Bark Beetles 1

Lecture Outlines/goals

• Introduction
  • Diversity
  • Roles and impacts
• General Biology
  • Brood (offspring) production
  • Emergence/dispersal
  • Colonization

Bark Beetles

(Coleoptera: Curculionidae: Scolytinae) Feed within subcortical tissues (e.g. phloem) of virtually all parts of woody and herbaceous plants

• Roots
• Stems/boles
• Branches/twigs
• Fruits/cones

Generally mono- to oligophagous

• They only like one (mono) to a few species (oligophagous)

Most commonly associated with conifers as disturbance agents

• High diversity in conifer ecosystems

Roles and impacts

Consumption of phloem tissue kills part or all of a tree

Ecological roles:

• Succession
  • Species removal
• Soil nutrient cycles
  • Decay processes (e.g. fungi)
• Fire cycles
  • Ecosystem renewal

Socio-economic impacts:

• Production forestry
  • Decaying dead trees non-merchantable
• Aesthetics
  • Loss of high-value trees on private lands
• Wildland-urban interface
  • Fuel loading, fire risk
  • Tree failure
• Biodiversity/endangered species
  • Ecosystem alterations
• Water quantity/quality
  • Watershed alteration
• Carbon dynamics
  • CO2 release from dead organic matter

General Biology

Brood production

• Mating
• Gallery construction
• Oviposition
• Brood Development

Males or females initiate attacks (species dependent) Male initiators = males excavate nuptial gallery for mating

• Multiple females (polygamous)
• Egg galleries (one per female) extend from nuptial chambers

Female initiators = mating at bark surface

• Most species +/- monogamous
• Females create single egg gallery

Gallery patterns (with host-tree identification) often characteristic of species

Gallery creation versus tunneling/scoring

• Galleries are created by multiple, related individuals (offspring/sibling) whereas tunnels are created by one individual

Females:

• Lay eggs singly or in groups in niches along margins of gallery
• Niches packed with frass
• Fecundity from several to 300 eggs/females

Males

• Frequently assist females by cleaning galleries
• Adaptations for clearing frass/boring dust:
  • Elytral declivity, flattened head capsule, hairiness

Brood production 3

• Larvae feed and develop through several instars Silviculture/Biotic Disturbances/Module-1/Definitions
• Pupation occurs in enlarged chambers at the end of larval galleries or in common feeding area
• Young (i.e. tuneral) adults often feed to complete maturation (increase flight muscles, acquire symbionts, sclerotize (harden))
• Mature adults cut exit hole to outside
• Maturation feeding may involve other plant parts (e.g. European elm bark beetle)

Brood production 4

• The number of generations per year (i.e. voltinism) may be less than 1 (semivoltine) to 7-8 (multivoltine)
• One generation per year (univoltine) most common in Canada

Brood Production 5

Temperature is very important (based on elevation or latitude or both)

• Within a species, the number of generations per year may vary with climatic conditions
• Climate-related population asynchrony may cause significant mortality
  • Delayed/advanced development cause cold susceptible stages to overwinter
  • Prolonged adult emergence reduces chances of mating

Emergence/dispersal

• Flight

• Host selection

• Environmental impacts

  • Temp, humidity, light

• Optimal temperature and humidity required for emergence and flight

• Avoid rain and winds > maximum flight speed

• Short window of emergence to maximize chances of finding mates/hosts

• Flight period may be required before beetles become receptive to cues from hosts or conspecifics (other beetles)

  • Ensures population dispersal, minimizes intraspecific competition.

• Dispersal both short and long range

• Most beetles fly beneath canopy (short-range dispersal, upwind flight when seeking hosts or mates)

• Small percentage fly above canopy (long-range dispersal)

Host selection (primary attraction)

• Cues
  • Light/temperature (stand density)
  • Visual (large vertical silhouettes)
  • Olfactory (e.g. ethanol, terpenes)
• Cascade of decisions:

1. Host versus non-host species
2. Susceptible versus resistant
3. High versus low quality
4. Presence/absence of competitor species
5. Presence/absence of conspecifics

Colonization

• Beetle-tree interactions

• Mortality factors

  • Tree defense
  • Competition
  • Predation

• Host acceptance based on gustatory stimulation (i.e. taste)

• Following acceptance (by pioneer beetles)

  • Release of aggregation pheromones (both sexes)
  • Response by conspecifics (secondary attraction)

Trees produce resin as defense against attacks

• Most bark beetles select trees with impaired defenses
  • Drought, disease, windthrow, suppression, age
  • "secondary species"
• Some species don't care
  • "primary species"

Primary beetles:

• Preferentially attack large diameter vigorous trees
• Succesful attack requires tree mortality
• Occasionally undergo population eruoptions (i.e. outbreaks) and cause mortality over landscapes

Secondary:

• Preferentially attack suppressed trees

Tree defenses

• succesful colonization contingent on tree death
  • Trees have evolved defensive responses
  • Constitutive resinosus
    • Preformed resin ducts or pockets exude pitch
    • Mainly physical barrier
  • Induced resinosus
    • Cellular necrosis (autolysis) and release of toxins
  • Defense = vigour

