Types of Parasites

Insects as Parasites:

Exploiting other animals.

Insects as Hosts:

Being exploited.

Insects as Vectors:

Carrying someone else’s parasite.

What counts as parasitism?

Resource exploitation that reduces host fitness.

Microparasites:

Brief feeding on many hosts.

Conventional Parasite:

Prolonged exploitation, host death is not required.

Parasitic Castrator:

Host survives but reproduction is suppressed.

Parasitoid:

Immature stage develops on/in one host and always kills it.

What are the filters to success?

Encounter (30%) → Acceptance/Entry (20%) → Establishment (50%) → Development/Reproduction (70%).

Encounter:

Habitat-habitat location and host location, find the habitat first (plant volatiles), then the host (frass, silk, contact chemicals, vibrations).

Acceptance/Entry:

Host acceptance and oviposition or invasion, encountered =/ accepted =/ suitable, acceptance is behavioral (examine, probe), suitability is physiological.

Establishment:

Survival in a physiologically compatible host, parasite must avoid detection and attack, parasitoid eggs must avoid encapsulation.

Development/Reproduction:

Completion of development and production of viable offspring.

What happens if a parasite fails one of the filters?

The interaction ends, each filter is a point where the host can defend itself.

What is ectoparasitism?

Living at the host surface.

Ectoparasitism - Fleas:

Laterally flattened body and backward-directed spines for moving through fur.

Ectoparasitism - Feather Lice:

Preening constraints establishment and host switching.

Ectoparasitism - Bed Bugs:

Temporary ectoparasites, emerging nocturnally for a blood meal.

Ectoparasitism - Botflies:

Lays eggs in wound, larvae infest tissue.

What is endoparasitism?

Living inside the host.

How are parasitoids ecologically important?

They are major regulators of herbivore populations and the basis of classical biological control.

What decisions determine the payoff of consuming a host (a finite meal) in development?

Host fate (does the host stop developing when attacked or keep growing) and brood size (does one parasitoid or a brood share the host).

Idiobiont:

Host paralyzed or arrested at attack, the meal is fixed.

Idiobiont Solitary (one adult per host):

Spider wasps (Pompilidae, e.g., Pepsis).

Idiobiont Gregarious (several adults per host):

Habrobracon hebetor on stored-product moth larvae.

Koinobiont:

Host keeps feeding and growing, the meal grows.

Koinobiont Solitary:

Aphidius ervi on pea aphid.

Koinobiont Gregarious:

Cotesia glomerata on cabbage white caterpillars.

What are entomopathogenic fungi?

Specialized parasitic microorganisms that infect, kill, and regulate arthropod populations.

How do entomopathogenic fungi enter the host?

Through the cuticle, not by ingestion, which is unusual among pathogens.

What is the typical sequence of entomopathogenic fungi?

Fungal spore adheres → germinates → penetration structures develop → mechanical force and enzymes breach the cuticle → fungus proliferates internally → host dies, under suitable conditions fungus emerges and sporulates.

Entomophthora - flies:

Parasitic fungus that infects adult flies, airborne spore lands on fly and pierces its exoskeleton, causes “summit disease” (forces fly to climb as high as possible on wall, stem, or blade of grass before sunset), fly assumes a specific position and fungal spores erupt, also attracts male flies, increasing spore spread.

Massospora - cicadas:

Specialized fungus that infects 13-and-17 year periodical cicadas, replaces lower abdomen and reproductive organs with white plug of spores while keeping host alive.

Insect Defense - Behavioral and Physical Barriers:

Before entry: Avoidance, grooming, cuticle, and gut epithelium.

Insect Defense - Humoral Immunity:

After entry: Antimicrobial peptides, prophenoloxidase cascade, clotting.

Insect Defense - Cellular Immunity:

Hemocytes performing phagocytosis, nodulation (many microbes), encapsulation (large targets like parasitoid eggs).

Resistance:

Reduces parasite establishment or burden.

Tolerance:

Reduces damage at a given parasite burden.

Why does resistance vs tolerance matter?

Different evolutionary consequences, resistance reduces parasite fitness and can drive antagonistic coevolution, tolerance does not, and can even raise parasite prevalence.

Resistant and Tolerant:

A tolerant and resistant variety will cope with attack and limit PCN multiplication.

Tolerant and Susceptible:

A tolerant but susceptible variety may cope with attack but will not limit PCN multiplication.

Intolerant and Resistant:

An intolerant and resistant variety is vulnerable to attack but will limit PCN multiplication.

Intolerant and Susceptible:

An intolerant and susceptible variety is vulnerable to attack and will not limit PCN multiplication.

Defense Provided by Symbionts:

Pea aphid + Hamiltonella defensa + APSA phage → protection against the parasitoid Aphidius ervi.

What is a vector?

An organism that transmits a pathogen between hosts.