Flying bugs often can’t fly when their wings are wet. They go in for a drink, get wet, then can’t get out. If there were some dirt on the edge, they could swim out and dry off, but a glass isn’t made of dirt, so it’s harder to get a grip on vertical glass or glazed drinkware.

Wine and sweet drinks are even worse, because the bugs are attracted to the scent, and they often don’t have as much surface tension as water, which means the bug can’t just land on it, and when they fall in the liquid fills their respiratory system (basically little holes in their exoskeleton).