Temporal Patterns in Populat ion Dynamics:
I Int roduction
II Populat ion cycles
III Chaos
IV Synchrony of Populat ion Fluc tuations
Plant-herbivore interactions
“an inordinate fondness for beetles” -Haldane
Interactions can be classified by the effect
they have on the species involved
General types of interactions and their
outcomes
Type
Consequence
partner 1
partner 2
competition
-
-
parasitism / predation*
-
+
mutualism*
+
+
commensalism?
0
+
amensalism?
0
-
* Directly relevant to plant-insect interactions
? May occur in plant-insect interactions
Lifestyles of herbivores:
Monophagous, single food type SPECIALIST
Oligophagous, few food types SPECIALIST
Polyphagous, many food types GENERALIST
What Do Plants Provide?
• Fats - low except seeds and some fruits
• Proteins - < 2% of biomass
– Amino acid balance different than animals’
• Carbohydrates - mostly indigestible
– cellulose, hemicellulose, in cell wall
• Vitamins and Minerals
• Defensive Chemicals and Structures
• All nutrients generally at much lower tissue
concentrations in plants than in their consumers
Generalists
Specialists
Pros
- Predictable defenses
-Easier decision-making
- Finding mates?
- Many food options
- Flexibility to avoid
predators
Cons
- Variable host quality
- Host-finding decisions
- Finding mates?
- Finding the right food
- Specific plant defenses
- Avoiding predators
- Competition for food?
90% of insect herbivores are specialized
1% of mammal herbivores are specialized
How do herbivores choose their food?
Generalists
1. Nutrient constraints
2. Detoxification limitations
Specialists
1. Endosymbionts
2. Detoxification enzymes
There are many ways to exploit a plant…..
Types of plant feeding
Leaf herbivory: most commonly studied
Root herbivory
Nectar robbing
Florivory
Types of plant feeding
Seed predation
Galls, stem borers
Types of plant defenses to herbivory
Constituative- defenses that are always
present regardless of the presence of herbivory
Induced- defenses that are only produced when there is
feeding by an herbivore
Physical- trichomes, spines, thornes
Chemical- plant compounds derived from primary
metabolism
Tolerance - some plants may be adapted to withstand
some degree of attack without fitness loss
Really cool plant defense examples
latex
Really cool plant defense examples
Bersera squirts toxic resin when fed upon -- terpenes under pressure
Blepharida has
evolved a behavior to
overcome this
defense
Squirt up to 2 meters!
Beetle trenches- takes 1hour to poke tiny holes in the mid-veins
to release toxic resin
After draining- the beetle takes 15 minutes to consume the leaf
Really cool plant defense examples
Indirect defenses are plant traits that attract predators and parasitoids
of herbivores which facilitate top-down control.
Examples:
constituative
Ant-acacia
Mutualism (+/+)
induced
Jasmonic acid and methyl
jasmonate (MeJA) induce
volitiles to attract natural
enemies of the plant
Beet armywormpest of cotton
Parasitoid of armyworm
Really cool plant defense examples
Plant communication??
Rick Karban – University of California, Davis
Levels of defense
chemicals in tobacco
near damaged or
undamaged sage
brush
Proportion tobacco
plants damaged
near clipped and
unclipped sagebrush
Proportion tobacco
plants damaged
near clipped and
unclipped sagebrush
Herbivore offense: behavioral
Feeding choices
- Larval:
Diet selection- chemoreceptors
Supplemental food
Diet mixing
- Adult: oviposition
Trenching
Herbivore offense: Physiological traits
Enzymes: reduce effects of plant compounds salivary enzymes
- inhibit induced plant defenses
metabolic enzymes
- e.g., metabolize nonprotein amino acids
Sequestration: incorporate plant defenses into herbivores’ tissues
to gain protection from predators/parasites
Symbionts: allow digestion of cellulose, produce missing nutrients
vector diseases to weaken plant defense
How well do plant defenses work and are herbivores
successful at overcoming plant defenses?
