>I suppose common sense would also suggest that bees foraging on poisonous
plants such as nicotiniana, atropa, etc. would encounter poisonous nectar.
Which they don't.

We should here define what one means by the term "poisonous" (Definition: (of
a plant or substance) Causing or capable of causing death or illness if
taken into the body).

The nicotine in the nectar of nicotiana would certainly be considered to be
a "poison," yet at the dose given is actually preferred by bees.    The
whole discussion about the neonics centers on the dose.

 >Nature is pretty good at segregating the toxins to the leaves and making
the nectar and pollen generally wholesome.

Jerry Bromenshenk's  research (in prep) has found that canola and melons
put lower concentrations of clothianidin in the nectar than in the leaves.
 But not all plants to the same with all allelochemicals--some concentrate
them in the nectar to the degree that they are indeed toxic to some
pollinators.

>
> >A plant which killed pollinators, would either die out or evolve into a
> self pollinator
>

Actually, there is a third route commonly found in nature--a pollinator
resistant to that particular toxin evolves to pollinate that plant.  The
plant benefits in not having to share its nectar or pollen with other
pollinators, and ensures that its dedicated pollinator pollinates it more
efficiently.

>
> >Hence, plants such as brassica species can be good sources of honey and
> pollen for bees, despite expressing insecticidal compounds in the leaves.


We do not normally think of brassicas (the cabbage/mustard family) as being
poisonous plants, but they are:
"Brassica oleracea includes common cultivated crops such as kale, broccoli,
Brussels sprouts, and cabbage. All these vegetables are capable of forming
toxic quantities of SMCO, a chemical that can cause hemolytic anemia in
livestock. These plants also contain glucosinolates, which can cause
goiter. In general, these widely used vegetables are safe for human
consumption."

"Poison" is all about the dose.  Amazingly, some humans intentionally
consume some of the most poisonous parts of these plants in mustard seed
and prepared mustard! (I recklessly do myself, heedless to the danger).

At a low dose, a potential poison may (as Allen suggested) have desirable
effects.

"Many vegetables considered beneficial to human health contain the amino
acid derivative, S-methyl-Lcysteine
sulphoxide. This plant secondary metabolite functions as a phytoalexin with
strong antimicrobial
activity preventing pathogenesis in growing plants. The potent biological
activity of S-methyl-Lcysteine
sulphoxide [SMCSO] was first identified in ruminant animals as a toxicant,
where it was demonstrated to
be the ‘kale anaemia factor’. Contrary to this toxicity, scattered reports
in the literature have since shown
anti-carcinogenic, anti-diabetic and cardiovascular effects in experimental
animals."

"There exists a general agreement that vegetables containing SMCSO,
consumed in
moderation, are beneficial to health, but the presence of glucosinolates
has dominated the scene. Any potential contribution
arising from SMCSO appears to have been overlooked or disregarded
as unimportant. However, this may change. Preparations
from vegetables (including broccoli and cabbage)
containing SMCSO (15–400 mg) have been patented already as
drugs and health foods and drinks for lowering serum cholesterol
levels." Toxicol. Res., 2013, 2, 11

We must all remember that the line between being a poison and being a
medicine is all about the dosage.

-- 
Randy Oliver
Grass Valley, CA
www.ScientificBeekeeping.com

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