ENTM Lec 13 Herbivory

what is the a long history of insects & plants

• insects & plants share associations that date back over 300 million years

• major radiation of insects coincided with the evolution & radiation of angiosperms (flowering plant) 100 million years ago

• ~ 35-50% of extant insects species are herbivorous (Phytophagous)

what is phytopagy

Herbivorous insects vary in the number of plant taxa that they will feed on:

• Polyphagous insects are generalists that feed on many plant groups

  • E.g. aphids, grasshoppers

• Oligophagous insects feed on a few

  • E.g. catapillas (feed on miilkweeds,

• Monophagous insects feed on one

  • E.g. silkworms

what are some examples of plants they eat

roots, zylom (cicadas), seeds, fruits, rosegal (living plant tissue), bark,

what are some relationships of plants & herbiovores

Reciprocal interactions between plants and herbivores over evolutionary time have shaped the plants and herbivorous insects we observe today (i.e. coevolution)

(include info about ehrlich & raven later)

what are some examples of plant defenses

• constitutiove defnese - always present

• induced defense - synthrnsized or otherwise acivated in response to injury

what are some induced defences

• when injury occurs:

  • up- regulation of genes for producing semiochemicals (&/or other defences)

  • down regulation of genes for photosynthesis & other metabolic processes

• methods for detecting injury vary

  • e.g salivary enzymes of herbivore increases rate of release of secondary compounds

what is the relationship of constitutive & induced defences

what are some mechanical constitutive defences

• wazy epidermal cuticles

• lead toughness

  • impedes feeding due to high levels of indigestible fiber & lignin

• trichomes

  • hairs

  • scales

  • glandular

• (tree bark as well)

what are some mechanical induced defences

• leaf drop & color change

  • defences against leaf miners, gall formers

  • undermine camouflagr

• ‘pitiching out’

  • a plant stem / tree trunk is a pressured tube & when bugs eat the bark, the tree ‘pitches’ out fluid killing or getting the bugs stuck.

what are some plant chemical defenses

• occasionallly primary metabolites:

  • amino acids, sugars, nucleotides, lipids

• e.g. High sugar content in fruits can deter feeding in some insects

• secondary metabolites

  • diverse array of compounds

  • often metabolic by products

  • found in related groups of species

  • e.g glucsinolates are only present in brassicaceae

• modes of action

  • feeding & oviposition deterents

  • toxicants

  • attractants (smell, color, taste for pollinators, parasitoids

  • antimicrobial activity

some examples of secondary metabolite

difference in ‘volatile’ in secondary metabolites

• non volatile terpenes like limonene can be distasteful to herbivores

• volatile terpenes like menthol warns herbivores that the plant is toxic to them before herbivore feeding commences

what are phytoecysones in secondary metabolites

• plant steroids that are chemically similar to insects molting hormones

• when ingested, can interfere with or inhibit molting

how to herbivorous insects avoid dying

• morphological adaptations

  • eg acorn weevil

    • they will rasp a hole in an acorn & insert an egg

• behavioral adaptations

  • eat parts of leaves to avoid defensive compounds

    • eg trenching

• detoxification

  • Detoxification enzymes

    • e.g. Depressaria pastinacella, the parsnip webworm, and Pastinaca sativa, the wild parsnip. Webworm has ability to metabolize furanocoumarins (Berenbaum and Zangerl 1998)

  • Saliva

    • Injected into plant tissues to reduce production of or break down induced compounds

    • E.g. corn earworm caterpillars feeding on tobacco plants (Musser et al. 2002)

• genetic resistance

  • There are 18 species of insect that feed on leaf tissues containing cardenolide poisons (e.g. milkweed plants)

  • Cardenolides bind to proteins pumps that animal cells constantly use impairing normal cell function

  • All insects examined (from 4 different orders) had an amino acid substitution in the gene that codes for the pump, providing resistance

  • These genetic substitutions had to have occurred completely independently at least 4 different times, in the 4 different orders

• sequestration

  • storage of toxic/distasteful secondary plant metabolites in tissues

  • eg turnip sawflies, milkweed bugs, and monarch butterflies all sequester cardenolides

    • Distasteful to predators

    • Reduce competition

    • Many also possess aposematic (warning) coloration

• (populations level responses?)

what are some population level responses

what are gall insects

• highly specialized herbivary

• galls result from specialized plant - insects interactions in which the morphology of plant parts is altered by the insects

  • provide food and/or protection for the inhabitants

• ~13,000 species of gall forming insects known

  • hemiptera - aphids, coccoids, psyllids

  • diptera - gall midges, tephritidae

  • hymentoptera - cynipoidea, tenthredinoidea

  • lepidoptera

what are the 2 processes of gall formation

• Initiation – galls can only be stimulated in living plant tissue, generally occurs on new growth

• Growth and maintenance – continued stimulation is required to maintain the nutritive tissues in the gall. Gall growth ceases when the gall insect completes it’s development

• Relatively little is known about the mechanisms of gall induction and growth

  • May involve wounds, plant hormones, secretions and foreign microorganisms