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Temperature responses of a plant-herbivore-parasitoid system using a ‘Tri-trophic food-web performance ratio’ approach Sandra 1Département 1 Flores-Mejia , Valérie de Biologie, Université Laval 2 Fournier , Conrad 2Département 1. Cloutier de Phytologie, Université Laval • INTRODUCTION Each component of a multi-trophic food web has its own thermal window1. Climate change is believed to have a higher impact in higher trophic levels of the food web, because they depend on the capacity of adaptation of the lower levels and because they have a smaller thermal window2, 3. One limitation when studying a food web is lack of a measure of performance that can be used to directly compare its components, including the host plant, which has a different biology than insect herbivores and carnivores. The product of rm * biomass has been suggested as a parameter to evaluate the performance of herbivores with respect of a plant4,5,6. Here we propose the “Tri-trophic food-web performance ratio” (φ), as a common measure to evaluate a plant base food web as affected by temperature. • • OBJECTIVES Determine the temperature profile of a tri-trophic food web and determine which trophic level is more likely to outperform the others under a wide range of different thermal conditions. RESULTS Biomass accumulation can be a useful to evaluate the performance of a food web, both for individual components and as a whole. Each level of our experimental food web has a different temperature profile, and thus they maximize biomass accumulation rate at different temperatures (Figure 1): • Plants (Figure 1A): have the widest temperature range, going above 28°C. In spite of the different temperature preferences between peppers and potatoes, both perform better at midtemperatures • Herbivore (Figure 1B): perform better at lower temperatures, with no growth above 28°C. Aphid productivity will depend on the host plant it is exploiting. • Parasitoid (Figure 1C): has the smallest temperature range, having very low performance at both low and high temperatures. It is capable to similarly exploit aphids raised on different host plants, showing its generalistic nature. Only sex ratio is significant for the parasitoid’s biomass accumulation. As females are bigger than males (data not shown), a 40♂:60♀ sex ratio will have a higher overall biomass than a 60♂:40♀. ♂ ♂ ♂ ♀ ♀ ♀ Develop a single parameter that can be used to evaluate the performance of a tri-trophic food web under different temperature conditions. MATERIALS AND METHODS The food web model tested was formed by: three different host plants, one cultivar of potato (S. tuberosum cv. “Norland”), and two cultivars of bell pepper (C. annuum cv. “Fascinato”© and “Crosby”©). The herbivore was the potato aphid (Macrosiphum euphorbiae Thomas), biotype POT; and finally the parasitoid wasp Aphidius ervi Halliday. The experimental temperatures were: 8, 12, 16, 20, 24, 28, 32, and 36°C, with 65% R.H. and 16D:8N photoperiod. We obtained the mean relative growth rate (mRGR)7of the three plants, when infested with 0, 10, 40 L1 aphids per plant type and per temperature. We obtained the net reproduction rate (Ro)8 and the adult biomass of both aphids and parasitoids, for each combination of host plant-aphid biotype per temperature. We then calculated a Net Generational Productivity (NGP)9 parameter for aphids and parasitoids, as the product of Ro*average adult biomass. The “Tri-trophic food-web performance ratio” (φ3) parameter was finally calculated using the formula: NGPh - NGPp φ3' mRGR * L Where NGPh NGPp stand for the net generational productivity of the herbivore (aphid) and carnivore (parasitoid) , respectively. The mRGR is the average relative growth rate of the plant, and L is a fixed time, chosen to be the average aphid lifetime (in days) for which the NGPh was calculated. Click here to enlarge figure Figure 1: Individual models for each of the components of the “Tri-trophic food web ratio”: A) mRGR of the three host plants; B) NGP of the herbivore, the potato aphid, (M. euphorbiae) and C) NGP of the parasitoid, the wasp A. ervi. All three components were tested at eight constant temperatures between 8 and 36°C. Click here to enlarge figure Figure 2: Tri-trophic food web ratio at six constant temperatures Temperature response of the tri trophic food web ratio, summarized in Figure 2, showed that: • At low temperatures (below 15˚C), the herbivores outperform both plants and parasitoids (positive values of φ3). • At mid-temperatures, the parasitoids have the advantage above the other two components (negative values), especially when the sex ratio is more female oriented. • At high temperature (30˚C or above), it is the plant that has a slight competitive advantage over both herbivores and parasitoids (φ3 values between -1 and 1). This is the first attempt to evaluate a food web performance with a single parameter allowing direct quantitative comparison of different levels. This parameter can be used to evaluate the food web under different scenarios, such as those of climate change. REFERENCES 1. Dixon, A.F.G., Honěk, A., Keil, P., Kotela, M.A.A., Šizling, A.L., & Jarošík, V. (2009) Relationship between the minimum and maximum temperature thresholds for development in insects. Functional Ecology, 23, 257-264. 2. Jeffs, C.T. & Lewis, O.T. (2013) Effects of climate warming on host–parasitoid interactions. Ecological Entomology, 38, 209-218. 3. Van Baaren, J., Le Lann, C., & van Alphen, J. (2010). Consequences of climate change for aphid-based multi-trophic systems. In Aphid Biodiversity Under Environmental Change. Patterns and Processes (ed. by P. Kindlmann, A.F.G. Dixon & J.P. Michaud), pp. 55-68. Springer Science & Business Media. 4. Fenchel, T. (1974) Intrinsic rate of natural increase: The relationship with body size. Oecologia, 14, 317-326. 5. Lamb, R.J., MacKay, P.A., & Migui, S.M. (2009) Measuring the performance of aphids: fecundity versus biomass. The Canadian Entomologist, 141, 401405. 6.Van Impe, G. & Hance, T. (1991) Étude comparative de la biomasse des populations d'Aphis fabae SCOPOLI (Homoptera: Aphididae) et de Tetranychus urticae KOCH (Acari:Tetranychidae). In 43rd International Symosium of Crop Protection, Vol. 56, pp. 343-354. Mededelingen van de Faculteit Landbouwwetenschappe, Gent. 7.Penaloza P, Palma B & Silva C (1996) Plant growth and reproductive development of Capsicum annuum L. Phyton-International Journal of Experimental Botany 59: 187-195. 8. Birch LC (1948) The intrinsic rate of natural increase of an insect population. Journal of Animal Ecology 17: 15-26. 9. Flores-Mejia, S., Fournier, V., & Cloutier, C. 2014 Temperature responses of a plant-insect system using a food-web performance approach. Entomologia Experimentalis et Applicata. ACKNOWLEDGMENTS We thank RZH Canada Ltd and Syngenta seeds for graciously providing us with the bell pepper seeds. We also thank J.F.Guay, A. Auger, Y. Gobeil, E. Morin and E. Lemaire for their assistance in experimental work, Funding for this project was provided by: NSERC-Strategic and Discovery Grants, Centre SÈVE, and Ministère de l’Éducation, du Loisir et du Sport du Québec. Figure 1: Individual models for each of the components of the “Tri-trophic food web ratio”: A) mRGR of the three host plants; B) NGP of the herbivore, the potato aphid, (M. euphorbiae) and C) NGP of the parasitoid, the wasp A. ervi. All three components were tested at eight constant temperatures between 8 and 36°C. ♂ ♂ ♂ ♀ ♀ ♀ Figure 2: Tri-trophic food web ratio at six constant temperatures Click here to return to poster