Ron, I can't ID these but this may be interesting. On May 24th, I posted what I think is the same beetle. I hope you don't mind ... I enlarged yours a bit to compare and they look 'pretty darn close' to me. I was told the leaf or plant they were on was important, which I could not do. The alder you mention may help others to identify it, so I'll be very interested if anyone can ID yours further.
edited ... just checked yours looks closer to this one in Denis' album than mine:
Hi Ron and Jimmy,
I think Jimmy is right. I also believe this is the same beetle of images of mine that have been ID'd on Bug Guide as Calligrapha rowena. BUT, I think they may indeed be Calligrapha vicina rather than rowena. This issue is far from resolved. I will need some expert help at some point from a beetleologist (aka an entomologist who really knows beetles, maybe Reggie Webster or Chris Majka). I am posting two links for you to check out on Bug Guide, one of my pics and one of Tom Murray's from Massachussetts:
"Calligrapha rowena DDoucet"
Thanks for your input. However, I still think that C.confluens is more likely IF the host plant is a reliable (although not fool proof, I am sure) indicator of the species.
The beetle was definitely found on an alder, and going by memory, there was no Cornus species in the immediate vicinity. According to Chris Majka's key (http://www.chebucto.ns.ca/environment/NHR/Calligrapha.html), three Atlantic Canada species occur on alder: C. confluens; C. alni; C. alnicola. Of these three species, C. confluence appears to be a very good match, much better than C. alnicola and quite different from C.alni, of which I also took a picture at the same location.
However, I have to admit that I considered "only" the three species identified as occurring on alder in Chris' key. In other words, I made no attempt to eliminate similarly coloured species such as C. rowena and C. vicina, which, while much more closely associated with dogwood, could perhaps occasionally be found on a plant other than their typical host(?).
In any case, it is fun trying to sort out through these and get input from others.
All the best,
Fair enough :) Some of my specimens were defintely photogrpahed on Dogwood so would most likley be another species with this philosophy, for sure.
Also, it is interesting to note that several experts on Bug Guide (including Chris Majka) also commented on the image of mine that Jimmy linked above for Calligrapha confluens. Check out that discussion on this link from Bug Guide: