A large amount of butterflies is causing great lag. Anyway to remove/kill them? Please help

[Barend]

Hello there.

I have an issue with an excessive amount of butterflies, which are causing lag. (Singleplayer)
http://i.imgur.com/VTHC6cm.jpg (look at the minimap)

I had been working on a project in minecraft far away from my base and when I came back I found these butterflies (close to my base). My world is lost unless I can get rid of these butterflies, as the lag is unbearable. Whenever I hit a butterfly with a weapon it takes over a minute for the damage to register. Therefore manually killing the butterflies is no option. Nor can I easily use explosives since the butterflies are located above water.
How can I get rid of them? Is there some kind of kill command or can I do something in the save files that deletes all the entities?

Please help

 

[UniZero]

I think you can use mcedit to remove them.

 

[Barend]

I think you can use mcedit to remove them.

Yeah I thought about that but the last time I used MCEdit (a while ago) it messed up blocks. e.g. Turning rubber logs to nether fences I think. I'm not sure how that in handled in the most recent build.
Anyway, I came accross a mod called singleplayercommands which includes a command (/killall) which kills all mobs. There are however 2 downsides to it.
1. It kills everything. 2. It sort of semi-crashes when the command is used (no interaction in the world, however, moving is possible). Simply exiting the world and rejoining fixes it, plus the mobs are gone (it works only on the loaded mobs so I had to repeat it a couple of times).
But it was definitely worth it, as a managed to get rid of the massive amounts of butterflies that had spawned (there were way more than can be seen in the screenshot).

Thank you for the suggestion.