I have a world of perpetual, consistent, and evenly distributed "daylight" (for purposes of this question, you don't need to worry about how this would be achieved). Left to its own devices, living on this world would be a lot like living in the temperate zone on Earth, but with no day & night, no winter & summer, no tropical & arctic zones, etc. But then an advanced race introduced biologically engineered organisms that are able to manipulate the world into having a summer/winter like cycle similar to that of Earth.
These organisms are divided into two categories: "summer fairies" and "winter fairies". They alternate between ~6 Earth months of activity and ~6 Earth months of hibernation and they each do something different to the environment of the world to raise or lower its temperature by an average of 12-14°C to bring the appearance of Earth like seasons. Summer and winter fairies do not necessarily have to be exactly 2 species. A symbiosis of species working together to make each season happen is okay, but I would like the process to require some action on the part of your stereotypical looking fairies (small, insect-winged, intelligent humanoids).
The mechanism I am looking for should not rely on the mechanics of the world itself; so, if you put the fairies and any symbiotic organisms they need onto another normal world, they should be able to influence the seasons there as well.
I am looking for a more-or-less scientifically sound explanation as to what these fairies are doing to control the seasons.