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Taxonomy and Biology of Insect

Pathogens Read Ch 6

Insect Pathogens

Bacteria (esp. Bacillus thuringiensis)

Viruses (esp. baculoviruses)

Fungi (several species of imperfect fungi

and microsporidia)

Nematodes (two main families)

Vertebrate viruses

Bacteria

Bacillus thuringiensis isolates

Bacillus sphaericus

Paenibacillus popilliae

Serratia 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. rapae

feeding and

frass

Bacillus thuringiensis cells contain a toxin crystal, a spore for passing

unfavorable conditions, and the genome

P. rapae

feeding and

frass

White grub infected with Paenibacillus popilliae (right), the cause of

milky spore disease vs. a normal grub (left)

P. rapae

feeding and

frass

Fungi

Fungi Imperfecti- such as species of

Beauveria, Metarhizium, Verticillium,

Hirsutella, Ashersonia

Entomophthorales- such as

Entomophaga maimaiga

Microsporidia - Nosema

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

Entomophaga maimaiga- a

successful classical

biocontrol agent for the

gypsy moth

n

Year

1995 2000 2005 2010

% I

nfe

ction

0

20

40

60

80

100

No.

egg m

asses /

ha

0

1000

2000

3000

4000

5000

6000

% E. maimaiga infection

egg masses/ha

Courtesy of Anne Hajek

Microsporidia

once thought to be protists, are

now considered fungi

they cause debilitating infections

in many Arthropods

they are often important

contaminants in lab colonies, such as in

mass rearings or quarantine colonies

for classical biological control

Microsporidia (Nosema sp.) spores in midgut

of cabbage looper (Trichoplusia ni)

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 show

typical head down

position, allowing

virus 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

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

stylet

Japanese beetle larvae killed by heterorhabditid

nematodes (note red color of cadaver)

stylet

Viral pathogens of vertebrates

few vertebrates have been targeted

for 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

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

1. Contact with new hosts

2. Host penetration

3. Reproduction in host

4. Escape from old hosts

5. Complex vs. simple life cycles

Step 1: 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 heterorhabditid

nematodes (note red color of cadaver)

stylet

Vertical transmission

Sirex noctilio, major pest of pines in Southern Hemisphere, for which

a nematode transmitted in the eggs is an effective control

Effect of nematode (Deladenus siricidicola )

on ovaries of Sirex noctilio

Ovary of

healthy

Sirex

Ovary of

Infected

Sirex

Healthy eggs (left)

vs collapsed eggs

with nematodes

inside (left)

Step 2: Host Penetration

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 of water molds encyst on contact with their

hosts, such as this mosquito, to begin 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 into the host by using their stylet to

cut a hole in the integument

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

Step 3: 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 stylet Symbiotic bacteria

Steinernematid and heterorhabditid nematodes reproduce

in dead host tissues. Symbiotic bacteria carried in gut of

nematodes kill the host.

Step 4: 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 stylet

Conidospores

on exit hyphae Hyphae

growing

out 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

Coryceps cf. kyusuensis

Active discharge of spores

Spore halo around dead Plutella larva

Channel of stylet

Underwater zoospore discharge by water molds

Zoospore discharge tubes in fungus-killed mosquito larva

Discharge

tubes

cadaver

water

Swim away

Steinernematid and heterorabditid nematodes swim away

from decomposing host cadaver in soil water

cadaver

Swim away

Mermithid (Romanomermis culicivora) nematodes

wiggle free of dying hosts and swim away

Emerging mermithid worm

Channel of stylet

Drip down

Baculoviruses exit hosts when cadavers liquefy and drip

virus onto foliage below

Douglas fir tussock moth larvae killed by NPV

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