microbial insectisides

7/26/2019 Microbial Insectisides

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MICROBIAL INSECTISIDES


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Why these???


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1. Bacillus thuringiensis


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Commonly known as Bt

A highly specific insecticidal bacterium

B. thuringiensissubsp. kurstaki & B.thuringiensissubsp.aizawai Used against caterpillars of the epidoptera as

butterflies ! moths

B. thuringiensis subsp.israelensis Used against "iptera as #imuliid blackfly $%ectorsfor ri&er blindness in Africa'( )ungus gnat lar&ae !some mos*uitoes $asAedes spp.)

B. thuringiensis subsp. san diego or B.t. vartenebrionis Used against some Coleoptera as Colorado potato

beetle.


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Commercially( a&ailable as powders

containing a mi+ture of dried spores !

to+in crystals

Applied to lea&es( etc where insect lar&ae

feed.

,enetic engineering of the to+in genes

into se&eral crop plants $&ia

Agrobacterium'.


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arget pests and crops

American Bollworm $!ellicoverpaarmigera'

-ink bollworm $"ectinophera species'

#potted bollworm $#rias insulana'

"iamond back moth $"lutela $ylostella)

Colorado potato beetle $%eptinotarsa

decemlineota %egetables( fruit( maie( small grain

cereals and forests( orchards .


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ode o' action

%egetati&e cells ha&e endosporesand crystals

of an insecticidal protein to+in.

/he crystals are aggregates of a large 0123042

k"a protein5 A protoxin to be acti&ated Under normal conditions( highly insoluble #o(

safe to humans( higher animals ! other insects.

#olubilised in reducing conditions when p6 78.95 /he condition in the mid3gut of lepidopteran

lar&ae.


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-roto+in is clea&ed by a gut protease to producean acti&e :2 k"a to+in5 Delta-endotoxin.

Binds to the midgut epithelial cells Creates pores in cell membranes ! leads to

e*uilibration of ions

,ut is rapidly immobilised ! the epithelial cells

lyse ar&ae stop feeding

,ut p6 is lowered by e*uilibration with the bloodp6.

ower p6 enables the bacterial spores togerminate

/he bacteria in&ade the host( causing a lethal

septicaemia.


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"elta3endoto+in has threedomains

Domain I:A bundle of ; 3sheets 3 Binds to receptors in thegut.

Domain III:A tightly packed >3

sandwich 3 -rotects the C3terminus end of the acti&e to+in(pre&enting further clea&age by gutproteases.


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Bt to$ins and their classi'ication

Bt produces ? types of to+in

Cr$crystal' to+ins( encoded by !r genes $7 92genes @@@@'

Ct$cytolytic' to+ins( to augment Cry to+ins


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(train development

Cry to+ins are encoded by genes on 93: different

plasmids of Bt

A sea o' combinations & ry to$ins * why?

-lasmids can be e+changed between Bt strains by aconugation3like process

Bt contains transposons $transposable genetic

elements that flank genes and that can be e+cised

from one part of the genome and inserted elsewhere'

#o( commercially( genetically engineered strains

with no&el to+in combinations


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Plants genetically engineered with Bt gene

,enetically engineering to contain the delta3endoto+in gene from Bt

Bt corn

Bt potato Bt cotton

Bt soybean

/he downside -erpetual e+posure of insects to to+ins

Creates a &ery strong selection pressure for thede&elopment of resistance to the to+ins.


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Ad&antages in e+pressing Bt to+ins in

transgenic Bt crops5

e&el of to+in e+pression can be &ery highthus deli&ering sufficient dosage to the pest

/o+in e+pression is contained within the plant

system and hence only those insects that

feed on the crop perish

/o+in e+pression can be modulated by using

tissue3specific promoters

eplaces the use of synthetic pesticides inthe en&ironment.


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+n an industrial scale

-roduced in controlled fermentor in deep

tanks of sterilied nutrient li*uid medium

Dndoto+ins ! li&ing spores are har&ested

as water dispersible li*uid concentrates for

subse*uent formulation.


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2. Bacillus sphaericus


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,ram3positi&e bacterium

Used primarily as a lar&icide

An obligate aerobe bacterium used as alar&icide for mos*uito control

)orms spherical endospores

Can be isolated from soil( leaf surfaces anda*uatic systems

-roduces a 022 k"a protein that acts as a

lar&icidal to+in.

6ighly effecti&e against the lar&a of the Eyeomyia

mos*uitoes( drastically reducing their population.


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Dffecti&e against ule$ spp.

ar&icides are more effecti&e and less

to+ic than adult mos*uito sprays

Unlikely to result in human e+posure


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ode o' action

B.sphaericusspores are eaten by

mos*uito lar&ae

/o+ins released into the mos*uitoFs gut

ar&ae stop eating

Dffecti&e against acti&ely feeding lar&ae(

and does not affect mos*uito pupae oradults.


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3. Bacillus popilliae


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,ram3negati&e spore3forming rod.

#pores of Bacillus popilliaeinfect lar&ae of

Gapanese beetles $"opillia ,aponica'


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#pores( residing in the soil are ingested bybeetle lar&ae

"ay ?5 ,erminate in lar&al gut.

"ay 1395 %egetati&e cells proliferate( attainingma+imum numbers.

"ay 93025 #ome penetrate the gut wall andgrows in the hemolymph

"ay 043?05 A few spores form lar&a de&elopsthe typical milky appearance.

6ost diesH


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#pores are ingested by Gapanese beetle lar&ae$grubs'

#pores become acti&e bacteria and multiply inthe grubs( which continue to li&e.

-re&ents lar&al maturation.

Ehen the lar&aeIs bacterial population reaches a

high enough density( bacterial spores arereleased to the soil to await ingestion by futurebeetle lar&ae.

=nfected beetle lar&ae die when the spores are

released. /hus( they greatly decrease the numbers of

grubs and adult beetles( thereby reducing plantdamage.


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#pores also infect lar&ae of some closely

related beetles


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Ad&antages %ery narrow host range $they are effecti&e

against Gapanese beetles( only'

Complete safety for man and other&ertebrates

Compatibility with other control agents

including chemical insecticides "isad&antages

6igh cost of production in vivo-

#low rate of action(

ack of effect on adult Gapanese beetles

Jeed for large areas to be treated for effect.

microbial insectisides

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