>1. Do bees collect nectar or pollen from poisonous plants? We have purple nightshade and, I am sure, other dangerous plants.
Bees work nightshade. I have never heard of it being poisonous to bees to work it. They sometimes work California Buckeye and it kills them if they do, but they tend not to work it if other things are available.
> 2. If they do collect nectar from such plants and make honey from it will it kill them?
Some of them... yes.
> 3. If bees collect nectar from plants that have been sprayed with pesticide and make honey from it can it cause a die off such as I have?
Most pesticides that are sprayed are short lived (lessons learned from DDT)... but yes, they can bring them home and they can affect the hive for a while.
There are other things that cause a "failure to thrive". One would be skunks. Look for soggy piles of dead bees around the hives and evidence of animals trampling down the grass and sometimes scratches on the front of the hive. A failing queen, of course can be an issue. Chalkbrood or European foulbrood can set things back. Not everything is a full blown case either. Sometimes it's a low level. A high level of Varroa can spread viruses that can cause a failure to thrive...
Bees work nightshade. I have never heard of it being poisonous to bees to work it. They sometimes work California Buckeye and it kills them if they do, but they tend not to work it if other things are available.
> 2. If they do collect nectar from such plants and make honey from it will it kill them?
Some of them... yes.
> 3. If bees collect nectar from plants that have been sprayed with pesticide and make honey from it can it cause a die off such as I have?
Most pesticides that are sprayed are short lived (lessons learned from DDT)... but yes, they can bring them home and they can affect the hive for a while.
There are other things that cause a "failure to thrive". One would be skunks. Look for soggy piles of dead bees around the hives and evidence of animals trampling down the grass and sometimes scratches on the front of the hive. A failing queen, of course can be an issue. Chalkbrood or European foulbrood can set things back. Not everything is a full blown case either. Sometimes it's a low level. A high level of Varroa can spread viruses that can cause a failure to thrive...