Bean weevils

Yesterday, my wife was in the process of preparing one of her multi-bean soups when she noticed some unusual material in a bag of South American red beans (above).  At first glance the debris looked very much like the frass that she and I are used to encountering with our caterpillars, but no larvae were present; the dark dots were the dead insects -

She got on the internet and immediately tracked down the culprit: bean weevils.  Then it was my turn to get out my Christmas present (a digital microscope), with which I got some nice photos of the malefactors:

The images show the beans with small and large holes.  I assumed the small holes were the sites of insertion of ovipositors, but no - these are the entrance sites of the first instar larvae.   Weevils lay their eggs on the outside of the beans; when the larvae hatch, they burrow into a nearby bean and consume it from the inside.

When they are mature, they eat their way out.  Or try to get out...

I found a nice report about them in a blog about Gardening in Mannheim, Germany, and a very thorough discussion of "pantry pests" at the extension service of the University of Minnesota.

And finally, this interesting observation:

A substantial amount of Third World hunger could be alleviated if farmers would shake their beans. Even carefully stored, they remain vulnerable to weevils. Weevil larvae bore through a bean's tough outer coat and feed inside the seeds; it takes a larva 24-48 hours of nearly continuous scraping to pierce the hull of the average bean. During that time, the insect braces itself against another bean or the side of the container for leverage. Michigan State grad student Martha Quentin tried jostling the beans; buckets or bags of beans shaken twice a day for two weeks had 97% fewer weevils. Larvae either starved or were crushed by the tumbling. The same procedure works on another less serious agricultural pest "thus helping also to control the lesser to two weevils." -- Washington Post, 9/19/91 

Interesting not just for the logic and simplicity of the solution, but for the remarkable location where I found this.   As we say, you learn something every day.  (and then you forget it...)