Abstract. Maize (Zea mays L.) releases speci®c volatiles in response to herbivory by caterpillars. These volatiles are known to serve as cues for parasitic wasps to locate the herbivores. In the present study the exact time of volatile emission after simulated herbivory (mechanical damage and treatment with caterpillar regurgitant) was measured for seedlings of the cultivars``Ioana Sweet Corn'' and``LG11''. Odours were collected every 0.5 h for a total of 12 h. Typical``green leaf odours'', (Z)-3-hexenal, (E )-2-hexenal, (Z)-hexen-1-o1, and (Z)-3-hexen-1-yl acetate, were emitted immediately upon damage and their amounts dropped rapidly after the ®rst collections. Several of the induced compounds were released within 2 h after treatment, while others (mainly sesquiterpenoids) started to be released after 4 h. The LG11 seedlings emitted several compounds (e.g. bmyrcene, (Z)-b-ocimene, benzyl acetate, b-caryophyllene, (E,E )-a-farnesene) that were not detected for Ioana. (E,E )-a-farnesene was continuously emitted by LG11 seedlings, even by undamaged plants. Timing of the release of volatile compounds that the two varieties had in common did not dier signi®cantly, with the exception of indole for which the peak production was considerably earlier for LG11. These ®ndings are discussed in the context of biosynthetic pathways and mechanisms involved in induced emissions of plant volatiles and the exploitation of the resulting odour by parasitoids and predators of herbivores.

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When maize plants, Zea mays L., are mechanically damaged and the damaged sites are treated with caterpillar regurgitant, the plants will release a specific blend of volatiles. It is known that these volatiles can be attractive to natural enemies of herbivores. We hypothesise that the plant volatiles constitute part of the induced plant defence and that herbivores will be affected by the odours as well. In laboratory and semi-field studies this hypothesis was tested for the aphid Rhopalosiphum maidis (Fitch) (Rhynchota, Sternorrhyncha, Aphididae).In a Y-tube olfactometer significantly more aphids chose the odour of healthy, undamaged maize seedlings when tested against clean air or plants treated with regurgitant. Clean air was chosen more often when tested next to the odour of treated plants. This apparently repellent effect of the odour of treated plants was significant for winged aphids, but not for the wingless aphids.In field experiments aphids were released in the centre of circles of eight potted maize plants. Four plants in each circle were damaged and treated with caterpillar regurgitant while the other plants were left unharmed. At different intervals after aphid release, the number of aphids was counted on each plant. Significantly fewer winged and wingless aphids were found back on treated plants than on healthy plants.We suggest that herbivores may be repelled by the odours because they could indicate that: 1) the plant has initiated the production of toxic compounds; 2) potential competitors are present on the plant; 3) the plant is attractive to parasitoids and predators. Aphids may be particularly sensitive to induced maize volatiles because one of the major compounds emitted by the plant is (E)-β-farnesene, which is a common alarm pheromone for aphids. Collections and analyses of the odours emitted by crushed R. maidis confirmed that it too emits (E)-β-farnesene when stressed. The results are discussed in context of plant defence strategies and their possible exploitation for the control of pest insects.

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