The nematode is a body snatcher. The reason why these caterpillars turn red is to warn off predators. This parasite is considered a pesticide. Scientist for 12 years still haven’t been able to figure out why it changes color. The scientists discovered that some birds don’t eat caterpillars with the infected larvae. The way the nematode takes over is it enters via the mouth or anus and injects a lethal chemical that kills the host.
This article seems rather interesting. I wish I could learn how the nematode turns the host color red. It’s cool how birds take the red as a warning sign. Scientists should put more time into this to figure out how it turns red. For more info click here.