In this model, bee decline is considered a problem of an incorrect population structure. The ratio between young bees and old bees is wrong. A population model that establishes this is based on this theory. See http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0018491
There are many reasons for increased reproduction or high mortality among foragers. If the average age of the bees is lower, this can result in an incorrect population structure. For example, if there is a strong presence of Nosema, the average age of the bees may be reduced by 9 days (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3590174/pdf/pone.0058165.pdf). This can affect summer bees as well as winter bees. In the summer, increased reproduction may result in a food shortage (pollen, protein) for the brood as foragers cannot gather in sufficient quantities. Such a pollen/protein deficiency may also arise if too many foragers fail to return to the hive due to exhaustion, disease or disorientation. In the autumn, this failure to return to the hive means that the colony becomes too small to keep itself warm in the winter months. If a colony is too small at the beginning of winter period as a result of low reproduction in the autumn (after winter feeding), the colony size may then diminish quickly to below the minimum viable population of 10,000 – 12,000 bees. If at the same time, the reproduction rate remains too high, the population structure will be far from ideal. It is not always possible to observe the process of bees departing and not returning because this takes place gradually and because beekeepers do not inspect their hives so frequently in the autumn.
The possible roles of the Varroa mite and Nosema in this scenario are as follows: 1. The bees are more inclined to forage 2. Due to the weakness, many infected bees die in the field and so only a small number return to the hive. 3. Reproduction increases (specific role of Nosema ceranae). In this way the ratio of foragers to nurse bees slowly deteriorates.
Administering extra iron suppresses the reproduction. Iron can have a favorable influence on excessive reproduction by reducing the reproduction rate when that is specifically intended (for instance after winter feeding).