Why are Bug type attacks effective against Dark type Pokemon? This question actually originated from the games, but it applies to the anime/manga as well.

Water is strong against fire because water can extinguish fire. Electricity is weak against Grass because grass is grounded, which nullifies electricity.

What is the reasoning for Bug to be effective against Dark type?


   And I've already heard about the Malaria in Africa joke, it's not funny!
If I had to guess, it was because of game balance more than logic.

There are only two types that are super-effective against Dark.


Fighting
Bug


Neither of which make much logical sense. But it would be fairly safe to say that Dark would be overpowered if it only had one or no weaknesses.

Another argument to support the game-balance motivation is that bug types would otherwise be considered weak since they are super-effective against few other types.

So in short, they probably needed to keep Dark in check and make Bug more competitive.



This sort of balancing isn't particularly new. Psychic is a good example.

In the first generation of games, Psychic had only one weakness - Bug. Despite ghost also being super-effective against Psychic, there weren't any "real" ghost attacks in generation 1 (and only a couple of bug attacks).

So generation 2 added Dark types to keep Psychics in check.