Temperature-Driven Development
Insects are ectotherms — their body temperature, and thus their metabolic rate and development speed, track environmental temperature. Unlike mammals, insects cannot maintain a constant internal temperature. This means that a caterpillar developing in a cool spring takes weeks longer to pupate than one in a warm summer, even though both traverse the same developmental program. The degree-day model captures this temperature dependence with elegant simplicity.
The Degree-Day Concept
Below a species-specific base temperature (T₀), development halts entirely. Above T₀, development rate increases linearly with temperature (up to an upper lethal limit). The total heat units needed to complete a life stage — measured in degree-days — is approximately constant regardless of the actual temperature trajectory. A codling moth always needs about 600 DD above 10°C to go from egg to adult, whether those DD accumulate over 30 warm days or 60 cool ones.
Predicting Emergence
By tracking daily temperatures and accumulating degree-days from a known starting point (biofix), entomologists can predict with remarkable accuracy when pest populations will reach specific developmental stages. First-generation egg hatch, peak larval feeding, and adult emergence can all be forecast days to weeks in advance. This transforms pest management from reactive spraying to precisely timed interventions that maximize efficacy and minimize environmental impact.
Climate Change Implications
Rising temperatures accelerate degree-day accumulation, compressing development time and potentially adding extra generations per year (voltinism shifts). Species previously limited to one generation in northern latitudes are now completing two — doubling their reproductive output and expanding their range. The mountain pine beetle’s shift from a 2-year to 1-year cycle in British Columbia contributed to the largest bark beetle outbreak in recorded history, killing over 18 million hectares of forest.