28 January 2016

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...)


  1. So, them beans are no good for us vegetarians ?

  2. Many people eat insects on purpose. How much research went into the Washington Post article? Is it possible that converting beans into weevils might actually increase the nutritional content?

    1. Certainly you could eat the weevils, but that presumes that the farmers can harvest them. If the weevil larvae eat the beans, mature into grown weevils and fly off, nobody gains.

      The WaPo article was written 25 years ago; I don't have a link for it.

    2. About 9/10 of energy consumed is lost before it can be transferred to the next step up on the food chain, so no. https://www.learner.org/courses/essential/life/session7/closer5.html

    3. That is to say, in a third world country, caloric intake is probably going to be the first order of business that needs taken care of, with nutritional variety being secondary.

  3. I had some friends who worked to find solutions to problems like these critters "in the bush" in places in Africa. One interesting solution was to give out tarps which they would then put the beans in. They would fold the tarp over and leave it in the sun, getting the beans really hot but not harming them. The heat killed the pests, but left the critters (whichever kind they were) dead from the heat.

  4. I normally cook lots and lots of beans -- all varieties -- but after seeing these pictures, maybe I never want to cook beans again.

    Although it's true that the weevils would give added protein. Hmmm... Maybe if I eat with my eyes shut.

  5. And here I thought they all died of gas.


  6. the ones in your beans - were any still alive?

    either way, you should report that. at least get your hard earned moolah, back from the seller? and maybe end up scheduling someone for an inspection?



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