Biochemical Systematics and Ecology 29 (2001) 1025–1047 Herbivory, induced resistance, and interplant signal transfer in Alnus glutinosa Teja Tscharntke a, *, Sabine Thiessen a,b , Rainer Dolch a , Wilhelm Boland b a Agroecology, University of G . ottingen, Waldweg 26, D-37073 G . ottingen, Germany b Max-Planck Institute for Chemical Ecology, Carl-Zeiss-Promenade 10, D-07745 Jena, Germany Received 9 April 2001; accepted 19 April 2001 Abstract Field experiments with manually defoliated black alders (Alnus glutinosa) showed that defoliation affected herbivory by the major alder antagonist, the leaf beetle Agelastica alni. Herbivore damage increased with increasing distance to the defoliated tree, suggesting induced resistance not only on the damaged tree, but also on the neighbouring trees. The beetles also avoided leaves from the nearest neighbours for both feeding and oviposition in a laboratory assay, so the alders showed interplant resistance transfer. Natural enemies did not appear to shape this pattern, because the number of entomophagous arthropods and predator–prey ratios even increased with increasing distance to the defoliated tree. The numbers of all specialist, but not the generalist, herbivore species paralleled the increase in the attack of the specialist A. alni, supporting the view that specialists are more affected by plant resistance than generalists. Mechanisms causing this pattern, found in the field, were studied in more detail using biochemical analyses and further bioassays. Responses of alder leaves to herbivory of A. alni were shown to include ethylene emission and the release of a blend of volatiles with mono-, sesqui- and homoterpenes. Changes in leaf chemistry after herbivory included increases in the activity of oxidative enzymes (polyphenoloxidase, PPO, lipoxygenase, LOX, and peroxidase, POD) and proteinase inhibitors (PIs), and an increase in the phenolic contents of the leaves. Quantification of the endogenous jasmonic acid (JA) showed the activation of the octadecanoid pathway following herbivory. The active components in mediating a possible interplant signal transfer via airborne volatiles may have included ethylene, b-ocimene, 4,8-dimethylnona-1,3,7-triene (DMNT), and 4,8,12-trimethyltrideca-1,3,7,11-tetraene (TMTT). The incubation with volatiles resulted in an *Corresponding author. Tel.: +49-551-399209; fax: +49-551-398806. E-mail address: ttschar@gwdg.de (T. Tscharntke). 0305-1978/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII:S0305-1978(01)00048-5