ENVIRONMENTAL RESISTANCE
This is the sum of all the
factors in an environment that reduce the reproductive potential of an organism,
can be organized into 4 groups:
| physical |
| nutritional |
| biological |
| host resistance |
Physical Factors (≈
climate/weather, we have no control)
1.
Temperature - (zones: optimal, effective/active, dormant, &
lethal)
| insects are
poikilotherms (internal
temperature varies with the environment
≈ cold-blooded) |
|
they cannot physiologically regulate body
temp – they regulate their temperature by behavior;
|
| internal body temp >
ambient air temp
| due to muscle activity |
| dark colouring |
| large surface area to absorb heat
|
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Ranges of temperature:
o
optimal range where insect does best (often a narrow range
around 26ºC),
o
active range (10/15ºC
- 38/45ºC),
o
two dormant (quiescence) ranges - excessive high and low
temperatures
o
extremes are lethal (e.g. >48ºC)
Low temperature
o
fatal temperature (both low & high) varies with
o
species (northern beer beetle vs. southern margarita weevil)
o
stage of development (which stages are more tolerant ... depends
on the insect)
o
duration of exposure (obviously longer exposure is worse - quick
polar bear dunk is OK but longer can have severe consequences)
o
previous weather conditions (previously mild worse than previously
cold)
o
time of year (mid-winter vs. late spring)
o
lower temperature means lower activity, thus low temperature means
that activity can stop (quiescence), thus an insect will stop all movement and could
starve to death if for a prolonged period of time
o low temperature & quiescence can vary with development stage (1st instar of
tent caterpillar feeds above 15C, but the 4th intar will stop feeding when
only 15C)
o
insects have a supercooling
point (= internal freezing temperature) below which ice forms in the body (this is
bad), supercooling point is lower than 0ºC
due to elimination of free water,
electrolytes, and/or glycerol (= antifreeze),
o
also snow insulates (so if eggs/insects are covered by snow they
will be warmer than if exposed)
High temp
o
high temp leads to hyperactivity, then stupor then
death due to
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denatured proteins
|
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accumulation of toxic stuff (metabolic by-products that cannot be eliminated
fast enough)
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starvation and/or
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desiccation
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o
fatal temperature varies with the same factors listed for low
temperatures
o
wood boring insect can’t move fast
\ since top of log exposed to sunlight
can reach temperatures of 60ºC,
such logs have a “sterile zone”
Insect distribution - “life zones”,
o
just because there is sufficient summer warmth does not mean the
insect will be there - extreme cold in winter can limit where an insect can
survive
|
excessive cold sets northern limit
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excessive heat sets southern limit
|
o
snow cover helps with northern distribution (insulation)
o
years of mild or extreme weather can affect the distribution of an insect ...
then consider global warming
Insect development
o
rate of development is affected by temperature
o
within the active temp range, an
of 10ºC
doubles rate of development (remember Nantucket pine tip moth - 1 generation
in Ontario & 4 in Georgia);
o
a cold spring means
time of larval development, exposure
to predation,
\
¯ adult pop’n
in summer
Radiant energy (IR - infrared energy)
o
IR occurs between visible light & radio waves = 0.7 - 500
µ
(can't see the energy but can feel it as heat)
o
some insects have heat sensors, i.e. wood boring beetles attracted
to forest fires (can be a problem to fire fighters)
o
some budworms attracted to purple reproductive bud instead
of green vegetative bud (heat difference is about 5-8ºC)
2.
Light
| insect reaction to
light is similar to temp |
| light & temp are terms
applied to adjacent EMR (0.4-0.7µ
is visible light, 0.7-500µ is IR)
|
| insects can detect heat
(IR), light and UV (shorter wave radiation) |
| flight – some insects
fly day not night, others visa versa, others only in a narrow range (dusk)
|
| an insect's reaction to
light may change over time (early instars are light positive, later instars
are light negative ... why?) |
| insects can detect heat
(IR), light and UV (shorter wave radiation) |
| reactions to stimuli
(e.g. temperature & light) are
| tropism
(orientation/direction - usually plant growth) |
| taxis
(movement towards or away) |
| kinesis
(level of activity - i.e. more or less active)
|
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3.
Moisture
| distribution &
development are also dependent on water |
| spittle bug as a
nymph has a mass of spittle to regulate moisture, however, as an adult it has a hard
exoskeleton and can therefore be exposed to the elements |
|
powderpost beetles cannot develop in moist wood
|
| ambrosia beetles
cannot develop in dry wood |
| European elm bark
beetle has a preference for freshly cut log but cannot live in
dried log (speed of shipping has increased introductions of exotics)
|
| rate of development
– there are documented cases of wood beetles that usually take only 1 yr
to complete its life cycle but have taken ~20 yr in unusually dry conditions (the
larva was in a piece of furniture, there were no entry holes, but an exit hole
that occurred 20 years after the furniture was purchased) |
| water saturation
affects activity (can stop activity), also affects oxygen & temperature
|
4.
Weather
| Weather is a
combination of temperature, light and moisture (wind, snow, etc.)
ie. a wet warm spring vs. a wet cool spring, etc. |
| abnormal weather
helps regulate pop’n
| late/early frost |
| very cool spring |
| unusually hot
summer |
| heavy
precipitation (especially during mating, also some larva stop feeding
when raining) |
| lack of snow,
etc. can all cause sig. mortality |
|
Nutritional Factors (≈
food source, we can control to a degree)
1.
