1 Commercial and novel pyret
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Figure 2 Pyrethroids referred to in
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4-position result in almost complet
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and their primary site of action is
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Figure 4 Folded and extended confor
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aromatic rings or methyl groups by
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pyrethroids) and consequently a sec
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1998), although the precise mechani
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polymorphisms in the protein. Of th
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observed resistance to pyrethroids,
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propoxur against Culex quinquefasci
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esistance in house fly. Insect Bioc
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Shan, G.M., Hammock, B.D., 2001. De
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A1.4. Resistance The increasing num
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B A IIS4-S5 linker, IIS5 helix and
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2 Indoxacarb and the Sodium Channel
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Figure 2 Structures of pyrazoline-l
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observed that both indoxacarb and i
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deflections resulting from stresses
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Figure 8 Dihydropyrazole block appe
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potential conduction in the CNS of
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known to affect blocker affinity, w
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Figure 13 Diagrammatic representati
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that the probable mechanism of ovil
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conventional chemistries and spinos
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Clare, J.J., Tate, S.N., Nobbs, M.,
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Tsurubuchi, Y., Karasawa, A., Nagat
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R1 N N O N H indoxacarb. Thus, whil
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3 Neonicotinoid Insecticides P Jesc
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esidues, like the pyrid-3-ylmethyl
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containing neonicotinoids, and neon
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Studies were also done using compar
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Becke, 1988; Klamt, 1995) level of
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sampling using forcefield methods (
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Figure 9 Stable conformations and p
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Figure 13 Binding to putative catio
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Figure 14 Systemicity of neonicotin
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site (Kayser et al., 2002). Clothia
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Figure 20 Important pest insects ta
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Barley yellow dwarf virus vectors s
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investigate the mode of action of n
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within the desensitized state over
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the accumulation of this acid in th
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3.8.1. Safety Profile The introduct
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Table 7 Environmental profile of co
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substance is bound irreversibly to
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imidacloprid conferring a high leve
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nAChR are comparable to the efficac
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Figure 25 Flea control achieved wit
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and the resolution of FAD was teste
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Brown, J.K., Perring, T.M., Cooper,
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In: Ishaaya, I. (Ed.), Biochemical
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Léna, C., Changeux, J.-P., 1993. A
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patterns in Colorado potato beetle
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nach Saatgutbeizung. Pflanzenschutz
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Today, photoaffinity labeling (e.g.
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for control of stinkbugs in soybean
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Biochem. Physiol., doi: 10.1016/j.p
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4 Insect Growth- and Development-Di
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The molting process is initiated by
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Table 1 Bisacylhydrazine insecticid
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4.2.2. Bisacylhydrazines as Tools o
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to the three EcRs with either muris
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of action of ecdysone agonists was
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thus inducing effects and symptomol
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and microsomes. Subsequently, Willi
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dipteran insects, like the midge, C
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eetle (Exomala orientalis) when app
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metabolic fate studies showed the i
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y specific proteins in signaling pa
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their reduced risk for the environm
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can be reversed by applying JH. JH
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door for a more rational approach t
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apparent that it was a bHLH-PAS and
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Table 9 Continued Bemisia tabaci (s
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Table 11 Insect control with some a
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Table 13 Ecotoxicological profile o
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Flucycloxuron Nonsystemic acaricide
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The discovery of diflubenzuron spaw
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Table 15 Environmental effects of s
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ecessive (R male S female) manner,
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esistance management programs due t
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IPS Paraconfusus lanier (Coleoptera
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larvae parasitized by Microplitis r
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Kostyukovsky, M., Chen, B., Atsmi,
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Three-dimensional quantitative stru
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Riddiford, L.M., 1994. Cellular and
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characterization of resistant clone
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Biological Approaches to Pest Contr
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M389 L308 M272 T304 T393 V404 L420
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5 Azadirachtin, a Natural Product i
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and Hemiptera and was related to ef
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eported, but this is rather nonspec
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important source of pest control at
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effects with physiological effects
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eing associated with an accumulatio
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Page 216 and 217: which interact directly with azadir
Page 218 and 219: 6 The Spinosyns: Chemistry, Biochem
Page 220 and 221: Table 1 Structures of the spinosyns
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298 8: Mosquitocidal B. sphaericus:
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A8 Addendum: Bacillus sphaericus Ta
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310 A8: Addendum essential to devel
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314 9: Insecticidal Toxins from Pho
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A9 Addendum: Recent Advances in Pho
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330 A9: Addendum Lee, S.C., Stoilov
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332 10: Genetically Modified Baculo
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384 A10: Addendum cysteine protease
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388 11: Entomopathogenic Fungi and
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Zare, R., Gams, W., Culham, A., 200
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434 A11: Addendum although a number
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436 A11: Addendum Examination of ge
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440 12: Insect Transformation for U
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448 A12: Addendum genetic transform
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452 Subject Index Aphis gossypii (c
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454 Subject Index Bisacylhydrazine
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456 Subject Index Cry toxins (conti
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458 Subject Index Entomopathogenic
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460 Subject Index Homoptera molting
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462 Subject Index Kinoprene (ENSTAR
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464 Subject Index Muscle(s) sodium
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466 Subject Index Photorhabdus (con
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468 Subject Index Sodium channel bl
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470 Subject Index Toxocara cati, im
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