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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Japanese beetle larvae killed by heterorhabditid
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
Japanese beetle larvae killed by heterorhabditid
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
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