Popular media tackling environmental problems can come off a little clumsy, a la Star Trek IV: The Voyage Home, which is generally good but has some distracting moments. Personally, environmental messages don't particularly resonate with me and proponents come off a bit shrill. "Darkness Falls" tries to be more evenhanded, presenting a character who is against eco-terrorism and environmentalists while the ultimate outcome of the episode is a firm message not to cut old growth trees.
Mulder and Scully investigate the disappearance of loggers in a remote forest in Washington. Eco-terrorists are the first suspects, but of course it's a supernatural explanation. The insects are scary and unlike the werewolves in "Shapes," they actually look cool. Stuck up on the mountain, Mulder and Scully are in imminent danger the whole time. There's no way to get down, dwindling light, and insects all around there. Boarding up the walls has no effect. Like in "Ice," being isolated and in danger riles up emotions and there are more than a few heated discussions.
In the end, they make it out alive, but come close to dying. As they drive down the mountain at night, the tires get punctured and the insects start swarming. Eco-terrorist, Doug Spinney, runs out and is swarmed and insects even come into the car and begin attacking Mulder and Scully before help comes.
The environmental angle doesn't quite work out and the pathos could have been pushed more, but Mulder and Scully are seriously injured in the episode and the stakes are possibly the highest they've been.