I think I read here somewhere that when the weather gets hot, nectar is produced in the mornings and evenings so the plants can still attract pollinators, but can conserve their energy by not producing nectar during the heat of the day.
I don't know what the temperature break is, though it's something I want to research over winter.
We had some problems with that a couple summers ago in areas that did not have much surface water. It caught a lot of new beeks off guard to find that even through the backberries were blooming, there wasn't a lot of nectar for the bees. Feeding in July wasn't even on their radar and there were several losses. We all look for it now.