My assumption is that the Hot Shot killed many of the insects and the dentist would remove them from his shed roof, allowing for close-up inspection.

If there are any still around, post a picture or a more complete description.

No photos are available, but he described them as being spotted. My questions: are there any hornets or wasps who display this behavior this early and why?

Since these insects have not been definitively identified as wasps or hornets, it is hard to speculate about the behavior.

It is not wasp or hornet behavior when hundreds or more of them react meekly to a single stream of poison spray, unless the temperature was quite cool.

Swarming insects of any type can focus on unlikely objects, at least temporarily. If the swarm found itself out in marginal temperatures, it might have chosen the shed for an overnight roost, that metal surface likely being the warmest around.

If the previous home of the swarm was disturbed, that could explain the out-of-season activity.

This post was edited by larry_gene on Sun, May 5, 13 at 22:54

Thank you for that explanation. It does seem the most likely one.