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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Sublethal effects of four pesticides (Pirimor@, active ingredient [a. i.] pirimicarb; Deck@, a. i. deltamethrin; Alaxon@ D, a. i. diazinon; KelthaneO EC, a. i. dicofol) on the web building behaviour of the common gray cross spider Larinioides sclnpetarius (Clerck) were investigated with the spider test of WITT and PETERS.Pirirnicarb applied in a dose of 0.64 pg/mg (corresponding to the application of 1.28 &mg Pirimor@) caused neither mortality nor aberrations in web geometry. AlaxonB D led in a dose of 5.pl/mg to a mortality rate of 17.6 %. Web building activity was only slightly affected. The webs themselves showed some changes. The area of the webs was reduced to 62 % of the original size. The regularity of the catching spiral and the median angles of the radii were also affected but in a lesser degree. Decis@ applied in a low dose of 5 .WUmg caused 10 % mortality. Web building activity was affected (average delay in web building was 2.32 days). The area of the webs was reduced to 72 %, the number of radii was reduced by 17 YO, the median angle of the radii was enlarged by 21 % and the regularity of the angles was reduced by 36 '4. KelthaneB EC, in a high dose of 9.6. pl/mg, resulted in 10 % mortality. No negative influence on web building activity was recorded. The webs themselves were hardly affected: only the number of radii was reduced by 5 70. The efficiency of the various pesticides can be ranked as follows: Deck@ (deltamethrin) >> Alaxon@ D (diazinon) > KelthaneB EC (dicofol) 2 Pirimor@ (pirimicarb). Substance specific differences of the effects are discussed.