WAGENINGEN UNIVERSITY LABORATORY OF ENTOMOLOGY Asclepias syriaca – Aphis nerii, One host-specialist ecosystem at intercontinental scale C. Rogé, Msc Plant sciences Minor and Major Entomology thesis, No: 010.10 No ............................................ 85 09 06 701 060 Name ...................................................... Cyril Rogé Study program ....................................... Msc Plants Sciences Period .......................................................2010-2011 Thesis/Internship ..................... ENT 80436 + ENT 80424 (+ 6) Thesis number ............................................................. 010.10 1e Examinator ......................................................... M. Dicke 2e Examinator ............................................................R. Gols Thesis title : Asclepias syriaca – Aphis nerii, one host-specialist ecosystem at intercontinental scale Key words : Milkweed – Danaus plexippus – Induction – Enemy release hypothesis – Local adaptation – Aphid performance Supervisors & advisors Martijn Bezemer Jeff Harvey Arjen Biere Tibor Bukovinszky Rieta Gols Thesis (major and minor) submitted in partial fulfillment of the requirements for the degree of Master of Sciences with the specialization "plant pathology and entomology". Research subject was originally proposed by Tibor Bukovinszky, Netherlands Institute for ecology (NIOO). Experimentations were done at the NIOO, Heteren, Netherlands, under the supervision of Arjen Biere, Jeff Harvey and Martijn Bezemer. Thesis process was supervised by Rieta Gols, Entomology dpt, Wageningen. This project started in September 2009 and finished in August 2010. 3 4 Acknowledgment Thanks Tibor to have given me (again) the opportunity to work on your projects Thanks to all my supervisors (Arjen, Jeff, Martijn and Rieta) and the NIOO people (Gregor, Roel and Vanes) for the help and all the fruitful advices throughout this past year. Thanks to K. Veltman and the Artis royal zoo have kindly provided Danaus plexippus individuals. Thanks to A.A.A. Agrawal to have provided the North American experimental- materials. Thanks to Olaf, Vanes, Floris and Sotos for all this interesting breaks. I had to say sorry to all “greenhouse workers” that I disturbed with loud heavy music but it was part of Jeff supervision! Thanks to the French team, The Black Belgian, my big sister and my tinkerbell for all them supports and consideration. Special thanks to all living beings that give their lives for this study. 5 6 Abstract Several theories have been developed to explain why a plant is becoming invasive in its new range. The enemy release hypothesis (ERH) links plant fitness optimization (gain) with a decrease in regulation by herbivores and pathogens. ERH is subject to discussion because herbivore pressure is usually not the largest threat to plants. Nevertheless it can be an important hypothesis when plants escape from specialist herbivores with whom they have co- evolved. The milkweed, Asclepias syriaca, is a toxic lactiferous plant from North America (NA) that has become invasive in Europe (EU). It is a highly specialized trophic system in NA where only 12 specialist herbivores are feeding on it, whereas in EU plants were released free from 11 native enemies. In EU A. syriaca is only attacked by a specialist phloem-sucking herbivore, the aphid Aphid nerii. We chose the A. syriaca- A. nerii system because it is occurring in two different continents. In this study, the performance of Aphis nerii from North America and Europe (i.e. US and HUN) was tested on milkweed populations from NA and EU. First, we conducted aphid performance bioassays on six different plant populations. The objective was to access the degree of continental aphid adaptation by determining whether aphid populations perform better on the plant populations originating from the same continent (i.e. home), compared to plant populations from the other continent (i.e. away). Second, we measured aphid performance on plants that had been induced by the Monarch-butterfly caterpillars, Danaus plexippus, to access the degree of plant induction and the subsequent impact on aphid performance. No local adaptation was demonstrated in the “home vs away” comparison. Aphids were not achieving higher fitness on their home plant. However, US aphids had a faster development time, higher fecundity and built larger populations than HUN aphids. Furthermore US aphids did better on EU plants than on NA plants, whereas HUN aphid performance was heterogeneous among NA and EU plants. Induction treatment differed between NA and EU milkweed. On the Quebec population it positively affected HUN aphid development time, whereas it negatively affected aphid performance on the Hungarian milkweed population. Our results suggests that the A. syriaca- A. nerii interaction differs from both the plant’s and the herbivore’s perspective. As suggested by ERH, our study pointed at a change in Milkweed metabolism. 