Friday, February 22, 2013

Bean Weevils

Bob Korpella sent me the picture on the right with the note below and a challenge to identify the beetles.  You can see that they had developed in the seeds and then chewed their way out as adults.
Seeds and Beetles- Click to enlarge
"These were found by some teachers here at the Nixa Early Learning Center. The teachers had brought in honey locust seed pods each year for their students, and this was the first time they found insects.  The bugs were inside the seeds and seed pods from a honey locust." Nothing warms the Master Naturalist heart like fielding a question from kids- but it is humbling if you can't find the answer.
Not having my Kaufman's insect field guide handy, I googled "honey locust seed larva" and came up with the picture of the locust seed beetle,  Amblycerus robiniae.  It is a member of the Bruchidae family of bean weevils or seed beetles.
"Bruchidae are found on every major land mass except Antarctica and New Zealand. Eggs are usually laid on the seed or fruit of a plant suitable for development of the larva. Immature stages are spent inside seeds that have been excavated by larval feeding. Adults live free and feed on pollen and nectar. 
Approximately 84% of the known hosts of Bruchidae are in the plant family Leguminosae (pea or bean). The remaining hosts are scattered among 31 other families. Sixteen plant families support larval feeding in the United States and Canada." John Kingsolver *
Bean Weevil - Amblycerus robiniae-Jon Rapp
With its bean like seeds in a pea-like pod, you might assume that the honey locust, Gleditsia triacanthos, is a member of the bean/pea family.  The honey locust seed beetle, aka bean weevil, like some children, apparently like its beans if you don't call them peas.

Silver spotted skipper- Linda Ellis
This is not the only connection the honey locust has in the food web.  Illinois Wildflowers has a list of 51 other insect species that feed on honey locust.  The common silver spotted skipper lays its eggs on the leaves for its caterpillar larvae to feed upon.  Extinct megafauna such as the giant sloths and wooly mammoth are suspected of feeding on these seed pods, digesting the fiber and spreading the seeds in their waste.**  With their extinction, this function has been assumed by domestic cattle.

This serves as a reminder of how interconnected our trees are with the whole ecosystem.  It is not however a one-way street. The large herbivores not only disseminate the seeds in their feces, their digestive enzymes break down the hard shell, helping the seeds to germinate.  The fecal material not only provides fertilizer but also protects the seeds from the seed beetle above which might otherwise lay its eggs on them.   Many butterflies, moths and other insects will feed on the nectar of its flowers, pollinating them as they do so.

George Sims' final reply to the pictures above- "Although I have seen larger weevil specimens, you should always choose the lesser of two weevils."

*    Handbook of the Bruchidae of the United States and Canada, John M. Kingsolver 
** There is much more on these relationships in The Ghosts of Evolution, pages 37-39.


  1. Bob,
    To avoid charges of plagiarism, I must confess that the "lesser of two weevils" quote was stolen from Patrick O'Brian's "Master and Commander" series of novels. I NEVER hesitate to steal a good line when I find one.

  2. Also, thanks for the pdf of the "Handbook of Bruchidae".