Bee Killing Chemicals?

This morning I read a story in the Sydney Morning Herald, "Bunnings to pull pesticide allegedly linked to bee deaths".  According to the story, cans of Yates "Confidor" which have been stocked by the homewares and hardware giant Bunnings, contains neonicotinoid, a class of compounds used as an insecticide that some studies have suggested affects bee's navigation and immune systems and ultimately leads to the death of the bee colony. Indeed, a story on this appeared in New Scientist in July 2017 in which Dave Goulson at the University of Sussex, UK, is quoted as saying, "Although the field trial results varied between countries, the overall evidence points to harmful effects for bees. I think you’d have to be pretty unreasonable at this point not to accept that, at least some of the time, these chemicals harm bees when used in normal farming practice.”  The same story quoted Richard Schmuck of Bayer, one of the makers of this class of insecticides, as saying, "We remain confident that neonicotinoids are safe when used and applied responsibly."

As the name neonicotinoids suggests, these are "new" molecules based on the molecular structure of nicotine shown below:

 Nicotine has been used as a pesticide for over 200 years. It is found lots of plants.  Up to 3% of the mass of the tobacco plant is nicotine, and trace amounts of nicotine are found in vegetables like eggplants, potatoes and tomatoes. When used as a pesticide,  it degrades rapidly in the environment and is not very selective so it is not really a good pesticide. For instance, a dose of 1mg per kg of body mas can kill a human.
Development of neonicotinoids began in the 1980s by Shell and1990s by Bayer. Neonicotinoids are generally less toxic to birds and mammals than they are to insects, and, some of the breakdown products are also toxic to insects, this is why they can be used as insecticides.
Consider the structural formula of imidacloprid, an example of a neonicotinoid and one of the most widely used insecticides:

 Imidacloprid was patented by Bayer in 1985 as the first commercial neonicotinoid. Traditionally insecticides were coated onto plants, "crop dusting", but neonicotinoids like  imidacloprid are water soluble and break down slowly in the environment so they are absorbed by plants. Bees are exposed to these compounds in the plant's pollen.
The early 2000s saw the introduction of two other neonicotinoid compounds;  clothianidin and thiamethoxam. 

clothianidin	thiamethoxam

Clothianidin can be used as a spray, dust or injectable liquid, depending on which plants it is being to protect.
In 2013 the European Union restricted the use of imidacloprid, clothianidin and thiamethoxam on crops that attract bees.

References:
http://www.smh.com.au/national/bunnings-to-pull-pesticide-allegedly-linked-to-bee-deaths-20180113-h0htzq.html
https://www.newscientist.com/article/2139197-strongest-evidence-yet-that-neonicotinoids-are-killing-bees/

Suggested Further Reading
IUPAC Nomenclature:  http://www.ausetute.com.au/namctut1.html
Introduction to Functional Groups: http://www.ausetute.com.au/fungroup.html
Molecular Formula: http://www.ausetute.com.au/molecularformula.html
2-Dimensional Structural Formula: http://www.ausetute.com.au/structural2D.html
Skeletal Structural Formula: http://www.ausetute.com.au/skeletal.html
 Introduction to Polarity of Molecules: http://www.ausetute.com.au/molpolar.html
Intermolecular Forces and Solubility:  http://www.ausetute.com.au/intermof.html
Aqueous Solutions (water as a solvent): http://www.ausetute.com.au/aqueous.html

Suggested Study Questions:

1. Use molecular model kits to build models of  the following molecules:
   • nicotine
   • imidacloprid
   • clothianidin
   • thiamethoxam
2. Draw a 2-dimensional (full display) structural formula for each of the following molecules:
   • nicotine
   • imidacloprid
   • clothianidin
   • thiamethoxam
3. Write the molecular formula for each of the following molecules:
   • nicotine
   • imidacloprid
   • clothianidin
   • thiamethoxam
4. Consider the nicotine molecule. Do you expect it to be soluble in water? Explain your answer.
5. Consider a molecule of imidacloprid. Do you expect it to be soluble in water? Explain your answer.
6. What property of clothianidin enables it to be used as a spray? Explain this property in chemical terms.
7.  What alteration to the structure of a nicotine molecule could you make so that it would become more soluble in water?
8. Carefully compare the structure of clothianidin and thiamethoxam (the molecular models you built could be useful here). In what ways are the molecules:
   • similar
   • different
9. Considering only the structure of nicotine and the neonicotinoids in this article, explain why nicotine might be more toxic to humans than the neonicotinoids.
10. Compare the quotes from Dave Goulson and Richard Schmuck in the article. In what ways are the two quotes:
    • similar
    • different
11. Imagine you have been asked by your government to decide whether or not to ban the use of neonicotinoids in agriculture. What would you advise? Explain why.