PATHOGEN:

Definition:

- a specific cause of a disease.
- a microorganism capable of producing disease under normal conditions of host resistance and rarely living in close association with the host without producing disease.
- any microorganism, virus, substance or factor causing disease.
- in terms of biological control, we are interested in those agents which we can manipulate and cause diseases in pest organisms.
- these agents include:

a. viruses, b. bacteria,  c. fungi, d. protozoa,  e. nematodes* (not microorganisms).

Characteristics

1)   may or may not require insect host for survival.
2) many have to be ingested (contaminated substrate) or encounter insect by chance.
3) requires several days to a week or more to kill host.
4) younger insects usually more susceptible.

VIRUSES

- obligate intracellular parasites.

- virtually every kind of virus that has been isolated in all other organisms (with the exceptions of the putative slow viruses) have been isolated from insects.

 HANDOUT A --Table 1

- as you might guess insect viruses which are similar to viruses which cause disease in humans or things precious to humans are not really considered for mass production and spraying.                                                                                                                                         See Handout

- however, one group of viruses which appear to be restricted to Arthropods:  these are the Baculoviridae.

- Baculoviridae include two groups:

i.  nucleopolyhedroviruses  (NPVs)
ii. granuloviruses  (GVs)

- the vast majority of the NPVs and GVs have been isolated from caterpillars;  (butterflies and moths); in fact GVs are restricted to the Lepidoptera.

- factors which separate these viruses from all other viruses are:

 i. the presence of an occluding matrix which surrounds the virions which contain double-stranded circular DNA.
ii. the replicative strategy in which two different virion phenotypes are generated.

 Replicative Strategy                                             see Handout

- NPVs infect after oral ingestion; can eventually infect many tissues.
- diseased insects often climb to highest part of plant before dying -- aids in   dispersal of pathogen.
- viruses are inactivated by UV portion of sunlight; remain viable in the soil for many  years.

Methods of Transmission (natural conditions)

1) dying/dead insects - dispersed by wind and rain; other mechanical contamination.
2) diseased insects - eaten by birds; pass through bird.
3) parasitoids are capable of transmission from diseased to healthy insect.

Comment: length of time for virus to kill its host.

- Why?

- Is this acceptable to the grower?->-> genetic engineering.
- narrow host range is another aspect of these viruses which has inspired the genetic engineers.
- latest success is to insert mite paralysis toxins.
- a baculovirus, isolated a field population of insects has been patented by the U.S. Patent Office.

BACTERIA

- bacteria common component of the insect environment.
- most diseased-looking insects are full of bacteria but often these are opportunistic infections-i.e. the bacteria are not the primary cause of the disease.
- isolating these bacteria will not lead to an effective control agent.

- insects are protected from bacteria by:

i.  cuticle/integument.
ii.  peritrophic membrane.
iii. immune system.

- loss of integrity in (i). and/or (ii) leaves only the immune system to protect insect -- effective against a whole range of bacteria -- stress reduces the effectiveness of the immune system.

- lab rearing/production facilities -- stress -- predatory mites infected.

- some bacteria have the capacity to overcome insect defenses.
- both Bacillus popilliae and B. thuringiensis have been studied and used extensively  against insect pests (Lepidoptera, Coleoptera and Diptera).

- Bt is being researched by dozens of companies; currently over40,000 "strains" have been isolated (or created).
- Bt is a spore-forming bacterium that also carries a plasmid encoded toxin; the toxin forms crystals within sporangium.
- the toxin is largely responsible for the effectiveness of this organism as a control agent but may be responsible for its downfall as well.
- Bt induced pathology is dependent in part on the species infected.

 Lepidoptera

TYPE   I    immediate gut paralysis, general paralysis ∂-endotoxin alone,
death in hours (even without bacteria).

TYPE  II    immediate gut paralysis, starvation, no general paralysis
death in days with bacteremia. (most leps).

TYPE III    requires crystals and spores.   
death in days.

TYPE IV   not susceptible to toxin.

-for species affected by toxin alone, the application of Bt is similar to a convential chemical insecticide.
-don't need to spray viable spores, just toxin.

QUESTION: Is this biological control?

-the effectiveness of Bt has led to its widespread use and a host of genetic engineering projects.

for example:

1) toxin has been placed in different bacteria - bacteria which are found in a different habitat. e.g. rhizosphere.
2) place toxin directly into plants --defoliators exposed upon ingestion of plant  material--cotton, tomato and tobacco.
3) toxin gene has been put into baculovirus genome.

QUESTION: Do you think there are any risks or potential problems with this type of Bt use?

                  = RESISTANCE!!!!

FUNGI

- many kinds of fungi cause disease in insects.
- some have wide host ranges (Metarhizium anisopliae 200 hosts known; another example- Beauvaria spp.) while others (Entomophora) can be quite specific, e. g. Entomophora maimaiga - gypsy moth - change due to recent publication.
- most transmission occurs by spores and entry occurs by entry through the                integument.                                                                                        see Handout

- disease cycle.
- proteinases and chitinases digest cuticle, however surface conditions critical for spore germination.
- a recent report that ingestion of the spores can produce an infection.

- recent work in Alberta.                                           see Handout

COMMENTS ON PATHOGENS

- increasing interest in the use of pathogens for managing insect pests in forestry and agriculture as well as in urban settings.

- there is a great need to understand how pathogens work with respect to:

(1) host range.
(2) transmission in nature.
(3) maintain viability for longer periods.
(4) improved production.

- probably will work best when integrated with other management tools-                       Integrated Pest Management

- unfortunately a great rush to commercialize - before the systems are completely understood.

- repeat same mistakes:

(1) resistance.
(2) competition - commercialization - reduces free exchange of information.
(3) patents - issued for virus, why not for parasitoid?

- microbial control refers to the use of natural enemies to reduce the populations of an insect pests--however
- microbes have or will be used in a variety of systems, including:

(1) plant pathogens have been used to control weeds.
(2) benign microorganisms used as competitors of plant pathogens.
(3) pathogens used to reduce animal (insect) populations.