Upload
doandung
View
217
Download
0
Embed Size (px)
Citation preview
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