Cotton plants attacked by herbivorous insect pests emit relatively large amounts of characteristic volatile terpenoids that have been Implicated in the attraction of natural enemies ofthe herbivores. However, the composition of the blend of volatile terpenes released by the plants varies remarkably throughout the photoperiod. Some components are emitted in at least 10-fold greater quantities during the photophase than during the scotophase, whereas others are released continuously, without conforming to a pattern, during the entire time that the plants are under herbivore attack. The diurnal pattern of emission of volatile terpenoids was determined by collecting and analyzing the volatile compounds emitted by cotton plants subjected to feeding damage by beet armyworm larvae in situ. The damage was allowed to proceed for 3 days, and volatile emission was monitored continuously. During early stages of damage high levels of lipoxygenasederived volatile compounds [e.g., (Z)-3-hexenal, (Z)-3-hexenyl acetate] and several terpene hydrocarbons [e.g., a-pinene, caryophyllenel were emitted. As damage proceeded, high levels of other terpenes, all acyclic [e.g., (E)-f-ocimene, (E)-,-farnesene], were emitted in a pronounced diurnal fashion; maximal emissions occurred in the afternoon. These acyclic terpenes followed this diurnal pattern of emission, even after removal ofthe caterpillars, although emission was in somewhat smaller amounts. In contrast, the emission of cyclic terpenes almost ceased after the caterpillars were removed.Plant odors have long been of interest because they attract phytophagous insects. Additionally, a rapidly growing body of evidence has implicated plant odors in the attraction of species that prey on or parasitize herbivorous insect pests (1). In the cases so far reported, plants that were nearly odorless before feeding damage emitted large quantities of volatile compounds in a delayed response to herbivore feeding (2). These induced odors have been shown to be powerful attractants for parasitic Hymenoptera (2) and predatory mites (3). However, this mechanism of self-defense by plants has been explored in only a few species.In recent studies of both corn, Zea mays (2), and cotton, Gossypium hirsutum (4), seedlings we found that plants release a significantly greater number of compounds and larger amounts of total volatile compounds after overnight feeding damage by insects than when they have been freshly damaged by the insects. However, there was no previous indication that plants respond to herbivore damage with a diurnal rhythm of volatile compound emission, although a number of investigations have demonstrated the rhythmic nature of volatile compound release from flowers (5, 6). In many cases it appears that the release of volatile compounds by flowers is timed to coincide with the period of greatest activity of their pollinators. For example, Heath et aL (7) found that emission of floral odor by night-blooming jessamine peaked in the first 2 hr of the scotophase, coincident with the pe...

Volatile compounds elicited by insect herbivore feeding damage in five cotton cultivars and one naturalized cotton variety were examined by allowing beet armyworm larvae to feed overnight on leaves and collecting volatiles from the plants in situ. Of 23 compounds identified from larval damaged leaves, terpenes and lipoxygenase-hydroperoxide lyase-derived volatiles predominated. No pronounced differences in the levels of volatile emission were noted from leaves of undamaged plants of the different varieties. However, average volatile emission from damaged leaves of the naturalized variety was almost sevenfold higher than from damaged leaves of the commercial cultivars. This was despite the fact that larvae preferred feeding on the leaves of commercial cultivars over those of the naturalized variety in choice tests.

Cotton plants (Gossypium hirsutum L.), attacked by herbivorous insects release volatile semiochemicals (chemical signals) that attract natural enemies of the herbivores to the damaged plants. We found chemical evidence that volatiles are released not only at the damaged site but from the entire cotton plant. The release of volatiles was detected from upper, undamaged leaves after 2 to 3 d of continuous larval damage on lower leaves of the same plant. Compounds released systemically were (Z)-3-hexenyl acetate, (E)-β-ocimene, linalool, (E)-4,8-dimethyl-1,3,7-nonatriene, (E)-β-farnesene, (E,E)-α--farnesene, and (E,E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene. All systemically released compounds are known to be induced by caterpillar damage and are not released in significant amounts by undamaged plants. Other compounds, specifically indole, isomeric hexenyl butyrates, and 2-methylbutyrates, known to be released by cotton in response to caterpillar damage, were not released systemically. However, when upper, undamaged leaves of a caterpillar-damaged plant were damaged with a razor blade, they released isomeric hexenyl butyrates, 2-methylbutyrates, and large amounts of constitutive compounds in addition to the previously detected induced compounds. Control plants, damaged with a razor blade in the same way, did not release isomeric hexenyl butyrates or 2-methylbutyrates and released significantly smaller amounts of constitutive compounds. Indole was not released systemically, even after artificial damage.