Comparative Innate Responses of the Aphid Parasitoid Diaeretiella rapae to Alkenyl Glucosinolate Derived Isothiocyanates, Nitriles, and Epithionitriles Tom W. Pope & Ralph Kissen & Murray Grant & John A. Pickett & John T. Rossiter & Glen Powell Received: 28 May 2008 / Revised: 22 July 2008 / Accepted: 24 July 2008 / Published online: 19 August 2008 # Springer Science + Business Media, LLC 2008 Abstract Cruciferous plants (Brassicaceae) are character- ized by the accumulation of a group of secondary metabolites known as glucosinolates that, following attack by pathogens or herbivores, may be hydrolyzed to one of a number of products including isothiocyanates and nitriles. Despite the range of hydrolysis products that may be produced, the toxicity of glucosinolates to pathogens and herbivores may be explained largely by the production of isothiocyanates. Isothiocyanates are also known to provide an indirect defense by acting as host finding cues for parasitoids of insect herbivores that attack crucifers. It has been speculated that nitriles may provide a similar indirect defense. Here, we investigate the olfactory perception and orientation behavior of the aphid parasitoid Diaeretiella rapae, to a range of alkenylglucosinolate hydrolysis prod- ucts, including isothiocyanates, nitriles, and epithionitriles. Electroantennogram responses indicated peripheral odor perception in D. rapae females to all 3-butenylglucosinolate hydrolysis products tested. By contrast, of the 2-propenyl- glucosinolate hydrolysis products tested, only the isothio- cyanate elicited significant responses. Despite showing peripheral olfactory detection of a range of 3-butenylgluco- sinolate hydrolysis products, naïve females oriented only to the isothiocyanate. Similarly, parasitoids oriented to 3- isothiocyanatoprop-1-ene, but not to the corresponding nitrile or epithionitrile. However, by rearing D. rapae either on Brassica nigra, characterized by the accumulation of 2- propenylglucosinolate, or Brassica rapa var rapifera, char- acterized by the accumulation of 3-butenylglucosinolate, altered the innate response of parasitoids to 3-isothiocyana- toprop-1-ene and 4-isothiocyanatobut-1-ene. These results are discussed in relation to the defensive roles of glucosino- late hydrolysis products and the influence of the host plant on aphid parasitoid behavior. Keywords Braconid . Brevicoryne brassicae . Glucosinolates . Isothiocyanates . Nitriles . Epithionitriles . EAG Introduction Secondary metabolism plays a role in a plant’ s survival in the environment, providing a defense against insect herbivores and plant pathogens, while also aiding repro- duction through the attraction of pollinators (Kliebenstein 2004). In cruciferous plants (Brassicaceae), glucosinolates, a group of amino acid-derived compounds that consist of a β-thioglucose moiety, a sulfonated oxime, and a variable J Chem Ecol (2008) 34:1302–1310 DOI 10.1007/s10886-008-9531-2 T. W. Pope : R. Kissen : J. T. Rossiter : G. Powell Division of Biology, Imperial College London, Faculty of Natural Sciences, Sir Alexander Fleming Building, South Kensington Campus, London, SW7 2AZ, UK M. Grant University of Exeter, Stocker Road, Exeter, EX4 4QD, UK J. A. Pickett Rothamsted Research, Harpenden, Hertfordshire, AL5 2JQ, UK Present address: T. W. Pope (*) School of Biological Sciences, University of Southampton, Bassett Crescent East, Southampton, SO16 7PX, UK e-mail: t.w.pope@Soton.ac.uk