<The available experimental evidence now appears to be sufficient to accept lithium as essential >

Yes, as a naturally occurring **micro-nutrient** in the environment. But as the author of your citation mentioned:

"The biochemical mechanisms of action of Li appear to be extraordinarily complex, multifactorial and strongly intercorrelated with the functions of other elements, drugs, enzymes, hormones, vitamins, growth and transforming factors. "

The only impact on bees that was studied was dose toxicity, ie, looking for changes in mortality rates. That's the lowest bar for safe chemical/drug/pesticide use.  Additional studies seem warranted for other potential effects above normal environmental micro-doses for bees.  
As the Nature article about lithium as an acaricide pointed out:

"Currently we do not know how LiCl is killing the phoretic mites, and there are few publications on the effect of LiCl in insects ... Field tests are just as necessary as analysis of sublethal and long-term side effects on adult bees and brood ..."

If longitudinal studies could demonstrate that there was zero adverse physio- and neurological impacts, it could be the anti-mite "silver bullet",  at least until mites develop a tolerance to lithium-laced haemolymph. :-)  

Speaking of silver bullets, here's one pharmacologist's article that makes lithium sound like a veritable "wonder supplement" for humans;  not coincidentally, he's also marketing it <raised eyebrow>. 

Lithium as a Nutrient, Timothy Marshall, PhD, Journal of American Physicians & Surgeons, Vol 20 Number 4, Winter 2015
www.jpands.org/vol20no4/marshall.pdf

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