© 2008 The Authors Entomologia Experimentalis et Applicata 128: 109–116, 2008 Journal compilation © 2008 The Netherlands Entomological Society 109 DOI: 10.1111/j.1570-7458.2008.00695.x Blackwell Publishing Ltd Differential effects of jasmonic acid treatment of Brassica nigra on the attraction of pollinators, parasitoids, and butterflies Maaike Bruinsma*, Harm IJdema, Joop J.A. van Loon & Marcel Dicke Laboratory of Entomology, Wageningen University, PO Box 8031, 6700 EH Wageningen, The Netherlands Accepted: 13 February 2008 Key words: Apis mellifera, syrphid flies, Cotesia glomerata, Pieris rapae, host plant selection, flower visitors, octadecanoid pathway, pollination, Lepidoptera, Pieridae, Hymenoptera, Braconidae Abstract Herbivore-induced plant defences influence the behaviour of herbivores as well as that of their natural enemies. Jasmonic acid is one of the key hormones involved in both these direct and indirect induced defences. Jasmonic acid treatment of plants changes the composition of defence chemicals in the plants, induces volatile emission, and increases the production of extrafloral nectar. However, few studies have addressed the potential influence of induced defences on flower nectar chemistry and pollinator behaviour. These have shown that herbivore damage can affect pollination rates and plant fitness. Here, we have investigated the effect of jasmonic acid treatment on floral nectar production and the attraction of pollinators, as well as the effect on the behaviour of an herbivore and its natural enemy. The study system consisted of black mustard plants, Brassica nigra L. (Brassicaceae), pollinators of Brassica nigra (i.e., honeybees and syrphid flies), a specialist herbivore, Pieris rapae L. (Lepidoptera: Pieridae), and a parasitoid wasp that uses Pieris larvae as hosts, Cotesia glomerata L. (Hymenoptera: Braconidae). We show that different trophic levels are differentially affected by jasmonic acid-induced changes. While the herbivore prefers control leaves over jasmonic acid-treated leaves for oviposition, the parasitoid C. glomerata is more attracted to jasmonic acid-treated plants than to control plants. We did not observe differences in pollinator preference, the rates of flower visitation by honey- bees and syrphid flies were similar for control and jasmonic acid-treated plants. Plants treated with jasmonic acid secreted less nectar than control plants and the concentrations of glucose and fructose tended to be lower than in nectar from control plants. Jasmonic acid treatment resulted in a lower nectar production than actual feeding damage by P. rapae caterpillars. Introduction Induction of defence responses in plants can alter the behaviour of associated insects. This is well studied for foliar herbivores (folivores) and their natural enemies associated with vegetative plants (e.g., Dicke et al., 1990; Turlings et al., 1990; Shiojiri et al., 2002; Bruinsma & Dicke 2008). However, the effects of induced defences on flowering plants, and consequently pollinator behaviour, have received much less attention. Root and foliar herbivory may indirectly affect plant fitness by reducing resources for reproduction through reduction of root volume or photosynthetic area. Floral herbivory (florivory) directly affects plant fitness by reducing the number of gametes (Poveda et al., 2003). Observed effects of foliar herbivory on flowering plants include a decrease in pollinator visitation, an increase in secondary metabolites in leaves, flowers, or nectar, fewer and smaller flowers, and decreased pollen production (Strauss et al., 1996, 2004; Lehtilä & Strauss, 1997; Mothershead & Marquis, 2000; Hambäck, 2001; Smallegange et al., 2007). Root herbivory can increase pollinator visitation (Poveda et al., 2003, 2005) and florivory can decrease the number of flowers, nectar production, seed set, and pollinator visitation (Krupnick et al., 1999; Adler et al., 2001). Although these examples show that herbivory can increase or decrease pollinator visitation, depending on type of damage, plant and pollinator species, the mechanisms * Correspondence: E-mail: maaike.bruinsma@wur.nl