Warmer years give insects more generations, not just faster ones
A spruce forest under attack from tiny beetles shows how a warming climate lets insects fit more life cycles into a single year.
Heat speeds development, so insects squeeze in extra generations
Insects are ectothermic, meaning their body temperature and rate of development are set by the surrounding environment.
Warmer temperatures speed up their development, so each life cycle finishes faster.
When cycles finish faster, a species can complete more generations per year, an increase in its voltinism (the number of generations produced in a year).
A species that once fit one generation into a year may now fit two or more.
Example
The spruce bark beetle (Ips typographus) often needs a full year per generation in cool climates.
In warmer conditions it can complete a generation faster and fit a second one into the same year.
Synchronised emergence turns many small broods into one large outbreak
Warmer temperatures also synchronise insect populations, so adults emerge together in simultaneous waves rather than staggered trickles.
A single synchronised wave can overwhelm host defences that scattered emergence would not.
Example
On a slower cycle, beetle emergence is staggered and each brood does limited damage.
On a faster cycle they emerge in synchronised waves that overwhelm the trees.
Longer breeding seasons open a window for one more generation
Climate change extends the warm breeding season, giving many insects time to raise an extra generation before winter.
Where beetles once produced one generation per year, a longer season may let them produce two or more.
Mosquitoes: extra generations widen the spread of disease
Mosquitoes such as Aedes aegypti are vectors, organisms that carry a pathogen from one host to another, for diseases like dengue and Zika.
As temperatures rise, eggs hatch faster and larvae develop more quickly, so more mosquito generations are produced each year.
More generations raise the mosquito population density, which increases biting and the risk of disease transmission.
The result is a greater incidence of dengue, Zika, malaria and other mosquito-borne diseases, including in areas that were previously too cool.
Spruce bark beetles: extra generations drive heavier forest damage
The spruce bark beetle (Ips typographus) is a pest that bores into spruce trees and damages forest ecosystems.
Warmer winters bring fewer frosts, so more overwintering beetles survive to breed the following spring.
Warmer summers let the beetles complete an extra generation within the year, which multiplies their numbers and the damage they cause.
Trees weakened by repeated attacks are more prone to disease and death.
Note
From one generation to two
In cool conditions the beetle typically manages one generation per year.
Warmer conditions add a second generation, so far more beetles are produced in the same year.
Note
Heat is not always a help
Warmer temperatures speed insect life cycles, but extreme heat can also be lethal.
The balance between faster development and heat stress decides the overall effect.
Active recall
Check your understanding
Why does being ectothermic make insect development sensitive to temperature?
What does an increase in voltinism mean for a population over a year?
How does a longer breeding season let an insect fit in an extra generation?
How do extra mosquito generations raise the risk of dengue, Zika or malaria?
How does a warmer year let Ips typographus damage more spruce trees?
