On a critical wing, eg Challenger, any contamination, on and behind the leading edge, of anything worse than quarter mm (fine sandpaper) can reduce lift by up to 50%. That includes bugs and sand as well as ice.
Keep it clean before flight.
Be aware of supercooled droplet icing, it's dangerous stuff and a number of manufacturers have issued Flight Manual amendments as a LIMITATION that if encountered you must depart it toute suite - how do you know you're in it? big clue when side windows start to ice over!