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Defoliators

General Introduction​

  • 2nd in economic importance to bark beetles
  • Many species of defoliators, few cause damage
  • Defolitation is ubiquitous in forests -- always some level of injury
  • Regulation of defoliator populations typically by natural enemies, seldom food limited
    • Predators
  • Weather is extremely important as sa density-independent source of mortality

Impacts​

  • Impacts / injury to individual trees
    • Feeding by larvae affects photosynthetic organs
    • Disruption of
      • Production
      • Transportation
      • Subsequent utilization of photosynthate

Reduced photosynthesis leads to:

  1. general weakening
  2. Growth loss (reduced ring increment)

General weakning and growth loss leads to:

  • Increased susceptibility to secondary insects and/or infection:
    • Douglas-fir: beetle infestation
    • Spruce:
  • Need more notes here from slides

Altered cone production:

  • Light defoliation = increased cone production
    • Stress response
  • Heavy defoliation = decreased cone production
    • Depleted reserves used for refoliation

Tree mortality:

  • Death of parts of a tree
    • Preferential feeding on new foliage
      • Top kill
      • E.g. western spruce budworm
  • Death of entire tree
    • Feeding on all foliage age classes
      • Douglas-fir tussock moth
      • Western hemlock tupper

Tree Response​

  • Trees adapted to some level of defoliation (tolerance)
    • Excess foliage (more than required for optimal growth)
    • Energy reserves (stored photosynthate)
  • Defence mechanisms to defoliation
    • Leaf toughness
    • Chemicals (constitutive and induced)
    • Hairs, thorns, etc. (trichomes)

Angiosperms are fairly tolerant

  • photosynthate reserves
  • Refoliation immediatelyh following defoliation
  • Multiple years defoliation may deplete reserves
  • Species differences
    • Aspen (tolerant) vs. maples (less tolerant)

Gymnosperms intolerant

  • Large species differences
    • Photosynthate reserves:
      • Pines less than spruces
  • Larch relatively tolerant

Defoliation impacts vary by timing and host tree species

  • Early season worse than late; ew foliage worse than old (conifers)
    • Late season = shoot/leaf growth completed; photosynthate reserves established
  • Old foliage = less efficient photosynthesis
    • Type of defoliation varies by insects: e.g. budworms (new foliage) vs sawflies (old foliage)

Impacts vary by host tree species

Old versus new foliage

  • Old foliage at end of season or older cohorts (low importance to tree)
  • New foliage, early season (highest importance to tree)
    • Significant loss of capacity for current & future photosynthate production/storage
      • E.g., spruce budworms -- early season defoliator, preference for new shoots
    • Relatively more new foliage in upper crown of conifers
      • Feeding concentrated in the upper crown; topkill
    • Conditions in upper crown promote injury
      • Sun leaves more nutritious than shade leaves
      • More heat accumulation in upper crown
  • Insect preference a guide for impact

Suppressed trees on poor sites

  • more likely to suffer
  • Crown closure and stand structure affect tolerance
    • Open grown white pine 75-100% defoliation = death
    • Shaded white pine
      • 25-50% defoliation = death
    • Poor crown on understory trees
      • Relatively less foliage
        • Fewer insects = growth loss/mortality

Environmental influences​

  • Synchronization (i.e. phenology) critical to many defoliators
    • Egg hatch/emergence synchronized to bud burst
      • Determined by "degree-day" accumulation (temperature)
        • Threshold temperature above which development begins
    • Spruce bud moth
      • White spruce buds suitable for colonization less than 1 week
    • Douglas-fir tussock moth
      • Hatch only following 77-97% bud burst

New foliage, why the preference?

  • Not as tough
  • More nutritious
  • But, very ephemeral (available for only a very limited time)

Phenological constraints: Early season defoliator adaptations to access new foliage

  • Synchronize feeding stages with bud burst
    • Egg hatch
      • Douglas fir tussock moth
    • Larval emergences
      • Spruce budworms
    • Or utilize older foliage
      • Conifer sawflies
    • Poor synchrony = larval starvation
    • Phenological adaptations by trees to avoid defoliation
      • E.g., black spruce
        • Late bud break
        • 10-14 days later than white spruce and balsam
        • "Immune" to spruce budworm

