Enzymatic decomposition of elicitors of plant volatiles in Heliothis virescens and Helicoverpa zea

Feeding by larvae of Heliothis virescens induces cotton, corn and tobacco plants to release blends of volatile organic compounds that differ in constituent proportions from blends released when Helicoverpa zea larvae feed on the same plant species. The same elicitors (and analogs) of plant biosynthesis and release of volatiles, originally identified in oral secretions of Spodoptera exigua larvae, were also found in oral secretions of H. virescens and H. zea. However, relative amounts of these compounds, particularly N-(17-hydroxylinolenoyl)-L-glutamine (volicitin), 17-hydroxylinolenic acid, and N-linolenoyl-L-glutamine, varied among batches of oral secretions, more so in H. virescens than in H. zea. This variation was due to cleavage of the amide bond of the fatty acid–amino acid conjugates by an enzyme, or enzymes, originating in the midgut. The enzymatic activity in guts of H. virescens was significantly greater than that found in guts of H. zea. Furthermore, H. zea frass contains N-linolenoyl-L-glutamine in more than 0.1% wet weight, while this conjugate comprises only 0.003% wet weight in H. virescens frass. These results indicated that physiological differences between these two species affect the proportions of volicitin and its analogs in the caterpillars. Whether this causes different proportions of volatiles to be released by plants damaged by each caterpillar species is yet to be determined.