Bark Beetle Biology

Bark Beetle Biology
- Overview
  - Phloem feeders
    - virtually entire life under bark
    - adults emerge only to find new host
  - Genera in BC
    - Dendroctonus (incl. mtn pine beetle)
    - Dryocetes
    - Ips
    - Scolytus
  - communications
    - semiochemicals
      - kairomone (vs. allomone)
      - aggregation pheromone ... also attracts secondary BB & PP
      - anti-aggregation pheromone
    - stridulation - sound (vibration)
  - Primary vs. secondary attack
    - normally on dead/dying (secondary attack) - little/no host defence
    - host material typically scattered (endemic) - hence kairomones
    - outbreak ... may attack healthy trees (primary attack)
    - tree defence: pitch out beetle & isolate fungus
    - require mass attack to overcome tree defence - hence pheromones
  - Epidemic when
    - favourable weather
      - mild winters (survival)
      - warm/ dry summers (dispersal)
    - abdundant suitable host material
      - e.g. large area of lodgepole pine in BC
      - we fight fires (natural disturbance agent) ... now have lotsa mature pine
    - current epidemic ...
      - fire fighting increased mature pine inventory by 3-fold
      - recent climate ... mild winters & warm dry summers
  - Critical Factors
    - Finding the needle in the haystack
      - Efficient host selection
      - - size is important
      - - its gotta smell right
      - - "tastes like chicken"
    - It makes "scents"
      - Sophisticated communications
      - - semiochemicals
      - - stridulation
    - I got the blues
      - Symbiosis with blue stain (Ceratocystis // Ophiostoma)
      - - provides critical benefits
    - Timing is everything
      - Phenology (seasons) & Synchrony (each other)
      - - stage-specific thresholds (OW & emergence/ flight)
    - I gotta go!
      - Varied dispersal behaviour
      - - spread = local (increases pop'n)
      - - dispersal = "to somewhere else"
      - ... risk spreading behaviour
- Generalized life cycle
  - Host selection & colonization
    - 'pioneer' beetles find suitable host
      - kairomones
        stressed trees & recently killed
        alcohol
        terpenes (pinene & myrcene)
      - tree size
        bark thickness
        "thermal mass"
      - final decision to colonize tree - taste
    - aggregation pheromones ... mass attack
      - pioneer attracts opposite sex ...
      - ... they in turn attract "the original sex"
      - ... party time
    - anti-aggregation pheromones
      - masking and/or repelling
      - "no vacancy"
      - avaoids over utilization
    - female excavates egg gallery, eggs in niches
    - sapwood inoculated with blue stain
      - helps to overcome host defences (i.e. resin flow)
      - protection from antagonistic fungi (via antagonsim)
      - improve moisture in phloem (good for larva)
      - provide nutrients to complete development (maturation feeding)
    - egg gallery placement may be regulated ... stridulation by females ("chirping")
  - Brood development
    - eggs hatch in ~2 weeks
    - larva tunnel at right angles to egg gallery - in phloem (=sugars & carbs.)
    - in BC typically OW as L3/L4 (most cold hardy stage) in the bark
      - L1/L2 can continue to feed/dev. @ lower temp
      - can 'catch-up' to L3/L4
      - synchrony - helps keep pop'n at same stage
      - phenology - majority at most frost hardy stage
    - continue feeding next spring
    - excavate a 'pupal cell' at end of larva gallery
    - larva - pupa - callow / teneral adult
    - maturation feeding (on fungus) to become fully mature - before emergence
    - mortality ... endemic if mort.=97.5%, epidemic if mort. <97.5% (surv.>2.5%)
  - Dispersal
    - emergence & flight in mid-late summer
      - warm for flight
      - dry ... moisture stress for trees
    - adults remain under bark until threshold temp.
      - thus "mass flight"
      - peak emergence lasts ~ 1 week
      - synchrony
      - flight temp, 25C is ideal (18-35C)
    - some only disperse a short distance ~30%
      - = spread w/in stand
      - more energy for reproducion
      - contributes to local pop'n increase
    - most disperse long distance - i.e.
      - require flight before attack (i.e. will ignore semiochemicals)
      - flight helps host-seeking ability
      - encourages dispersal out of the stand
      - often 10's of km's, can be 100's of km's
      - may lead to finding habitats with higher qual. hosts
      - ensures new areas get colonized
    - short dist. + long dist. = risk spreading strategy
    - flight characteristics
      - flight ht. - typically ~5m, most "open" and least wind (but few above canopy)
      - NO flight if wind>flight speed (~7 km/hr)
      - fly downwind until kairomone ... then upwind to source
      - lower temp and light are conducive to attack (IBM)
      - tend to "keep flying" if bright & warm ... think about that ...
      - kairomone attracts pioneer beetle ... think about that
      - final decision based on taste
- Cold Tolerance
  - glycerol in blood
  - L3/L4 > L1/L2 > adult > pupa > egg
  - shoulder seasons ... -20C is lethal
  - winter ... -40C (or -30C for several days) is lethal
- Tree Size/ Age
  - Size
    - larger (>25cm)
      - thicker bark
        more food
        greater insulation
        protection from predators & parasitoids
      - larger bole = larger thermal mass
      - however, usu. higher vigour ... need mass attack ... hence pheromones
      - if successful attack ... big 'beetle source'
    - smaller (<25cm)
      - thinner bark (food, insulation)
      - less thermal mass
      - easier to attack ...
      - ... but less success ... 'beetle sinks'
  - Age
    - 40-60 yr - increasing resistance
    - 60-80 yr - start of decline in resistance (still high)
    - 80+ yr - become very susceptible
    - susceptibility increases with age ... think about it
- Key Points
  - sophisticated communication
    - aggregation
      - typically conversion of host chemicals
      - to overcome defences of a preferred (vigourous) host
    - anti-aggregation
      - prevents over-utilization
      - diverts attack to neighbours
    - stridulation
      - communication btwn females
      - prevents "over crowding" of egg galleries
  - mutualistic relationship with blue stain
    - helps overcome host defenses
    - protection from antagonistic fungi
    - improve moisture in phloem
    - provide nutrients to complete development
  - stage specific thresholds
    - ensures "good timing"
      - with each other (synchrony)
      - with the seasons (phenology)
    - encourages OW as L3/L4
    - encourages mass flight (threshold temperature)
  - varied dispersal behavious
    - (limited) local spread within stand
    - dispersal to another (better?) stand
    - (limited) very long distance spread (flight above canopy)
  - efficient host selection
    - kairomones to find host
    - sight - size of stem (larger stems preferred)
    - taste to determine quality
- S&S
  - Tree Level
    - External Bole
      - pitch tubes
      - borring dust
        bark crevices
        base of tree
      - bark flaking - wood peckers (esp. in winter)
      - emergence holes (no pitch)
    - Crown
      - (fade to yellow-green)
      - red crowns 'next year'
      - gray crowns 2-5 years after attack
    - Under Bark
      - galleries - central egg gallery with larva galleries at 90 degrees
  - Stand Level (landscape)
    - endemic
      - scattered smaller diameter, suppressed trees
    - building
      - small scattered patches - 'peppered' (not all trees attacked)
      - some larger trees get attacked
    - epidemic
      - less 'peppered' as higher proportion opf trees attacked
      - patches grow & coalesce