Defoliation Impacts​

Sampling and management​

Detection and sampling

  • reconnaissance flights
  • Satellite images

Aerial sketch mapping

  • GPS

Ground sampling

  • Egg sampling
    • Late summer/early fall
    • Nt+1 population prediction
  • Larval sampling
    • Spring/early summer
    • Parasitism
    • Disease
    • Developmental rates

Management​

Direct Control​

  • Biological insecticides
    • BTK
      • Bacillus thuringiensis var. kurstaki
    • Must be ingested
    • Soil bacterium (natural)
      • Bacteria produce toxin
    • Specific to Lepidoptera
      • Budworms and gypsy moth
      • Timing is critical:
        • Application late enough to ensure sufficient ingestion, but not too late to prevent significant defoliation
  • Viruses (biological insecticides continued)
    • NPV (nuclear polyhedrosis virus)
      • Highly specific
      • Douglas-fir tussock moth, sawflies
    • Spray decision
      • NPV is highly effective

Budworm complex​

The phenological window: ephemeral resources

The budworms Genus Choristoneura

  • complex of species
  • Diverse hosts
    • Most north american conifers
  • FINISH NOTES FOR THIS LIST

Budworms vs bud burst: importance of phenological synchrony

  • Larval development longer than food availability
  • Larval development = 6 instars
  • Duration of development longer than shoot elongation
  • Resource requirements for large larvae greater than small larvae
  • Adaptation = abandon feeding as L1, mine buds/old needles as L2, synchronize L3 to L6 with newly flushed foliage.

Life cycle (C fumiferana, C Orae, C Freemani)

  1. Eggs lay: August
  2. L1's spin hibernacula: August
  3. L2 Feeding: May
  4. L2 - L6 Feeding: May - July
  5. Pupation: July - August (Holometabolist)

Life Cycle (C biennis) Semivoltine

  1. Egg lay: August
  2. Egg Hatch/L1s spin hibernaculae: August
  3. L2-L4 Feeding May-July
  4. L4's spin hibernaculae (overwinter): July
  5. L4-L6 Feeding: May-June
  6. Pupation: June-July

(finish life cycle)

Budworm injury patterns

  • Older may feed on old foliage if young foliage is depleted
  • crowns of damaged trees appear reddish-brown from June to Sept
  • Initial symptoms of defoliation visible in tree tops and at branch tips
  • After several years of defoliation -- reduced cones, growth loss, top kill or mortality (esp. immature/suppressed trees)

Western spruce budworm​

BC's most important defoliator

  • Focuses on Douglas-fir
    • Common-name misleading

Outbreak first recorded 1909, Southern Vancouver Island near Victoria

  • Apparent increase in frequency, severity, and distribution during past century
  • Decreasing outbreak intervals, increasing outbreak duration
  • Fire suppression
  • Climate change

Fire suppression (retake notes for all of this for fuck sakes):​

  • Remove low intensity ground fires
  • Fewer ground fires allow establishment of dense understory
    • Shade tolerant douglas-fir saplings
  • When outbreak populations deplete overstory foliage, they drop into the understory
  • increased food supply sustains large populations
    • Outbreaks persist for longer priods

WSBW outbreak range shift

Observational evidence:

  • Apparent northward shift during 3 most recent outbreak periods
  • Between 1994 and 2011, epidemic infestations detected further north than previous records
  • real or atefact?

Emperical evidence:

  • Aerial overview survey data
  • Centroids
  • Determined effective latitude (Latitiude + elevation/122 m) for each centroid
    • Effective latitude accounts for variation in climatic conditions associated with both latitude and elevation
    • 122 m in elevation = 1 degree of latitude
    • Lets us figure out effective latitude

Hypothesis: The northward range shift is a consequence of a climate change-induced shift in optimal synchrony between larval emergence and Douglas-fir bud burst.

Shifting synchrony

  • converging synchrony at the highest effective latitudes
  • Stable synchrony at moderate effective latitudes
  • Diverging synchrony at lower effective latitudes

past/present outbreaks​

changes in outbreak characteristics​

Control efforts​

  • BTK Application (biological insectisides)
  • 61,966 ha treated in 2008
  • Treatment priorities based on:
    • Predicted defoliation
      • Based on aerial surveys and ground assessments of larval populations
    • Potential for stands to recover
    • Stand value (investment)
    • Stand structure
      • Potential for understory to maintain populations
    • Wildlife habitat
    • Recreation/aesthetic values