7 8 Table of contents Introduction 13 The milkweed system as a story between invasiveness and specialism 17 Research questions and hypothesis 21 Materials and methods 23 Plants 23 Aphid 25 Lepidoptera 25 Experimentations 27 Bioassay 1 25 Bioassay 2 29 Measurement of various plant traits 29 Statistics 29 Results 33 Bioassay 1: Home versus Away comparison 33 Measurement of various plant traits 33 Measurement of Aphid performances 29 Bioassay 2: induction by monarch caterpillar 35 Dry-weight measurement 35 Measurement of Aphid performances 35 Discussion 37 References 45 9 10 Table of contents, figures and annexes Figure 1: Asclepias syriaca (drawing) 15 Figure 2: Some North American Milkweed-herbivores (pictures) 16 Figure 3: Herbivores feeding on A. syriaca in Europe (pictures) 18 Figure 4: Milkweed populations and related coding (table) 22 Figure 5: Danaus plexippus caterpillar (picture) 24 Figure 6: Aphid settling procedure 26 Figure 7: Bioassay 1, Aphid development time (graphic) 32 Figure 8: Bioassay 1, Aphid fecundity after three days (graphic) 32 Figure 9: Bioassay 1, Aphid population densities after two weeks (graphic) 34 Figure 10: Bioassay 2, Aphid development time (graphic) 34 Figure 11: Bioassay 2, Aphid fecundity after three days (graphic) 35 Figure 12: Bioassay 2, Aphid survival (graphic) 36 Figure 13: Bioassay 2, Aphid logarithmic dynamics on three weeks (graphic) 36 Figure 14: Statistics tables from SPSS 49 Table 1: Bioassay 1, aphid development time, general linear model (GLM) Table 2: Bioassay 1, aphid fecundity over 3 days, GLM repeated measure of variances Table 3: Bioassay 1, aphid population dynamics, GLM repeated measure of variances Table 4: Bioassay 2, stem dry weight, Table 5: Bioassay 2, aphid development time, GLM Table 6: Bioassay 2, aphid survival, GLM Table 7: Bioassay 2, aphid population dynamics 11 12 Introduction Alien species invading new ecosystems are becoming a worldwide problem, affecting biodiversity and leading to economic issues when these exotics become a plague. Numerous hypotheses taking into account abiotic and/or biotic conditions have been formulated to explain how an exotic becomes invasive e.g. allelopathy, competition, human activity etc. The enemy release hypothesis (ERH) postulates that introduced plants experience in the new ecosystem a decrease in regulation by local herbivores and pathogens. Therefore, they can spread more easily and rapidly in its new range than in its native range (Keane and Crawley, 2002; Muller-Scharer et al., 2004). ERH is subject to discussion (Collautti et al., 2004) because herbivore pressure is not ultimately the largest threat to plants, but it remains an important hypothesis when plants escape from host-specific herbivores (i.e. specialist) with whom they have co-evolved (review in Thompson 1989, 1999). For example, if an exotic plant species experiences a significant decrease in herbivore-pressure, i.e. when it is free of specialist, these plants may survive and disperse, leading to a possible outbreak in its new range. Successful establishment of exotic plants can be explained by two mutually non-exclusive hypotheses 1) plants harbor pre-adapted traits, to the new range context, insuring successful colonisation and 2) after establishment, plants rapidly evolve traits contributing to successful expansion (Muller-Scharer et al., 2004). It is usually assumed that plants are able to allocate resources in a way that maximizes individual inclusive fitness (Optimal defense hypothesis 1, Stamp, 2003). It is commonly stated that defenses (e.g. secondary metabolites) are costly and are allocated (constitutively and inducible) in proportion to risks and costs with regard to plant fitness (Stamp, 2003). At the same time, defense processes divert resources from other needs e.g. growth and reproduction, resulting in a tradeoff between defense and other plants traits, especially if no crosstalk occurs between defense metabolites and primary chemistry. The most prominent change with respect to enemies experienced by the exotic in its new range is a shift towards an assemblage dominated by generalist foe, although generalist pressure can vary tremendously (Muller-Scharer et al., 2004). Subsequently, an alien species released in a range free from its specialist foe is expected to decrease investment in defenses against specialists to maximize growth and dispersal; (i.e. Evolution of Increased Competitive Ability (EICA) Muller-Scharer et al., 2004), especially if defenses are toxic at low doses and costly to produce. 13 14 Asclepias syriaca (Figure 1) is a toxic