My understanding, from whatever nature TV show when I was younger, is that the beetles don't actually die. They literally freeze in ice. I also remember reading that certain countries are releasing tens of thousands of birds (possibly hummingbirds?) to deal with them as well. I don't have links so possibly grain of salt here.
If you look at California as an example, you can see hundreds of acres of trees that are dying year over year. I remember going camping in lush green forests that are all gone now. I'm in the PNW now and from what I've seen and heard from others the summers are getting longer and hotter so it's only a matter of time before what's happening to California comes here.
EDIT: From what I remember as well, every tree comes with defenses to keep beetles away. This is why trees are sappy. In order to produce sap the trees need water, and they produce their own sugars through photosynthesis (sap being a mix of water and sugar). Trees in certain areas are adapted to the length of their winters. A longer winter means more time without dealing with beetles but might also mean less time spent in dry soil.
> For example,
an under-bark temperature of –37°C will
kill 50 per cent of a mountain pine beetle
population, even in mid-winter; however, a
low temperature of –20°C in the fall, be-
fore the beetles are prepared for winter,
or in the spring, when beetles are start-
ing to become more active, will also kill
beetles if it is preceded by temperatures
above 0°C. The relatively warmer temper-
ature causes the larvae to start to lose its
natural antifreeze.