Quantity of Food - if other conditions are
favourable then insect can multiply to the limit
of the food supply! the more numerous a tree species (age a factor too) the
greater the risk:
| MPB in
Chilcotin plateau |
| spruce budworm in east
due to abundance of overmature balsam fir |
| spruce terminal weevil
with Ss plantations |
| red pine (s. states)
used to considered insect resistant, then they planted lots and it got hit
with spittlebug, sawflies, shoot moths and bark beetles [this is similar to
our Ss and the terminal weevil]
|
2.
Kind & Quality of Food - insect abundance & development is
limited to their food supply, i.e. leaf feeding insects have to develop
quickly while leaves are available; wood borers may need 'fresh wood' and cannot
utilize rotten wood
| leaf feeders
limited to develop in one season - some
limited to even shorter period when leaves/needles are most succulent (these
would need to 'be in sync' with bud burst and develop quickly) |
| soil nutrients/moisture
affects quality of tissues (i.e. a good food source) and a tree's general
health (and ability to withstand attack) |
| wood borers food
is fairly stable \ can develop over >
1 year; many wood borers have young larva feed in phloem, then feed in wood
(their digestive ability advances with age) |
| ambrosia beetle
has symbiotic relationship with fungus (ambrosia) – they use the log as a home
and the fungus as food [termites raise fungus as well] |
3.
Host Selection – one/few hosts vs. many hosts
| most forest insects
restricted to one tree species or small group of tree species, e.g. locust
borer - only black locust, Nantucket pine tip moth - only pines and eastern
spruce budworm - only balsam & spruce; these are easier to manage as you can
more easily manipulate species composition if only one (or a few) host species
are affected |
| other forest insects
are general feeders, e.g. gypsy moth – they feed on a great number of species
– these pests cannot readily be managed by altering species composition
|
Biological Factors (interaction among organisms)
1.
Competition – for food, space or shelter
| competition only
occurs when a required resource is in short supply (i.e. demand of insect
population exceeds resource) |
| a tree "can afford"
to supply food only what is in excess of its’ own requirements (e.g. trees are
able to produce more food/energy than they actually require for a good life)
| insects at endemic
levels eat only this “surplus amount” of food |
| in an epidemic
there are more insects, \ more is
eaten, and now there is competition for a scarce resource
| if some other
factor does not control the pop’n they will “eat
themselves out of house and home” |
|
| the previous points
dealt with competition between individuals of the same species at an
epidemic level, however, there is also competition between 2 species
for same food source
|
Ips beetles and spruce bark beetle prefer
windfallen spruce
|
Ips develop more quickly and destroy
the habitat for spruce beetle |
| the spruce beetle
can only succeed in logs that are not yet infested by
Ips or are too moist for
Ips beetles
|
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2.
Predators, Parasites & Pathogens - subject for “biological
control” (e.g. IPM) – this topic was covered earlier and will be discussed again
later
Host Resistance Factors
1. Preference
| this is due to
responses of insects to chemical stimuli produced by tree; i.e. +
chemotrophic: positive flight to the chemical
| Douglas-fir beetle to
α-pinene
|
| ambrosia beetle (and
forestry students) to ethanol |
| wood borers to
turpentine |
| preference is bad
for the tree |
|
2. Non-preference
– in this case trees emit no attractive chemicals
| bark beetles are
attracted to stressed/over mature trees over healthy mature trees, thus
healthy trees are not preferred (they don't emit the same chemicals) |
| moisture is a factor
for wood borers – e.g. wood gets too dry and insect will leave current host
for a better one |
| it is important to note
that non-preference trees can be attacked during an outbreak (esp. true for
bark beetles) |
| the tree has no "active
resistance" - it is just not attractive to the insect
|
3. Antibiosis -
resistance mechanism that is detrimental to insect ["active resistance"]
| distasteful or
poisonous chemicals
| Douglas-fir beetle is repelled by
ß-pinene |
| juglone in walnut leaves for bad gypsy moth |
| chrysanthemums produce pyrethrum which is used as an insecticide |
| marigolds give off volatile insecticides that keep insects away - one of
the chemicals, thiophene, kills malaria & yellow-fever mosquitoes |
|
| antibiosis may not
be equal in all individuals of a species (i.e. genetics) -
| spruce terminal weevil - some Sitka spruce (Ss) trees are capable of
producing a juvenile hormone (JH) such that when a female terminal weevils
feeds on the tree it is unable to produce viable eggs [there is a breeding
program for Ss and JH is a desired trait]
|
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4. Tolerance -
ability of tree to grow, reproduce or repair injury in
spite of an insect pop’n that would damage a
susceptible tree (a.k.a. “withstand the attack")
o
age and vigour are important factors for bark beetles, but
not so for defoliators
o
vigorous tree have copious amounts of resin and sap, a
single boring beetle can’t
handle this
§
pitching out of bark beetles often successful
§
also galleries can be overgrown by vigorous tree;
o
tolerance is related to site and growing conditions (i.e.
remember soil moisture/nutrients and weather)
5. Pseudo-tolerance
– lucky to escape attack (due to brief susceptibility period or not in sync)
| vigorously growing tree
is more tolerant (repel/repair attack) BUT can also evade an outbreak –
if a certain ‘life stage’ is susceptible, then it is better if that life stage
is a short period |
o
pine shoot tip moth attacks young stands until crown
closure - the quicker a stand closes the shorter the susceptible time;
o
eastern spruce budworm
(which prefers balsam but will also attack spruce) -
black spruce buds generally burst 2 weeks after spruce
budworm eggs hatch, tissue is susceptible but if high proportion of stand is
Sb, most budworms may starve before bud burst of
Sb - key point ... tissue is susceptible, but it is
typically available awhile after larva hatch
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