The researchers were investigating whether or not fungal glow follows a set schedule. Their bioluminescence, which relies on chemical processes inside the mushrooms' cells and presents itself in different parts of the mushroom -- sometimes the thready mycelium that shoots through the dirt like a web of roots, sometimes the fruiting body of the mushroom itself, and sometimes the spores that float off on the wind -- is obviously more visible at night. You don't get much fun out of a glow stick cracked in broad daylight, after all.
But it turns out that in some species, the luminescence is actually timed around the evening so that the chemical reactions that produce glow aren't wasted during the day. That suggests it's not just a beautiful evolutionary fluke: If mushrooms budget their bioluminescence, it probably serves some greater purpose. And given how attractive these beacon-like glows are, it seems reasonable to start with the hypothesis that mushrooms are making a "come hither" gesture.
To test that, the researchers created fake mushrooms -- baited with green LED lights and rigged with sticky traps -- and put them in the same Brazilian forest as the real mushrooms they were studying. They also put the same sticky fakes in without any lights. As expected, many more insects ended up stuck to the glowing traps.
Mushrooms generally aren't in the business of trapping bugs. But because the dusty spores that allow them to reproduce can be carried away to new breeding grounds by animals and bugs, mushrooms have other reasons for wanting to look pretty to flies and beetles. This study helps support the idea that mushrooms glow in order to become a prime landing target for such creatures, and it follows that the evolutionary benefit of spore distribution would be a big motivator in learning how to shine.