Do lupins produce nectar? According to this study it seems like there are conflicting conclusions regarding this. And in this study the supposed inability for Lupinus argenteus to produce nectar is utilized to study whether the addition of nectar changes pollinator behaviour.
If the answer to this question is no, then why do these bumblebees insert their tongue inside the base of the flower, because it certainly looks like that is what they are doing. Do they do so regardless of whether a flower contains nectar or not, just to verify whether there is any present in the flower? But if pollinators only visit lupins for pollen to eat then why do they stick their tongue inside the banner where it seems to me that there should be either trace amounts or no pollen at all.