There are many potential factors  that affect biodiversity pollinator abundance and diversity in particular, and different environmental drivers rarely act in isolation. Interactive, non-additive effects, where one sub-lethal driver increases the severity of another driver, can help explain ongoing declines in wild and managed pollinators. Potential factors can be :  pests and pathogens, environmental stressors such as lack of quality food resources, agrochemicals, apicultural mismanagemet etc. But in my opinion,  all of these factors have strong interactive effects rather than when they act in single. Here is one paper that explains about the interactive effects.

http://www.sciencedirect.com/science/article/pii/S0169534710000364

Dear Ramzi Mansour

Viral diseases.Pls. find the attached files

http://www.tandfonline.com/doi/abs/10.1080/0005772X.1996.11099306

http://articles.extension.org/pages/71172/honey-bee-viruses-the-deadly-varroa-mite-associates

http://caes2.caes.uga.edu/bees/disorders/viral-diseases.html

http://link.springer.com/chapter/10.1007%2F978-3-642-17815-3_6#page-1

https://www.sciencedaily.com/releases/2016/02/160204150617.htm

http://onlinelibrary.wiley.com/doi/10.1111/j.1744-7348.1967.tb05920.x/abstract

http://www.academia.edu/download/39877490/A_metagenomic_survey_of_microbes_in_hone20151110-20304-1y51ixt.pdf

Hoping this will be helpful

Regards

Prof. Houda Kawas

 That is lots of information Dr Kawas. Thanks for sharing. Yes, viral diseases are one of the most that takes its fair share to cause pollinators decline. But if we look at the micro scale analysis, I think use of pesticides, apicultural mismanagement, habitat fragmentation for agricultural practices such as monoculture, altering habitat such as converting natural prairies to agricultural field in combination might have multi folded the effects to pollinator decline.  

you most welcome

yes, but the changes in the last few years in different country not pesticides or monoculture...that was decades ago, may its the accumulation of mismanagement

Regards

Houda

 Yes! agreed. Thanks.

Dear All,

According to a recent EPA report possible causes of Colony Collapse Disorder of honey bees may be:

- „Increased losses due to the invasive varroa mite (a pest of honey bees).

- New or emerging diseases such as Israeli Acute Paralysis virus and the gut parasite Nosema.

- Pesticide poisoning through exposure to pesticides applied to crops or for in-hive insect or mite control.

- Stress bees experience due to management practices such as transportation to multiple locations across the country for providing pollination services. 

- Changes to the habitat where bees forage.

- Inadequate forage/poor nutrition.

- Potential immune-suppressing stress on bees caused by one or a combination of factors identified above.”

https://www.epa.gov/pollinator-protection/colony-collapse-disorder

I propose to read the following paper and other information:

http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0070182#authcontrib

https://www.epa.gov/pollinator-protection/stakeholder-conference-bee-health

https://www.ars.usda.gov/news-events/news/research-news/2015/bee-survey-lower-winter-losses-higher-summer-losses-increased-total-annual-losses/

 Great information Dr. Andras! Yes, interactive effects caused by those factors caused the huge loss in pollinator diversity. And obviously, it is very important to quantify which factor contributed how much loss at what location. Which could be different in space and time.

Dear all, @ Dr András Bozsik, @ Dr Houda Kawas and @ Dr Chet Bhatta,

Many thanks for your very interesting contributions and key links you've provided. It's clear that  there are cumulative impacts of several biotic and abiotic factors resulting in bees's decline. But do you think that there are from a side MAJOR factors and from another side MINOR factors behind the occurrence of such alarming decline in bees? The impact of a factor A would not necessarily be similar to the impact of a factor B. This may be linked for example to the nature of the agro-ecosystem (geographical localisation of the country) where bees occur.

Best,

Ramzi

Dear Ramzi, 

Yes there are many factors that contribute to the pollinators decline. I'm gonna talk about honey bees which is my research field  . Insecticides in particular pose the most direct risk to honeybees. The observed, sub-lethal, low-dose effects of insecticides on bees are various  for exemple physiological effects, (i.e. the time required to reach adulthood), and malformation rates (i.e. in the cells inside the hive), perturbation of the foraging pattern, interference with feeding behaviour,

impacts of neurotoxic pesticides on learning processes  and many others. Now research are focused on the effects of pesticides on drone fertility and queen fecondity 

https://www.researchgate.net/publication/307577202_Drone_exposure_to_the_systemic_insecticide_Fipronil_indirectly_impairs_queen_reproductive_potential

And to answer your question: in my opinion, the suspect number one  are pesticides. Bees exposed to pesticides become fragile and susceptible to attacks by pathogens and viruses.

I hope this will be helpful 

Regards 

Faten

Article Drone exposure to the systemic insecticide Fipronil indirect...

Hi dear Faten,

Thank you so much for your high quality contribution in this discussion, I’m pleased to hear from you and I hope you are doing well !!

Yes absolutely, I agree with your statements, some pesticides, but mainly neonicotinoids (neurotoxicity issues), proved to be harmful to bees (honeybees, bumblebees: considering both lethal and sub-lethal adverse effects) and this should be taken into account (as MAJOR factor inducing bee decline) when using them for managing target pests. Many thanks for sharing your interesting article on non-target effects of Fipronil on drone fertility and queen fecundity (two key biological parameters indeed), there are quite original data inside, great !!

Best regards,

Ramzi