Biology of Insect Pathogens...Mycelia inside insect Hyphae crossing integument For some fungi, exit...

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