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
|
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
|
denatured proteins |
|
accumulation of toxic stuff (metabolic by-products that cannot be eliminated fast enough) |
|
starvation and/or |
|
desiccation |
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 |
|
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
|
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
|
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)
|
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
|
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
| ||||||||
| antibiosis may not
be equal in all individuals of a species (i.e. genetics) -
|
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