The following insects affect the rope as indicated.

Ants: When the fire touches an ant, the flame will change color to match the burned insect. This is how colored ropes are burned, as the flame color must match the rope color.
Beetles: Must be burned with a matching flame color to count towards bonus points.
Firebugs: When the fire touches a firebug, the insect will explode and scatter flames around it. This is how isolated strands of rope are burned.
Lightning Bugs: When the fire touches one of the lightning bugs, its partner will start a new flame of the same color.
Water Bugs: When the fire touches a water bug, it will be extinguished, possibly ending the level.