Entomology 10

Parasitoid wasp (Tetrastichus) ovipositing in leaf-cutting bee larva

Behavioral Ecology: Parasitoid Reproductive Strategies

I. Contrasts between predator, parasite, and parasitoid

II. Strategies for successful parasitization

1. Host finding
   1. Locate macro- and microenvironments of host
   2. Locate host
2. Host acceptance
   1. Host stage (age)
   2. Reject marked (parasitized) hosts [Sensillae on antennae & ovipositor]
3. Host availability
   1. Decreases with host age group
   2. Those on early larval stages
      • High fecundity (ovarioles/ovary, eggs stored)
      • High mobility
   3. Those on overwintering pupae (cocoons)
      • Lower fecundity & mobility
      • Better competitors
4. Host suitability
   1. Host defenses (encapsulation)
   2. Nutritional quality
   3. Competition due to multiple parasitoidism or superparasitoidism

III. Decisions by a foraging parasitoid

1. How many eggs to lay
2. What sex ratio to produce (proportion of males:females)

IV. Parasitoid life cycles

1. Parthenogenesis
   1. Facultative- Males from unfertilized eggs (Haplodiploidy)
   2. Obligate- No males
      • Fusion of polar body and egg nucleus
      • Incomplete reduction (meiotic) division
2. Endoparasitoid vs Ectoparasitoid
   1. Lay eggs inside host, larvae develop internally (Endoparasitoid)
   2. Lay eggs externally, larvae feed on outside of host (Ectoparasitoid)
3. Solitary vs Gregarious
   1. Female lays 1 egg per host (Solitary)
   2. Female lays >1 egg per host (Gregarious)
      • Polyembryony = Many larvae from one egg [Rare]
      • Monembryonic = Each egg gives rise to one larva [Most common]
4. Primary parasitoid vs Hyperparasitoid
   1. Larvae feed on non-parasitic host (Primary)
   2. Larvae feed on another parasitoid (Hyperparasitoid)
5. Multiple parasitoidism vs Superparasitoidism
   1. >1 species/host (Multiple parasitoidism) [Interspecific competition]
   2. >1 individual of same species/host (Superparasitoidism) [Intraspecific]

V. Clutch size

1. Solitary species
   1. Avoids intraspecific competition
2. Gregarious species
   1. Survivorship
      1. Host size
      2. Density dependent mortality
   2. Fecundity, Longevity, Searching ability
      1. Larger clutch, smaller size adults, lowered fitness
   3. Probability of female finding a host
      1. Low- large clutches
      2. High- small clutches

VI. Sex Ratio

1. Local Mate Competition
   1. Inbreeding vs Outcrossing
2. Host Quality
   1. Size of adult female solitary parasitoid is host related
   2. Fitness often size related

VII. Implications

1. Population Growth
   1. Depends on numbers of females produced
2. Biological Control
   1. Need sufficient parasitoid females to control host populations
   2. Mass-rearing: Need to produce large numbers of females for release

VIII. Case Histories

• Parasitoids of post-defecating larvae of alfalfa leafcutting bees
• All have facultative parthenogenesis (Haplodiploidy)

1. Leucospis [Leucospidae, Chalcidoidea, Hymenoptera]
   Solitary, Primary, Ectoparasitoid
2. Monodontomerus [Torymidae, Chalcidoidea, Hymenoptera]
   Gregarious, Primary, Ectoparasitoid
   ca 15 eggs/host; 1 male/5 females
3. Melittobia [Eulophidae, Chalcidoidea, Hymenoptera]
   Gregarious, Facultative Hyperparasitoid, Ectoparasitoid
   ca 60 eggs/host: 1 male/20 females
4. Tetrastichus [Eulophidae, Chalcidoidea, Hymenoptera]
   Gregarious, Primary, Endoparasitoid
   ca 160 eggs/host; 1 male/20+ females

Readings in Gullan, P.J. and Cranston, P. S. (1994). The Insects: An Outline of Entomology:

• Read: pp. 332-333 [12.2.2]; pp. 334-340 [12.3-12.3.2].

Return to the Course Syllabus