That's why it helps to have some sort of insect repellent handy to keep mosquitoes, flies, and other unwelcome bugs at bay. If you still have a stray bottle or two at home from last summer, you may be wondering: Does bug spray expire? Though insect repellent may not have a listed expiration date, the summer staple does lose its effectiveness over time. Ahead, we talked to pest experts who answer all our questions about old insect repellent.