Politicians who blame “speculators” in futures markets for the run up in oil prices — such as Sen. Byron Dorgan (D-N.D.) writing in this morning’s USAToday — should consider a lesson from the lowly onion.

Onions are one of the few commodities in the United States for which there are no futures markets, according to an item published Friday in Fortune magazine. (Futures markets allow the sale of commodities for set prices at future dates.) It seems that in the late 1950s domestic onion producers blamed those same speculators in futures markets for driving onion prices DOWN. They successfully lobbied Congress to ban all futures trading in onions, a ban that is still in place a half century later.

So has the absence of futures-market speculation kept onion prices low and stable? Quite the contrary. According to Fortune:

And yet even with no traders to blame, the volatility in onion prices makes the swings in oil and corn look tame, reinforcing academics’ belief that futures trading diminishes extreme price swings. Since 2006, oil prices have risen 100%, and corn is up 300%. But onion prices soared 400% between October 2006 and April 2007, when weather reduced crops, according to the U.S. Department of Agriculture, only to crash 96% by March 2008 on overproduction and then rebound 300% by this past April.

Sen. Dorgan and his allies will need to find someone else to blame for volitale and rising oil prices.