If the nectar is inaccessible, low in the floral tube (but plentiful and reachable in most years with adequate moisture), then bumblebees will bite holes to reach it. Other bees then follow, often not having mandibles strong enough themselves to do it on their own. Repeated visits eventually destroy the flower. I seem to remember Grant mentioning this in Hummingbirds and Their Flowers, with respect to red-flowered plants whose floral tubes are too narrow for bees. Bob Nold Denver, Colorado, USA