If i were to leave a watermelon in my backyard for say, 9 hours overnight in western Canada during autumn, would it attract rats or lead to other negative effects? I'm just thinking because it has the rind I don't know that wildlife would be able to smell it. If it would lead to a negative experience in anyway, is there some kind of spray I can smear it with to detract critters?
If a watermelon wouldn't work out my plan would also work with a cantaloupe or really any other melon.
If it's intact and undamaged it should be fine. After all they're grown outdoors (in your climate and mine related species like pumpkins may do better). Growers do suffer some losses but that's over the whole growing season. If you do attract rodents or similar creatures, they're not coming from far away, so it's not like you'd be bringing them into the neighbourhood with a one-off treat.
If it's cut, it's better to put it in a sealed plastic box. Birds, slugs, and snails are likely to go for cut fruit. If a frost is likely, better leave it indoors.
I would avoid spraying it with anything unpleasant - even if you wash it afterwards there's a chance of being able to taste it, either from transferring it, or a little getting in through the skin
I can answer only for northern Virginia, which I suppose is warmer than western Canada. If I left an intact ripe fruit on my patio overnight, it would not be intact by morning. A watermelon might be more intact than a cantaloupe, which, when ripe, has an aroma that says "Eat Me" even to creatures with a poor sense of smell, such as humans.
But I have raccoons, foxes, and the occasional coyote in addition to many small nibblers. No rats. And no bears, which would make quick work of fruit left outdoors.
Point of curiosity -- why do you want to leave the fruit in your backyard?
