Martin, If the season has been abnormally hot and dry, bumblebees and other bees forage on a wider variety of flowers due to low or inaccessible nectar, then bite them to access the nectar if it's too low to reach "properly". One or two things are happening causing the nectar to refrain from raising up the floral tubes, and becoming accessible to bees. First, the heat could be evaporating the nectar. Very few plants have a defense against nectar evaporation. The other thing that may be happening, if there is drought or for some reason water is not as available to the plants, is nectar production could be slowed or even stopped completely. The only plant I know of that resists nectar evaporation is Echium, having dense hairs (trichomes) in the floral tube, and being good nectar producers, even in drought. 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. Travis Owen Rogue River, OR amateuranthecologist.blogspot.com http://www.pacificbulbsociety.org/