insect pathogen 2

8/8/2019 Insect Pathogen 2

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Taxonomy and Biology of Insect

PathogensRead Ch 6


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Insect Pathogens Bacteria

Viruses

Fungi

Nematodes

Protozoa

Vertebrate viruses


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Bacteria

Bacillus thuringiensis isolates

Bacillus sphaericus

Paenibacillus popilliae

Serratia entomophila


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Bacillus thuringiensis isolates

kurstaki- against caterpillars

tenebrionis- against scarab and chrysomelid larvae

israelensis- against mosquito and blackfly larvae


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Here we see a caterpillar killed byBacillus thuringiensis (top),compared to a healthy caterpillar (bottom).

P. rapae

feeding and

frass


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Bacillus thuringiensis cells contain a toxin crystal, a spore for

passing unfavorable conditions, and the genome.

P. rapae

feeding and

frass


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Scarabeid larvae infected (on right) withPaenibacillus popilliae vs. a

normal grub (right), the cause of milky spore disease in scarabs

P. rapae

feeding and

frass


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Fungi

Fungi Imperfecti- such as species ofBeauveria, Metarhizium, Verticillium,Hirsutella, Ashersonia

Entomophthorales, such as

Entomophaga maimaiga

I i di h i h hi h l i h idi f i h i


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In a petri dish with high relative humidity, fungi such aBeauveria

bassiana are highly infective to many insects

M li t di f th i kill d b B i b i


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Mycelia extending from a thrips killed byBeauveria bassiana

S f B i b i th li d t


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Spores ofBeauveria bassiana are the applied stage

S A h i f i t th i hit fl h t d


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Some Aschersonia fungi turn their whitefly hosts red


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Viruses

Baculoviruses are specializedviruses that only attack Arthropods

No other insect virus group is

manipulated for biological control

G th i i t i l b l i (NPV)


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Gypsy moth virus is a typical baculovirus (NPV)

Virus-killedcaterpillars show

typical head down

position, allowingvirus to drip from

cadaver onto foliage


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Codling moth virusis a granulosis type

virus

Here, we see a cell

with viral bodies

inside the nucleus

Virus

bodies

Young codling moth larva killed by granulosis virus


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Young codling moth larva killed by granulosis virus


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Protozoa

Microsporidia are aredebilitating protozoa that attack

many Arthropods

Important contaminants in

importation projects

Microsporidia (Nosema sp ) spores in midgut


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Microsporidia (Nosema sp.) spores in midgut

of cabbage looper (Trichoplusia ni)


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Nematodes

Many families of truly parasitic nematodes

(e.g., Mermithidae and others) exist and are

part of natural control

Nematodes in two families

Steinernematidae and Heterorhabditidae

are massed reared as biopesticides

Infective juvenile nematode


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Infective juvenile nematode

Japanese beetle larvae killed by heterorhabditid


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nematodes (note red color of cadaver)

stylet


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Viral pathogens of vertebrates

few vertebrates have been targetedfor classical biological control

examples are rabbits, mice, cats

pathogens employed have been

viruses or internal metazoan parasites

Feral cats on uninhabited sea islands with seabird colonies


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Feral cats on uninhabited sea islands with seabird colonies

are severe ecological pests. Feline leukemia was released

on Marian Island, South Africa, to reduce cat densitystylet


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Night hunting of feral

cats on uninhabited sea

islands complements useof pathogens

Myxomatosis virus was released in Australia and Europe


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y p

in the 1950s for rabbit suppression. In the 1990, another

virus (calicivirus) was released to combat resistance.

rabbit index

Bi l f I t P th


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Biology of Insect Pathogens

Contact with new hosts

Host penetration

Reproduction in host

Escape from old hosts

Complex vs. simple life cycles


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Host Contact

At the end of one generation,

pathogen propagules will be

released back into the environment The new pathogen generation

begins when these propagulescontact a new host

Host contact- gypsy moth larvae congregating under


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gyp y g g g

burlap spread virus from larva to larva.

Called horizontal transmission

H i t l t i i


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Horizontal transmission


Vertical transmission


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nematodes (note red color of cadaver)

stylet

a a

Musca domestica female on left is infected with nematode;


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stylet

note swollen abdomen (nematodes in ovaries).

Fly on right is uninfected.

