Valerie FINIDORI-LOGLI-1, Anne Geneviève BAGNERES-1, Dirk ERDMANN-2 Wittko FRANCKE-2 and J.L. CLEMENT-1
1CNRS - UPR 9024 Lab Neurobiologie, 31, chemin J. Aiguier 13402 Marseille
cedex France
2Institut Organishe chemie und Biochimie, Universitat Hamburg,
Martin Luther King Platz 6, D - 20146 Hamburg 13 BRD
Diglyphys isaea Walker is a larval ectoparasitoid which is used as a means of biological pest control against the american serpentine leafminer Liriomyza trifolli Burgess. We studied the Diglyphus parasitoid's host searching behavior, using olfactometric methods. Our data shows that the parasitoids locate the diptera's larvae on the basis of the volatile signals released by the plant-host complex. The female Diglyphus is strongly attracted to the odours arising from damaged bean-plants, whereas she shows practically no response to intact plants. The results of our chemical analysis showed that fifteen or so components were present, two of which, Cis-3-Hexen-1 ol and 4-Hydroxy 4- Methyl 2-Pentanone, were present in significantly larger quantities in the leaf extracts from mined or damaged bean plants than in those from hearty plants. The damage inflicted by the host larvae on these plants therefore triggers the release of larger amounts of these substances, which probably lead the parasites to their hosts, thus acting as synomones. Recognition of the female by the male occurs at close range on the mined plants. Courtship behavior is triggered when males are in the presence of a living female regardless of her age, as well as in the presence of a female killed by freezing although courtship is not always completed. Courtship behavior is not observed using a dead female washed with organic solvents as a lure. Using the polar fraction, of an organic extract of a female, courtship behavior was practically the same as with a living female. Analysis of the organic extract by gas chromatography (GC) revealed chemical dimorphism between the two sexes. Our findings demonstrate that each sex develops a specific chemical signature that can be dissolved in hexane and transferred to a lure. Gas chromatography coupled with mass spectrometry (GC-MS) showed that the main signature components in females were esters of medium chain fatty acids and long chain 11-alcohols.
Long range signals will probably just convey information about the host's habitat: while more short-range semiochemical substances emanating from infested plants or from the host's faeces or other secretions will provide more direct and more reliable information about the availability of hosts and the sites where they are to be found. A parasitoid searching for a host uses a continious, dynamic flow of information to locate and invade its hosts. Parasitoids therefore need to be able to mahe use of what they have previously experienced and learned in order to take advantage of this complex system of interactions.
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