University of Michigan Biological Station

Biology 442 - Biology of Insects

Lecture Notes - Herbivory

1. Herbivory - eating plants
   1. Types of herbivory
      1. Chewing - skeletonizers, miners, tiers, rollers, folders, borers, external feeders
      2. Sucking - Homoptera, Hemiptera
      3. Gall formers - Diptera, Hymenoptera
   2. Coevolution with respect to herbivory - 1st popularized by Ehrlich and Raven 1964 with butterflies and plant families.
      1. See tight association between groups. e.g. Monarchs and Milkweeds, Heliconius and Passiflora.
      2. Diffuse coevolution - suites of species influencing each other.
   3. Plant adaptations
      1. Physical - toughness, spines, hairs
      2. Chemical - nutritional differences, secondary chemicals (alkaloids, terpenoids, phenols)
   4. Insect adaptations
      1. Physical - mouthparts, size, orientation of mouthparts (miners)
      2. Chemical - detoxification (MFO's), sequestration, secretion of growth factors (galls)
      3. Behavioral - Leaf miners that mine through veins or in a circle; leaf feeders that clip veins or leaf; avoidance of sun in insects feeding on photosensitizing plants.
   5. Example - Monarchs and milkweeds, black swallowtails and carrots, pharmacophagy
2. Apparency Theory - 1st major idea to explain patterns of herbivory.
   1. Feeny in 1970's.
   2. Apparency of plant depends on size, generation time, chemical characters.
   3. 2 types of defenses depending on apparency.
      1. Quatitative - found in apparent plants; dose dependent; function pre-digestively, e.g. tannins; generalist herbivores probably more prominent.
      2. Qualitative - found in unapparent plants; not dose dependent (at least not at low levels in which they are found); act as toxin; specialist herbivores prominent.
3. Resource Concentration Hypothesis - defense depends on quality of environment.
   1. 1980's - Coley.
   2. High resource environment leads to lower defense because parts are less expensive (easy to replace).
   3. Low resource environment leads to higher defense levels because parts are valuable (difficult to replace; there is a lower limit to this where plants will stop spending energy on defense).
4. Plants eating insects.
   1. Evolved several times.
   2. In this area we have 4 genera; Sarracenia (pitcher plant), Drosera (Sundew), Dionea (venus' flytray), Utricularia (Bladderwort).
   3. Often associated with low nitrogen environments.

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