A guy goes into the gas station and says, "I need a box of rubbers with pesticide."
The cashier said, "Pesticide? Don't you mean spermicide?"
The guy says, "No! My old lady has had a bug up her ass all week, and I am going to kill it."
Alright, listen up buttercup. This joke is about a dude who wants to off a bug that's been living in his wife's butt. Instead of asking for something that kills sperm (spermicide), he asks for pesticide, which kills bugs. It's funny 'cause it's a messed up way to solve a problem, and you're probably laughing 'cause you have the brain of a walnut.