Hi Suze
Having had bees on and in spitting distance of areas that use large amounts of weed Killer I’ve never had an instance of any direct deaths, I can’t answer for sub lethal or hidden aspects other than to say colonies thrived! That aside the pic shows dead drones any poisoning would not solely kill drones.
As others have suggested this pic is a common site if a queen excluder is incorrectly used and traps drone bees and prevents access to the outside.
Given you say the colony was in poor condition and the number of drones, is there a possibility the queen was a drone layer?
What leads you believe other than the dead drones this is a poisoning?
On the subject of supering add when your bottom box is 3/4 full. This season with your expanding nuc your single brood box may be sufficient for the queen to lay. However your bees will require both room for stores and room to lay. A point often missed by flow suppliers is that queens/bees will often require more than a single box to lay in! and perhaps even additional supers. That generally makes the costs considerable.
If your queen is laying on 7-8 frames early to mid season I’d suggest in the future a second brood.
Can I ask if you’ve attended a course, if not you and perhaps more importantly your bees will benefit.
Good luck Ian
I believe they have moderated the language since! But keeping bees and doing it well does involve knowledge and some practicality and I’d suggest if your bees are on an allotment some hands on guidance is essential!!
