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Parasitoids
(insects whose larvae are the actual “predator”)
Parasites are obligately
dependent on their hosts –
so better adapted parasites
get what they need without
killing their host!
cercaria and
metacercaria are
just names for
different fluke life
stages
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Evolution of protective or
defensive adaptations help
species avoid exploitation
Flight, aggressive posturing,
cryptic colouration, herd
behaviour, chemicals
(animals)
Thick bark, spines, thorns,
chemical defenses, cryptic
colouration (plants)
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katydid
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Williams, et al. Ecology 74:1143-52
The surface of the
leaves of many
plants are covered
with protective
trichomes that can
negatively impact
wandering
herbivores by
physically
obstructing their
movements
giant stick insect
A stone fly, a predatory
The herbivorous mayfly
aquatic insect
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Kathy Williams monitored bird predation
rates on cicadas
Her data support a predator satiation
hypothesis
Cicada abundance peaked in late May (50%
of emergences were in 4 days) declining
through June
Predation is significant in early emergence
when cicada density if low
At peak emergence, predation declines
markedly
Predation climbs to 100% as cicada density
declines
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Protection through barriers
Protection through crypticity
spanworm
*Peckarsky. Aquatic insect
predator-prey. Ecology 55:
1104-11
Emerge as adults once every 17 years (although broods
emerge every year at different geographic locations in
eastern NA location)
Millions emerge from the ground over a period of only
a few days with up to 4x106 or 3,700 kg of cicadas.ha-1!
Males fly up into the tree tops, singing to attract
females who lay their eggs in the twigs of trees and
shrubs
When the young cicadas hatch, they drop to the
ground, burrow down to a tree root and begin to feed
without moving around much for 17 years!!
Periodic Cicadas
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Sometimes just looking bigger
might confer protection from
predation: Stoneflies can be
injured by large mayflies and
tend to avoid them. Barb
Peckarsky’s* data suggest that
smaller mayflies, who present
no danger to a stonefly, may
protect themselves by adopting
the “scorpion stance” i.e. trying
to look larger.
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Live Cicadas (#.ha )
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Protection in numbers: Predator satiation
Cicada mortality (%)
Recall that the young of many
species may be vulnerable to
predators, while the adults are not
(recruitment limitation)
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Nevertheless, exploitation is a two way street:
prey are not passive victims!
Pathogens can mimic flowers
Protection by size
ƒ A form of the parasite passes out of the infected sheep/cow in its feces
ƒ Cionella, a snail, picks up the parasite while feeding on sheep/cow poop
ƒ The parasite metamorphoses ending up in the snail’s “lungs” and is
released along with the mucous in the snails’ slime trails
ƒ Ants pick up the parasite while feeding on the snail slime
ƒ One of the parasites migrates to the ant’s “brain”
ƒ [Ants normally retreat to their burrows late in the afternoon remaining
there until the next day.] However, parasitized ants climb up onto the
tips of grass blades, clinging on by their jaws, throughout the night.
ƒ Consequently the ant becomes a prime target for accidental ingestion by
grazing cows/sheep, thus completing its life cycle.
Note only does
the liver fluke
require the cow
(and 2 other
hosts), it has
evolved a
mechanism to
alter the
behaviour of
those hosts to
move it around!
Parasitoids differ from
parasites in that they almost
always kill their hosts. The
adult parasitoid only needs
the host as food for its
offspring
ƒ Not all bright alpine meadow
flowers are quite what they seem!
ƒ This bright yellow, sweet smelling
“flower” is actually a fungus
parasitizing a mustard plant
ƒ Insects, attracted to the structure,
carry fungal reproductive cells to
other infected mustard plants,
promoting the cross fertilization
prerequisite to fungal reproduction
ƒ The fungal structure prevents the
mustard from flowering and
producing its own seed – leading to
significantly reduced mustard
success.
Dicrocoelium: the liver fluke
Parasites can alter the behaviour of their hosts
Scanning electron micrograph of trichomes on the
leaves of the passion flower, P. lobata
Fig. 4-5A (KR8C)
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Protection by chemistry: A plant can ‘identify’ a
specific herbivore from the nature of the damage
it does to the plant
Once the plant has ‘identified’ the insect
eating its leaves, it ‘retaliates’
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Tortoise beetles nibble at the
edges of leaves
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Flea beetles gnaw holes in the leaf’s centre
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How might this work? (hopefully you know)!
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A random mutation could occur in a plant
that creates a chemical that ‘inactivates’ an
insect herbivore.
The leaves of the plant with that chemical are
in better shape than the leaves of plants
without the chemical (i.e. they’re not being
chewed).
Healthier leaves increase photosynthesis
(energy), ultimately leading to enhanced seed
production for the plant (i.e. it leaves more
offspring)
Plants with these defensive chemicals are
likely to be ‘selected for’ and come to
dominate the population.
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An insect which acquires a counter ploy (or
defense) to plant chemicals will feed more
successfully than insects without counter
chemicals
These lucky insects will be able to acquire
more food than their peers and leave more
offspring (i.e. they will be selected for).