Symbionts

Evolved to carry one or more species of fungi

Symbiosis: two unlike organisms living together for mutual benefit All bark beetles carry symbiotic organisms - mostly fungi Mutual benefits

fungi	beetle
Transport to new host	tree pathogen
blank	habitat modification
blank	nutrition

Mycangia (adj. = mycangial)

• structures that carry and sometimes nurture fungal spores
• Located mainly on head or mouthparts

Phoresy (ad. = phoretic)

• Spores carried on body

Semiochemicals:

• Chemical messengers
  • Insect-modified tree defenses
  • Pheremones: Cause a specific reaction in a receiving organism of the same species
    • E.g. aggregation, anti-aggregation
  • Kairomones: Evoke in the receiver a reaction that favours the receiver but not the emitter
    • E.g. host/prey location
  • Allomones: Evoke in the receiver a reaction that favours the emitter, but not the receiver
    • E.g. interspecific territoriality
  • Potential multifunctionality
    • E.g., one species' pheremone = another species' kairomone

Competition

• Intraspecific competition significant density-dependent mortality
• Optimal attack density to overcome tree resistance before competitive effects
• MEchanisms to minimize overcrowding: stridulation (chirping)

Resource quality:

• Tree quality = phloem thickness
  • Better nutrition
  • Less competition
  • Faster development
  • More offspring

Abiotic mortality factors

• Largest source of mortality following colonization
• Primarily temperature related
  • Direct effects on beetles
    • Growth and development
    • Survival
  • Indirect effects on host trees
    • Phloem moisture
  • Beetle adaptations:
    • Diapause
    • Cold tolerance
  • Cold-related mortality greatest during unseasonable events
    • E.g. cold snaps in autumn or spring

Bark Beetle Management

Sampling bark beetles

• Pheremone-baited multple funnel traps (i.e. Lindgren traps)
  • Best for presence/absence detection (i.e. monitoring)
    • Pheremones only 1 cue considered by foraging beetles
• Aerial and ground surveys
  • Aerial (low altitude fixed- or rotary-wing aircraft) detection of last years red attacked trees followed by ground surveys to detect new green attacks
    • Trees attacked in one year do not reliably show symptoms in crowns until the following year once beetles have finished development
    • Red attack tree = where beetles were not are
    • Green attack trees = signs of beetle colonization on bole only
• Aerial overview surveys
  • Since 1959, BC has employed annual aerial surveys. Some of the best data in the world.
  • Broad (high altitude fixed-wing aircraft); surveys of landscapes to assess population distribution and tree mortality
  • Suitable for extensive epidemics
• Remote sensing
  • Much promise, slow adoption

Two approaches

• (indirect) Modify existing stands to make them less good for beetles

• (direct) Or look for problematic populations and manipulate those populations

• Indirect control (proactive) = management of populations through tactics that reduce forest susceptibility (not yet widely operational, but is what we should be doing)

  • Long term
  • Species and/or age mosaics
    • Harvesting/prescribed fires
  • Stand modification (silviculture)
    • Host removal from mixed stands
    • Vigour maintenance for "pre-susceptible" stands (e.g., pre-commercial thinning)
      • Spacing to increase vigour of susceptible stands
      • Indirect control is irrelevant during outbreaks because too many beetles

• Direct control (reactive) = management of populations through tactics that kill populations early

  • Short term
  • Reduction of beetle pop'n
  • Strategy = population reduction greater than yearly population increase
  • Tactics:
    • Chemical tactics irrelevant and unacceptable
      • Bark beetles live under the bark of trees and are not exposed to any chemicals we could apply
      • Not legal in Canada
      • Environmental concerns
    • Cultural/mechanical tactics
      • Fire
        • Single trees felled, bucked, and burned on stump
        • Broadcast burning - unsuitable for direct control
          • Crown fires required for significant beetle mortality
      • Debarking
      • Harvesting and processing
        • Single trees
        • Stands - sanitation logging
        • Trap trees (note: spruce beetle, Douglas-fir beetle only)
          • Leave highly susceptible trees/logs behind for beetles to infest, followed by prompt removal and destruction
      • Semiochemical tactics (in combination with above)
        • "post-logging mop up" -- aggregation pheremones in residual trees after logging
        • "Containment and concentration" - aggregation pheremones in harvest blocks before logging
        • "Push-pull" - anti-aggregation pheremones in unharvested areas, aggregation pheremones in harvest blocks

Management Theory

Direct control: a framework for success

• Rate of Increase (R) = 2 == roughly 500 trees killed by year 10
  • 190 rees saved by removal of 3 of 8 infested trees in year 4
    • Population continues to increase though
• Control must exceed rate of increase
• P greater than 1-1/R (R = rate of increase) (P = proportion of trees treated)
• How much is enough?
  • Use credit card payment formulas

Examples of success

Banf, Alberta - 1940's

• Incipient MPB epidemic
• Entomologist + very large crews
• All trees surveyed over 2 years, infested trees destroyed
• Epidemic suppressed by year 3

Take home messages

• Successful direct control strategies require detailed knowledge of:
  • Population size (and location)
  • Potential rate of increase
    • Look under bark and record how many eggs laid by females
    • Alternatively, look at spread
  • Direct control tactics must:
    • Be early and aggressive
    • Treat more than the anticipated growth in the pop'n given its size and rate of increase
    • Persist as long as causes of pop'n increase remain operative (or until management objectives are achieved)