Clean beans created the conditions that caused outbreaks of soybean aphids in the same way that pouring an accelerant (for example, gasoline) around the outside of a building would cause the conditions for a fire.
The soybean aphid is Aphis glycines. It is originally from Asia. It was first discovered in the United States in 2000 when it was discovered in Wisconsin. It spread like wildfire, and in 2001 was causing extensive damage to crops in the upper Midwest. It was inhibited a little bit by the weather conditions in 2002, but had another huge outbreak in 2003. The aphids were so thick they even interfered with ball games.
The soybean aphid has as many as 15 generations per year on the soybean plant. It overwinters on buckthorn, and at least one species of buckthorn that it uses is also from Asia originally.
Photo via Wikimedia
Usually, when an alien insect or weed invades, part of the reason for its success is a lack of natural predators. A. glycines has a number of natural enemies in Asia that help keep its numbers in check. But there are a number of insects here in the United States that eat aphids as well. Lacewings, syrphid fly larva, lady beetles and their larva, and a whole host of other insects eat aphids. And the only defense that aphids have is to reproduce faster than they are eaten.
Those predators normally live on the vegetation in a bean field. Some live on the beans, and some live on the weeds that live between the rows. Eliminate the weeds and you eliminate a major portion of the predators.
When a third of the landscape is in soybeans, a soybean aphid outbreak is a landscape scale problem. Had the weeds not been eliminated between the bean rows would soybean aphids still have been a problem? Probably. Can you have clean beans and no soybean aphids? Yes, when the conditions are right. But the fire burns so much hotter with the accelerant.
Removing every weed (and the predators associated with those weeds) from a soybean field is an accelerant for the soybean aphid outbreaks.