Evaluation of the effect of induced chemical defenses on plants
Wild radish (Raphanus sativus), insect herbivores, and
induced defenses by Anurag A. Agrawal
Experimental design
3 treatments:
control = no defense
clipped = simulated herbivory
induced= defended by chemicals
noctuid moth
flea beetle
aphids
Results:
Increased levels of mustard oil glycosides in induced plants
Significantly higher leaf damage on control plants
Female fitness in induced plants was over 60% higher than controls
sampling date
treatment
Costs to plant to maintain defenses
Frequency of leaf response (squirting)
Insects and plants affect each other’s fitness
Parsnip webworm and wild parsnip
May Berenbaum
Parsnip webworm and wild parsnip
Parsnip established in NA in 1609 by European colonists
Webworm invaded in 1869
In Europe (native range for
both species) two
furanocoumarins, sphondin
and bergapten defend
parsnip from herbivory
Once in NA without insect,
parsnip evolved lower
levels of furanocoumarins
(herbarium specimens
studied)
After introduction of
webworms to NA,
furanocoumarin levels
increased
# of plants examined
Evolution in furanocoumarin levels possible:
Furanocoumarins produced by plant have heritable variation
Plants with higher furanocoumarin levels produce more offspring
Evolution in insects possible:
Furanocoumarin detoxification enzymes in insects have heritable variation
Insects with higher levels of enzymes produce more offspring
Plant defenses as selective agents
Defenses include:
mechanical traits (thorns, spines, latex)
digestibility reducers
toxins
recruitment of predators/parasitoids
Conditions for defenses to exert selection on herbivore
traits: (when you might expect evolutionary changes in the
herbivore in direct response to the plant)
- Defenses reduce herbivore fitness
- Herbivores have genetically based traits to
overcome defenses that improve herbivore fitness
-These traits are heritable and show variation
How do plant defenses reduce
herbivore fitness?
Direct effects:
- Death from mechanical defense, toxins
-Slow growth, reduced fecundity from
digestibility reducers
- Starvation/dehydration from deterrence due
to toxins
Indirect effects:
- Increased ‘window of opportunity’ for parasitoids
- Greater detectability by predators or parasitoids
Outcome: decrease in herbivore fecundity or number of
offspring they produce
Possible outcome of the reciprocal
selection pressure exerted on plants
and herbivores
Coevolutionary arms race
Attack --> Defense --> Counterattack
-Strong selection on plants to not be eaten because
herbivory reduces plant fitness
-Drives selection on herbivore ability to overcome defense
because plant defenses reduce herbivore fitness
-Initiates new round of plant defense
- May lead to diversity of plant defenses and specialization
between plants and their herbivores
Consequences:
Increased diversity of plants and herbivores
Hessian fly, Mayetiola destructor
The most destructive pest of wheat worldwide
Control: 27 resistance genes have been developed for wheat
Different populations of flies have evolved to overcome these
defenses
Deployment of new resistance genes in a population of wheat
are only effective against the fly for 8-10 years
Sometimes a single amino acid change enables resistance in the fly
Adaptive radiation of beetles on flowering plants
http://evolution.berkeley.edu/evosite/evo101/VIIB1bBeetles.shtml
Brian Farrell (1998)
-once beetle lineages switched to angiosperms, some of them
diversified into lineages that specialize, feeding on different
parts of the plant (root, seed, leaf, etc.)
Beetles
Coevolution of mutualism (+/+)
Star orchid
Long spur forces moths to pollinate flower
Moths evolve longer probocis to feed effectively/ efficiently
Plant evolves longer spur
Hawkmoth
Interested in plant-insect interaction courses/ research?
Lynn Adler
lsadler@ent.umass.edu
http://www.people.umass.edu/lsadler
ENTOMOL 597A: Insect-Plant Interactions