Parasitoids of herbivorous hosts face the problem that they have to utilize reliable and detectable cues during host location. It has been shown that larval parasitoids can solve this problem by orientating towards plant odors that are released after herbivore feeding damage. The role of induced plant volatiles for egg parasitoids is almost unknown. Its elucidation, however, may contribute to the enhancement of performance of egg parasitoids utilized as biological control agents. The egg parasitoid Oomyzus gallerucae (Hymenoptera, Eulophidae) attacks eggs of the elm leaf beetle Xanthogaleruca luteola (Coleoptera, Chrysomelidae), which is a serious defoliator of different elm species in Southern Europe, North America and Australia. O. gallerucae has been repeatedly introduced into the United States for biological control of the elm leaf beetle. In Southern France 30 -70 % of elm leaf beetle egg masses sampled on different locations and during different years were parasitized by O. gallerucae. These high discovery efficiencies of female egg parasitoids might be mediated by volatile synomones. Our olfactory studies on host location of O. gallerucae proved that these egg parasitoids are attracted to the odor of field elm (Ulmus minor) leaves that are carrying eggs of the elm leaf beetle. Oviposition of X. luteola induces a change of the volatile pattern emitted by the field elm leaves resulting in the attraction of the egg parasitoids. The plant's reaction to oviposition is systemic; the leaves without eggs adjacent to those with eggs also emit attractants. Females O. gallerucae also react to odor of elm leaves treated with jasmonic acid. Jasmonic acid is known as mediator of plant responses induced by feeding of herbivorous arthropods. Plants that are able to respond to oviposition of herbivores may defend themselves by emission of volatiles which “call” for egg parasitoids. This potential might be used to enhance the performance of egg parasitoids for semiochemical control of herbivores.

Index terms: Oomyzus gallerucae, elm leaf beetle, egg parasitoid, synomone induction, biological control

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