Infected flyNormal fly

Swollen

abdomen

Effect of nematode (Paraiotonchium muscadomesticae)


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on ovaries of house flies

Ovary of healthy flyOvary of

infected fly

Nematodes (Paraiotonchium muscadomesticae)


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emerging from infected house fly ovary


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Host Pentration

Once propagules have physically

contacted the host, they must cross

the integument and reach tissues

subject to infection


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Mode of action ofBacillus thuringiensis


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Shape of Bt toxin protein

Fungi contact hosts when spores land on cuticle. Spores

i t d t ti h h h th h ti l


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germinate and penetration hyphae push through cuticle

spore

Germination tube

(= penetration hypha)

Penetration hyphae use enzymes to chemically digest

ti l d th h d t ti t b k th h


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cuticle and then hydrostatic pressure to break through

Cuticle being broken

Micrograph of cross section through integument

ofDiprion similis being infected byEntomopthora

tenthredinidinis

Outside of insect

inside

Oospores encyst on contact as first step in host penetration


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Coelomomyces dodgei in cuticle of mosquito larva (Anopheles quadrimaculatus)

Encysted oospores-purple

Germ tubes from oospore cysts penetrate host


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cyst

Germ tube

Host integument

Nematodes penetrate host integument with a stylet


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Coelomomyces dodgei in cuticle of mosquito larva (Anopheles quadrimaculatus)

Encysted oospores-purple

stylet

Cross section of insect integument, showing channel

formed by nematode stylet


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formed by nematode stylet

Channel of stylet


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Reproduction in host

After host penetration, pathogens

must reproduce to be successful

Some pathogens kill hosts andthen reproduce (nematodes)

Others reproduce in living hosts(virus, fungi)

Virus reproduction requires living host cells.

Baculoviruses reproduce in nuclei


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Baculoviruses reproduce in nuclei.

Channel of stylet

Cross section of insect tissue showing baculovirus stainedred and clearly localized inside cell nuclei

Steinernematid and heterorhabditid nematodes reproduce

in dead host tissues Symbiotic bacteria carried in gut of


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Channel of styletSymbiotic bacteria

in dead host tissues. Symbiotic bacteria carried in gut of

nematodes kill the host.


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Exiting the host

After reproducing, most pathogens (except

vertically transmitted species) must

physically leave the host, enter the

environment, disperse and find new hosts

Mechanisms for exit, dispersal and

persistence outside of the host are critical in

pathogen success

Fungi exit hosts through hyphal growth and production of

special spores that become airborne


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special spores that become airborne

Channel of styletConidospores

on exit hyphaeHyphae

growing

out of cadaver

Outline of

host cadaver

Moldy appearance of dead caterpillar is caused by

overgrowth of outside of body by exit hyphae, produced by


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Channel of stylet

overgrowth of outside of body by exit hyphae, produced by

the mycelium inside of the cadaver

Here, we see a spruce budworm larva killed by the fungus

Zoopthora radicans

Cross section of insect body wall, showing fungal hyphae

growing through cuticle


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g g g

Channel of stylet

Outsideinsect

Hyphae emerged through cuticle to air

Mycelia inside insect

Hyphae crossing

integument

For some fungi, exit hyphae combine to form larger

structures. Here, the horns on this dead leafhopper


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, pp

Channel of stylet

The role of the exit hyphae is to provide a means of

ejecting spores (conidiospores) into the air. Here, we see a


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Channel of stylet

j g p ( p ) ,

Entomophthora sp. spore halo around a deadPlutella

larva. Halo is shaped by discharge radius of spore ejection.

Underwater zoospore discharge by water molds


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Channel of stylet

Zoospore discharge tubes in fungus-killed mosquito larva

Discharge

tubes

cadaver

water

Mermithid (Romanomermis culicivora) nematodes

wiggle free of dying hosts and swim away


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Emerging mermithid worm

Baculoviruses exit hosts when cadavers liquefy and dripvirus onto foliage below


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Channel of stylet

Douglas fir tussock moth larvae killed by NPV

Steinernematid and heterorabditid nematodes swim awayfrom decomposing host cadaver in soil water


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cadaver

Before exiting the host cell baculoviruses must getdressed for the weather. Viruses get coated by protein


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Channel of stylet

and form occlusion bodies that provide uv protection

Douglas fir tussock moth larvae killed by NPV

Reproduction maybe based on simple


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Channel of stylet

or complex life

cycles. This

Coelomomycetes

fungus requirestwo hosts, a

mosquito larva

and a copepod, tocomplete its life

cycle.

The parasitic wormRomanomermis culivorax requiresonly one host, but also has free living stages


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Channel of stylet

insect pathogen 2

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