Temporal Patterns in Population Dynamics: I Introduction II Population cycles III Chaos IV Synchrony of Population Fluctuations

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 Consequencepartner 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 tissueconcentrations in plants than in their consumers

SpecialistsGeneralists

- Variable host quality- Host-finding decisions- Finding mates?

- Finding the right food- Specific plant defenses- Avoiding predators- Competition for food?

Pros

Cons

- Many food options- Flexibility to avoid predators

- Predictable defenses-Easier decision-making- Finding mates?

90% of insect herbivores are specialized1% of mammal herbivores are specialized

How do herbivores choose their food?

Generalists1. Nutrient constraints2. Detoxification limitations

Specialists1. Endosymbionts2. Detoxification enzymes

There are many ways to exploit a plant…..

Types of plant feeding

Leaf herbivory: most commonly studied

Root herbivory

Florivory

Nectar robbing

Galls, stem borers

Seed predation

Types of plant feeding

Types of plant defenses to herbivory

Constituative- defenses that are always present regardless of the presence of herbivory

Physical- trichomes, spines, thornes

Chemical- plant compounds derived from primary metabolism

Induced- defenses that are only produced when there is feeding by an herbivore

Tolerance - some plants may be adapted to withstand some degree of attack without fitness loss

Really cool plant defense examples

latex

Bersera squirts toxic resin when fed upon -- terpenes under pressure

Blepharida has evolved a behavior to overcome this defense

Really cool plant defense examples

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:

Ant-acacia Mutualism (+/+)

Jasmonic acid and methyl jasmonate (MeJA) induce volitiles to attract natural enemies of the plant

constituative

Beet armyworm-pest of cotton

Parasitoid of armyworm

induced

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

Feeding choices- Larval:

Diet selection- chemoreceptorsSupplemental foodDiet mixing

- Adult: ovipositionTrenching

Herbivore offense: behavioral

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 nutrientsvector diseases to weaken plant defense

How well do plant defenses work and are herbivores successful at overcoming plant defenses?

by Anurag A. Agrawal

Wild radish (Raphanus sativus), insect herbivores, and induced defenses

Experimental design

3 treatments: control = no defense clipped = simulated herbivory induced= defended by chemicals

noctuid mothflea beetle

aphids

Evaluation of the effect of induced chemical defenses on plants

sampling date treatment

Results:

Female fitness in induced plants was over 60% higher than controls

Increased levels of mustard oil glycosides in induced plants

Significantly higher leaf damage on control plants

Costs to plant to maintain defenses

Frequency of leaf response (squirting)

Parsnip webworm and wild parsnip

May Berenbaum

Insects and plants affect each other’s fitness

Parsnip webworm and wild parsnip

Parsnip established in NA in 1609 by European colonistsWebworm 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

Furanocoumarins produced by plant have heritable variation

Plants with higher furanocoumarin levels produce more offspring

Furanocoumarin detoxification enzymes in insects have heritable variation

Evolution in furanocoumarin levels possible:

Insects with higher levels of enzymes produce more offspring

Evolution in insects possible:

Plant defenses as selective agents

Defenses include:mechanical traits (thorns, spines, latex)digestibility reducerstoxinsrecruitment 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

Consequences: Increased diversity of plants and herbivores

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

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

Beetles

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.)

Adaptive radiation of beetles on flowering plants

Coevolution of mutualism (+/+)

Star orchid HawkmothLong spur forces moths to pollinate flowerMoths evolve longer probocis to feed effectively/ efficientlyPlant evolves longer spur

Interested in plant-insect interaction courses/ research?

Lynn Adler lsadler@ent.umass.edu

http://www.people.umass.edu/lsadler

ENTOMOL 597A: Insect-Plant Interactions