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

Feeding by larvae of Heliothis virescens induces cotton, corn and tobacco p lants to release blends of volatile organic compounds that differ in consti tuent proportions from blends released when Helicoverpa zea larvae feed on the same plant species. The same elicitors (and analogs) of plant biosynthe sis and release of volatiles, originally identified in oral secretions of S podoptera 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, an d N-linolenoyl-L-glutamine, varied among batches of oral secretions, more s o 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 enzyme s, originating in the midgut. The enzymatic activity in guts of H. virescen s was significantly greater than that found in guts of H. zea. Furthermore, H. zea frass contains N-linolenoyI-L-glutamine in more than 0.1% wet weigh t, while this conjugate comprises only 0.003% wet weight in H. virescens fr ass. These results indicated that physiological differences between these t wo species affect the proportions of volicitin and its analogs in the cater pillars. Whether this causes different proportions of volatiles to be relea sed by plants damaged by each caterpillar species is yet to be determined. (C) 2001 Elsevier Science Ltd. All rights reserved.