According to this arstechnica article fruit flies lacking (or with extremely low levels of) dopamine suffer from extreme lethargy, and a type of masochism!
Flies that apparently lack dopamine signaling manage to live just as long as their peers. They do, however, end up turning into lethargic masochists. [...]
However, they’re nowhere close to normal. Compared to their normal peers, they were very lethargic, moving relatively little through the course of their normal lives, and not expending the effort to move out of the path of a mild electric current. They spent a lot of their time in the fly equivalent of sleep, even during daylight hours, when they are normally active. Despite the lethargy, however, caffeine was still able to give them a kick start, suggesting that there’s an intact activity control that is simply not getting the sorts of cues it needs without dopamine around.
The same sorts of issues showed up when feeding and visual activity were tested. The flies that lacked dopamine ate only about a third as much as their normal peers and weren’t interested in sugar water even after having been starved. They were still able to respond to sugar and eat, however, which suggests that the animals’ brains still have a feeding capacity that ends up going unused without dopamine. The flies also had normal vision and could use it for spatial orientation, but would no longer move towards light (a phenomenon called phototaxis).
The weirdest effect, however, came when the flies were tested for learned aversion, in which an electric shock is associated with a particular odor. Instead of learning to avoid the shock, however, the dopamine-deficient Drosophila ended up being attracted to the odor, hence the authors’ use of the term “masochistic” to describe the flies’ behavior. (via.)