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Taxonomy and Biology of Insect Pathogens Read Ch 6
Insect PathogensBacteriaVirusesFungiNematodesProtozoa
Vertebrate viruses
Bacteria
Bacillus thuringiensis isolatesBacillus sphaericusPaenibacillus popilliaeSerratia entomophila
Bacillus thuringiensis isolates
kurstaki- against caterpillars
tenebrionis- against scarab and chrysomelid larvae
israelensis- against mosquito and blackfly larvae
Here we see a caterpillar killed by Bacillus thuringiensis (top), compared to a healthy caterpillar (bottom).
P. rapaefeeding and frass
Bacillus thuringiensis cells contain a toxin crystal, a spore for passing unfavorable conditions, and the genome.
P. rapaefeeding and frass
Scarabeid larvae infected (on right) with Paenibacillus popilliae vs. a normal grub (right), the cause of milky spore disease in scarabs
P. rapaefeeding and frass
Fungi
Fungi Imperfecti- such as species ofBeauveria, Metarhizium, Verticillium, Hirsutella, Ashersonia
Entomophthorales, such asEntomophaga maimaiga
In a petri dish with high relative humidity, fungi such a Beauveria bassiana are highly infective to many insects
Mycelia extending from a thrips killed by Beauveria bassiana
Spores of Beauveria bassiana are the applied stage
Some Aschersonia fungi turn their whitefly hosts red
Viruses
Baculoviruses– are specialized viruses that only attack Arthropods
No other insect virus group is manipulated for biological control
Gypsy moth virus is a typical baculovirus (NPV)
Virus-killed caterpillars showtypical head down position, allowingvirus to drip from cadaver onto foliage
Codling moth virus is 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
Protozoa
Microsporidia– are are debilitating protozoa that attack many Arthropods
Important contaminants in importation projects
Microsporidia (Nosema sp.) spores in midgut of cabbage looper (Trichoplusia ni)
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
Japanese beetle larvae killed by heterorhabditidnematodes (note red color of cadaver)
stylet
Viral pathogens of vertebrates
few vertebrates have been targeted for classical biological control
examples are rabbits, mice, catspathogens employed have been
viruses or internal metazoan parasites
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 density stylet
Night hunting of feral cats on uninhabited sea islands complements use
of pathogens
Myxomatosis virus was released in Australia and Europe in the 1950s for rabbit suppression. In the 1990, another
virus (calicivirus) was released to combat resistance.
rabbit index
Biology of Insect Pathogens
Contact with new hosts Host penetrationReproduction in hostEscape from old hostsComplex vs. simple life cycles
Host Contact
At the end of one generation, pathogen propagules will be released back into the environment
The new pathogen generation begins when these propagules contact a new host
Host contact- gypsy moth larvae congregating under burlap spread virus from larva to larva.
Called horizontal transmission
Horizontal transmission
Japanese beetle larvae killed by heterorhabditidnematodes (note red color of cadaver)
stylet
Vertical transmission
stylet
Musca domestica female on left is infected with nematode; note swollen abdomen (nematodes in ovaries).
Fly on right is uninfected.
Infected fly Normal fly
Swollen abdomen
Effect of nematode (Paraiotonchium muscadomesticae) on ovaries of house flies
Ovary of healthy fly Ovary of infected fly
Nematodes (Paraiotonchium muscadomesticae) emerging from infected house fly ovary
Host Pentration
Once propagules have physically contacted the host, they must cross the integument and reach tissues subject to infection
Mode of action of Bacillus thuringiensis
Shape of Bt toxin protein
Fungi contact hosts when spores land on cuticle. Spores germinate and penetration hyphae push through cuticle
spore
Germination tube (= penetration hypha)
Penetration hyphae use enzymes to chemically digest cuticle and then hydrostatic pressure to break through
Cuticle being broken
Micrograph of cross section through integument of Diprion similis being infected by Entomopthora tenthredinidinis
Outside of insect
inside
Oospores encyst on contact as first step in host penetration
Coelomomyces dodgei in cuticle of mosquito larva (Anopheles quadrimaculatus)
Encysted oospores-purple
Germ tubes from oospore cysts penetrate host
cyst
Germ tube
Host integument
Nematodes penetrate host integument with a stylet
Coelomomyces dodgei in cuticle of mosquito larva (Anopheles quadrimaculatus)
Encysted oospores-purple
stylet
Cross section of insect integument, showing channel formed by nematode stylet
Channel of stylet
Reproduction in host
After host penetration, pathogens must reproduce to be successful
Some pathogens kill hosts and then reproduce (nematodes)
Others reproduce in living hosts (virus, fungi)
Virus reproduction requires living host cells. Baculoviruses reproduce in nuclei.
Channel of stylet
Cross section of insect tissue showing baculovirus stained red and clearly localized inside cell nuclei
Channel of styletSymbiotic bacteria
Steinernematid and heterorhabditid nematodes reproduce in dead host tissues. Symbiotic bacteria carried in gut of
nematodes kill the host.
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
Channel of styletConidospores on exit hyphaeHyphae
growingout of cadaver
Outline of host cadaver
Channel of stylet
Moldy appearance of dead caterpillar is caused by 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
Channel of stylet
Outside insect
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
Channel of stylet
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
Entomophthora sp. spore halo around a dead Plutella larva. Halo is shaped by discharge radius of spore ejection.
Channel of stylet
Underwater zoospore discharge by water molds
Zoospore discharge tubes in fungus-killed mosquito larva
Discharge tubes
cadaver
water
Mermithid (Romanomermis culicivora) nematodes wiggle free of dying hosts and swim away
Emerging mermithid worm
Channel of stylet
Baculoviruses exit hosts when cadavers liquefy and drip virus onto foliage below
Douglas fir tussock moth larvae killed by NPV
Steinernematid and heterorabditid nematodes swim away from decomposing host cadaver in soil water
cadaver
Channel of stylet
Before exiting the host cell baculoviruses must get “dressed” for the weather. Viruses get coated by protein
and form occlusion bodies that provide uv protection
Douglas fir tussock moth larvae killed by NPV
Channel of stylet
Reproduction may be based on simple
or complex life cycles. This
Coelomomycetes fungus requires
two hosts, a mosquito larva
and a copepod, to complete its life
cycle.
The parasitic worm Romanomermis culivorax requires only one host, but also has free living stages
Channel of stylet