Eventually the population of herbivores will
be dominated by individuals with the ability to
release counter defense chemicals.
Since ploys/counterploys can only develop over evolutionary time,
they are often referred to as “co-evolution”
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Becerra, et al. 2001. Interactions Between Chemical and Mechanical Defenses in the Plant
Genus Bursera and Their Implications for Herbivores. American Zoologist. 41:865-876
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Arachnidomyia lindae
This fly-spider interaction suggests that
the behavior of these two ecologicallylinked species co-evolved in a stepwise
fashion
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Suppose a plant does produce a chemical
that interferes with a herbivore that is
attacking it
Herbivores also can produce saliva
containing chemicals that move into the plant
Such herbivore induced chemicals can alter
the plant’s anti-herbivore chemicals, shifting
the advantage back to the herbivore.
Ploy counterploy in the parasitoid fly:
A rather dramatic anti-herbivore
defense by a species of Bursera: The
leaves have a network of pressurized
canals – when an herbivore who is
chewing on a leaf severs one of
these, a spray of sticky terpenes
shoots out. Herbivores hit by the
sticky stream abandon the leaf
quickly, try to clean themselves.
Often they die.
The spider can recognize this predatory
fly by the ‘buzzing” of its wings, a wingbeat signature. (Think about the +
fitness consequences for a spider that
could do this! How would it happen?)
When facing this particular fly the spider
starts a sequence of egg sac guarding
behaviours that include:
• “shuttling” (the spider maneuvers around
its egg sac to keep itself between the fly
and its egg sac);
• “grooming” (the spider searches the
surface of its egg sac removing any eggs
the fly managed to deposit).
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The ploy: the “squirt gun” defense
The counter-ploy
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Hey remember
these guys?
So there is an obvious advantage to a prey
population that can defend itself against
exploitation (often referred to as ploys), but of
course there is a corresponding pressure for the
exploiter to counter-attack (the counter ploy)
Of course, the same thing could happen
with the development of the counter ploy
in the herbivorous insect
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n
It might release a chemical
compound that interferes
with the herbivore’s
digestion, reproduction or
otherwise kills the
herbivore
Or the plant might release
‘perfumes’ (volatile
compounds) that signal the
presence of the herbivore
to a predator of the
herbivore – usually a
parasitoid – who finds and
destroys the herbivore
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Flies deposit their
eggs onto the egg sac
of the spider. The
developing fly larvae
eat the spider’s eggs
(not good for a
spider’s fitness!)
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Unfortunately for Bursera … the squirt
gun is a bit of a one-trick pony
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After one leaf fires, no
other leaves in a 20-30
cm radius seem to be
able to respond for 2448 hours
However, herbivores
seem to be effectively
deterred as the first leaf
they choose responds
This raises questions
1. How come insects don’t
hang around, waiting for
somebody else to get
squirted and then move in
for the feast?
2. Is herbivory not a
sufficiently strong pressure
to drive selection for more
leaves that can respond?
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Maybe there has just not been
sufficient time (?) b
A half-hearted counter-ploy: an
evolutionary work in progress?
nBoth the larva of nymphalid
butterflies (a) and Chrysomelid
beetles (b), common herbivores
on Bursera, attempt to sabotage
the squirt gun defense by
severing the terpene canals
before feeding (ok – that will
work)
nUnfortunately for the larva, this
takes more time (30-90 minutes)
than it would to consume a leaf
(10-20 minutes)
The pressures on competitors vs predators/prey
vs responses to disturbance may be different . . .
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• Reducing time/energy spent in competitive activities frees up more time
time/energy to engage in other activities that promote survival and
reproduction for an individual.
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The pressures on potential competitors for specializations that
create unique ways to access resources
The increased
“handling time”
leaves the
larvae subject
to predation
(c), slows
growth and
time to
pupation
Hmmm . . .
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The complicated relationships we have been
exploring can only develop over extremely long
periods of time
They help us understand:
1. Why anthropogenic activities (events
occurring in microseconds of Earth’s history)
are so antagonistic to the web of biological
interactions that maintain K-system
persistence
2. The futility of trying to preserve/protect
biodiversity with anything other than habitat
preservation
3. The devastating impacts of species
introductions
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The pressures on prey for adaptations that make them less
conspicuous or otherwise capable of avoiding exploitation.
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The pressures on exploiters that enhance their ability to
capture prey
The pressures on species to successfully weather disturbances
whether that “disturbance” is fire, drought or etc.
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• Dying prior to reproduction definitely reduces an individual’s fitness.
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But in spite of the variety of “pressures” acting
on species, the outcome is the same:
adaptation
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Individuals with various traits that promote
reproduction (and the number of offspring
produced) will come to dominate the
population over time
Individuals without these adaptations are
likely to disappear (eventually).
Remember: this is what we mean by “survival
of the fittest”
• Where “fitness” is defined by the number of
offspring particular individuals put into the next
generation.
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