Pest insects can be unpredictable, arriving in unexpected places yet failing to show up in regions where they ought to thrive. The famously defensive Africanized honey bees, for example, took more than a decade to move into Florida after establishing in nearby Texas.
Argentine ants (Linepithema humile) are a subtropical species from flood plains in the warmer, wetter regions of Argentina. Yet a persistent mystery about their world conquest has been their success in dry Mediterranean climates and in more northerly locations than predicted by their South American range. Now, though, a paper by Brightwell & Silverman in Environmental Entomology has answered the northern mystery, at least in part: native pine trees give the ants a winter refuge.
Figure one from the study shows pine trees maintaining high levels of ants during the winter while other trees lose them:
Apparently, those tall trunks are ideal for capturing winter sunlight. The bark stays warmer than ambient temperatures, creating a livable micro-climate, and ants continue foraging along the trunks throughout the winter.
These sorts of microhabitat effects are hugely important, especially for small organisms like ants. For those attempting to make future range predictions from crude climate data (as though I would ever do that!), these quirks of the real world are worth bearing in mind.
*update* smart comments over at G+.
source: Brightwell, R.J., Silverman, J. 2011. The Argentine Ant Persists Through Unfavorable Winters Via a Mutualism Facilitated by a Native Tree. Environmental Entomology, 40(5):1019-1026. 2011.