The assumption is that manganese is available in much greater quantities these days and too much manganese has various adverse influences. There is a variety of evidence to support this model:

•         Manganese is more mobile than iron. As the pH of the soil rises, manganese will become relatively more soluble than iron. More manganese is then absorbed by plants. There is, therefore, more manganese in the pollen. This process plays an important role in the alleged adverse effects of neonicotinoids.
•         As a result of the higher concentration in the air. Manganese becomes airborne via exhaust gasses from cars and airplanes and from coal fires. Manganese is a component of catalysts added to petrol and kerosene.
•         As a result of the additives in animal feed. Manganese is added as manganese sulphate to almost all animal feed to stimulate reproduction, and to increase egg production. This enters the soil via manure.
•         In bacterial disease processes manganese is exchanged for iron. This is a fundamental and commonly occurring process. There are numerous diseases for which this has been demonstrated.
•  Manganese stimulates foraging behavior in bees (bees leave the hive), in particular for gathering nectar, rather than pollen. This has been demonstrated in experiments many years ago.

Manganese levels in bees can be found in the article Spatial and temporal variation of metal concentrations in adult honeybees http://www.ncbi.nlm.nih.gov/pubmed/21823048 or http://edepot.wur.nl/176086. The quantities manganese detected in bees were significantly higher than the norm. The manganese level in bees varies considerably depending on the location and the period in the year.

Administering an iron supplement leads to a better ratio of manganese and iron. This counters the presumed adverse effects of high levels of manganese (stimulating reproduction, reduced flight capacity in foragers, increased urge to forage).