Insect Pathogens. Vertebrate viruses

Transcription

1 Taxonomy and Biology of Insect Pathogens Read Ch 6

2 Insect Pathogens Bacteria (esp. Bacillus thuringiensis) Viruses (esp. baculoviruses) Fungi (several species of imperfect fungi and microsporidia) Nematodes (two main families) Vertebrate viruses

3 Bacteria Bacillus thuringiensis isolates Bacillus sphaericus Paenibacillus popilliae Serratia entomophila

4 Bacillus thuringiensis isolates kurstaki- against caterpillars tenebrionis- against scarab and chrysomelid larvae israelensis- against mosquito and blackfly larvae

5 Here we see a caterpillar killed by Bacillus thuringiensis (top), compared to a healthy caterpillar (bottom). P. rapae feeding and frass

6 Bacillus thuringiensis cells contain a toxin crystal, a spore for passing unfavorable conditions, and the genome P. rapae feeding and frass

7 White grub infected with Paenibacillus popilliae (right), the cause of milky spore disease vs. a normal grub (left) P. rapae feeding and frass

8 Fungi Fungi Imperfecti- such as species of Beauveria, Metarhizium, Verticillium, Hirsutella, Ashersonia Entomophthorales- such as Entomophaga maimaiga Microsporidia - Nosema

9 In a petri dish with high relative humidity, fungi such a Beauveria bassiana are highly infective to many insects

10 Mycelia extending from a thrips killed by Beauveria bassiana

11 Spores of Beauveria bassiana are the applied stage

12 Some Aschersonia fungi turn their whitefly hosts red

13 Entomophaga maimaiga- a successful classical biocontrol agent for the gypsy moth

14 6000 n 100 No. egg masses / ha % Infection 0 Courtesy of Anne Hajek Year % E. maimaiga infection egg masses/ha

15 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

16 Microsporidia (Nosema sp.) spores in midgut of cabbage looper (Trichoplusia ni)

17 Viruses Baculoviruses are specialized viruses that only attack Arthropods No other insect virus group is manipulated for biological control

18 Gypsy moth virus is a typical baculovirus (NPV) Virus-killed caterpillars show typical head down position, allowing virus to drip from cadaver onto foliage

19 Virus bodies Codling moth virus is a granulosis type virus Here, we see a cell with viral bodies inside the nucleus

20 Young codling moth larva killed by granulosis virus

21 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

22 Infective juvenile nematode stylet

23 Japanese beetle larvae killed by heterorhabditid nematodes (note red color of cadaver) stylet

24 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

25 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

26 Night hunting of feral cats on uninhabited sea islands complements use of pathogens

27 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

28 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

29 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

30 Host contact- gypsy moth larvae congregating under burlap spread virus from larva to larva. Called horizontal transmission

31 Horizontal transmission

32 Japanese beetle Vertical larvae transmission killed by heterorhabditid nematodes (note red color of cadaver) stylet

33 Sirex noctilio, major pest of pines in Southern Hemisphere, for which a nematode transmitted in the eggs is an effective control

34 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)

35 Step 2: Host Penetration Once propagules have physically contacted the host, they must cross the integument and reach tissues subject to infection

36 Mode of action of Bacillus thuringiensis

37 Shape of Bt toxin protein

38 Fungi contact hosts when spores land on cuticle. Spores germinate and penetration hyphae push through cuticle spore Germination tube (= penetration hypha)

39 Penetration hyphae use enzymes to chemically digest cuticle and then hydrostatic pressure to break through Outside of insect Cuticle being broken inside Micrograph of cross section through integument of Diprion similis being infected by Entomopthora tenthredinidinis

40 Oospores of water molds encyst on contact with their hosts, such as this mosquito, to begin host penetration Encysted oospores-purple Coelomomyces dodgei in cuticle of mosquito larva (Anopheles quadrimaculatus)

41 Germ tubes from oospore cysts penetrate host cyst Host integument Germ tube

42 Nematodes penetrate into the host by using their stylet to cut a hole in the integument stylet Encysted oospores-purple Coelomomyces dodgei in cuticle of mosquito larva (Anopheles quadrimaculatus)

43 Cross section of insect integument, showing channel formed by nematode stylet Channel of stylet

44 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)

45 Virus reproduction requires living host cells. Baculoviruses reproduce in nuclei. Cross section of insect tissue showing baculovirus stained red and clearly localized inside cell nuclei Channel of stylet

46 Steinernematid and heterorhabditid nematodes reproduce in dead host tissues. Symbiotic bacteria carried in gut of nematodes kill the host. Symbiotic bacteria Channel of stylet

47 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

48 Fungi exit hosts through hyphal growth and production of special spores that become airborne Hyphae growing out of cadaver Channel of stylet Conidospores on exit hyphae Outline of host cadaver

49 Moldy appearance of dead caterpillar is caused by overgrowth of outside of body by exit hyphae, produced by the mycelium inside of the cadaver Channel of stylet Here, we see a spruce budworm larva killed by the fungus Zoopthora radicans

50 Cross section of insect body wall, showing fungal hyphae growing through cuticle Mycelia inside insect Channel of stylet Hyphae crossing integument Outside insect Hyphae emerged through cuticle to air

51 For some fungi, exit hyphae combine to form larger structures. Here, the horns on this dead leafhopper Coryceps cf. kyusuensis

52 Active discharge of spores Spore halo around dead Plutella larva

53 Underwater zoospore discharge by water molds Zoospore discharge tubes in fungus-killed mosquito larva water Discharge tubes Channel of stylet cadaver

54 Swim away Steinernematid and heterorabditid nematodes swim away from decomposing host cadaver in soil water cadaver

55 Swim away Mermithid (Romanomermis culicivora) nematodes wiggle free of dying hosts and swim away Emerging mermithid worm

56 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

57 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