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D4.3.11 (HL)—Increased insect life cycles

Warmer conditions can shorten insect development, increase annual generations, and intensify outbreaks such as spruce bark beetle attacks on forests.

Syllabus
First assessment 2025
Objective
D4.3.11
Level
HL

Warming Can Add Spruce Bark Beetle Generations

HL only

Within their tolerance range, warmer seasons accelerate insect development and can increase the number of complete life cycles per year.

Spruce bark beetles such as Ips typographus or Dendroctonus micans can complete additional generations or attack periods when summers are longer and winters milder.

Warmer development period → faster egg–larva–pupa–adult cycle → more reproducing adults in the same year → more attacks on spruce phloem → greater probability that tree defences are overwhelmed.

A second annual generation can attack trees already weakened by the first, allowing fewer beetles to cause lethal phloem damage.

Warming is beneficial only within physiological limits; extreme heat, drought effects on hosts and predators can alter the outcome.

HL Timing and Selection

HL only

HL D4.3 adds timing and evolution. Phenology tracks when seasonal events happen; warming can desynchronize interacting species, shorten insect development, add generations, and change selection pressures so phenotype frequencies shift.

  • timing of flowering, migration, breeding, nesting or insect emergence
  • interacting species shift timing by different amounts
  • warmer temperatures can add generations and attack periods
  • milder winters alter selection and phenotype frequencies

HL Phenology and Climate Selection

HL only

HL climate questions require reasoning from timing to ecological or evolutionary consequence. The answer starts with a seasonal event or life-cycle stage, explains how warming shifts timing or selection pressure, and states the effect on interaction success, number of generations, attack periods, survival, or phenotype frequency.

  • Define phenology and identify the seasonal event being shifted.
  • Explain timing mismatch or faster development using named examples such as plant-reindeer, great tit-caterpillar, or spruce bark beetles.
  • Link climate-altered selection pressure to phenotype frequency change using the tawny owl example.

Concept essentials

  • Temperature can control insect development rate and reproductive timing.
  • More warm time can allow extra generations or longer attack periods each year.
  • Increased insect cycles can amplify damage to forests, crops, or disease-host systems.