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Abstract
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.