A cyanide-laced diet would seem to be a recipe for disaster; but not for fall webworm larvae. According to Terrence Fitzgerald, the larvae thrive on black cherry leaves packed with the cyanide-producing chemical prunasin. How fall webworm larvae stomach their toxic diet puzzled Fitzgerald. Were the insects immune to cyanide, or maybe they'd found a way to hamper cyanide production in the chewed leaves()?
Fitzgerald tested the larvae's resistance to cyanide fumes. Half of the larvae curled up and died, while the other half survived, only to die later from their injuries. The larvae weren't immune to the toxin.
Next he measured the cyanide production potential of black cherry leaves,larvae faeces and gut contents, as well as the defensive vomit the larvae produce when threatened, and found that the concentration of cyanide precursors in the insect's faeces was even higher than in the leaves. The insects were able to survive their toxic diet because they had somehow prevented the breakdown of prunasin into cyanide. But how?
Fitzgerald measured the pH of the insect's foregut and faeces and found that both were extremely alkaline at a pH of 12. Could the high pH prevent prunasin from releasing its fatal toxin? Fitzgerald tested cyanide release from crushed leaves over pHs ranging from 7 to 11 and found that prunasin readily released cyanide at pH 7, but the rate fell significantly at pH 10 and had fallen to zero by pH 11. The insects are protected from their toxic diet by their incredibly alkaline gut, which halts cyanide production as the poisonous plant passes through them.
