This line is the first half of a commonly quoted joke from The Simpsons. It appears in the sixth season episode Bart's Girlfriend (2F04), in which Bart becomes enamoured with Jessica Lovejoy, the local reverend's daughter. He tries to clean up his act in order to impress her, but it turns out she's just as much (if not more) of a ne'er-do-well as he is.

In one scene, Jessica teases Bart by holding his hand and then pulling the fire alarm with it. As students and teachers rush into the hallway in order to evacuate the building, Groundskeeper Willie also rushes out exclaiming "If I don't save the wee turtles, who will?"

He breaks down the door of a classroom and enters, but rushes out seconds later, turtles attached to every part of his body.

"Ack! Save me from the wee turtles!" 

The joke is a classic example of Simpsons-type humour: it begins as something that's reasonably funny in its own right, and once the viewer thinks the joke is over it expands into something funnier. It also gives us a glimpse into the character of Groundskeeper Willie, because he's more concerned about the school's pet turtles when children are potentially in danger.

The joke is often referenced in popular culture, even inspiring the name of a rock band called the Wee Turtles. A quick Google search using the quote suggests that it is not quite among the most popular quotes from the series; it yields 104 results ("I call him Gamblor!" brings up 4,880, "My cat's breath smells like cat food" yields almost 22,000), which is still reasonably respectable. And besides, there aren't many occasions for people to use a line about wee turtles outside of the Simpsons context.

It's just funny. 

BrevityQuest07 

References:

http://www.answers.com/topic/the-wee-turtles
http://www.google.ca/search?q=%22if+i+don%27t+save+the+wee+turtles,+who+will%3F%22&hl=en&start=0&sa=N
http://www.lardlad.com