https://phys.org/news/2018-05-cool-weather-amplify-tree-killing-bark.html

In the study, the Dartmouth research team shows how the colder fall and winter temperatures encountered in northern latitudes influences the growth and development of immature southern pine beetles, leading to a more synchronized emergence of adults once the weather warms.

The behavior detailed in the research raises the risk for pine forests because the emerging beetles kill trees by attacking in large numbers. The more beetles that are active at the same time, the better the chance they will overwhelm tree defenses and produce even more beetles for the next generation of attacks.