Mono-culture has its advantages as you mention, but it has one big disadvantage, the trees are only in flower part of the year and are pollinated by bees. Bees pollinated about 1/3 of the food that we eat (including almonds) but need to have flowers all year around (or at least spring, summer and fall) to survive. In nature this is done by various plants coming into flower at different times of the year. In olden days, this was done by farmers growing different crops on their land, or farmers living next to each other planting different crops, this permitted the bees to get pollen all year around.
The problem with miles and miles of any one crop, is to get them pollinated you must move the bees to the area of mono-culture when the tree go into flower and then to other areas as those crops come into flower. People were doing this as early as the late 1800s (At that time by Rail-road) but this has really become the ways on most domesticated bees since WWII.
This has lead to problems for the domesticated bees over the last several decades. Most of the problems have been manageable, but in the last 2-5 years we have seen a huge die off of commercial Domesticated bees. Wild bees (Bumble bees and other native non-honey bees) do NOT seem to suffer this same die off (But wild honey bees seem to have disappeared since about 1971). The biggest problem with the wild bees are that they are NOT readily transferable like honey bees, nor are they as good as pollination as the honey bee.
The leading theory is the widespread adoption of nicotine based herbicides, through the biggest killers of bees seems to be mites. The problem with the mite theory is that the mites have been around for several decades and have been managed to several decades, but only in the last ten years have you seen Colony Collapse disorder (CCD). As to the herbicide theory, the honey has been tested and contain the herbicide but at below levels that should cause harm to the bees.
The best Theory I have read is a combination of the theories. The miles are what is killing the bees, but only do to the introduction of the herbicide AND that movement of the bees bring the bees into contact with other bees with the mites (and other diseases). This exposure to other bees spreads the mites and disease to other bees quicker then would be the case. This speed of exposure prevents the bees from having any time to recover from one disease before hit with another disease. The herbicide is just the icing on the cake, one one push that causes the colony to collapse.
The solution? Stop moving bees around. In fact the biggest movement of bees is to the California Almond crop. The movement is from ALL of the US, including the East Coast. Once the Almonds flowers have died off, the bees have NO food, for they are in the middle of a sea of almond trees. The bees must be moved. This brings most of the bees in the US into one place, diseases are spread and exchanged and then carried to the rest of the country affecting the bees that never made it to California. This brings us to the solution that people have been avoiding, to stop moving the bees around. This means mono-culture of any crop that depends on bees to be pollinated must stop. The almond trees of California must be cut down and replaced with crops that come into flower later in the year, to give the bees other flowers to work with once the Almonds no longer have flowers.
Other solutions are possible, there is even an outside chance of a cure, but the movement of the bees, at least to some degree has to stop and with that stoppage, mono-culture of crops that depend on bees must also stop.
More on bees and Almonds:
http://www.sfgate.com/cgi-bin/article.cgi?f=/c/a/2007/10/14/CM2SS2SNO.DTLhttp://www.hcn.org/servlets/hcn.Article?article_id=16891More on Honey bees:
http://en.wikipedia.org/wiki/Honey_beeMore on Colony Collapse Disorder:
http://en.wikipedia.org/wiki/Colony_Collapse_DisorderMore cites:
http://www.freshplaza.com/news_detail.asp?id=17081http://seattlepi.nwsource.com/opinion/322781_focusbees08.html