Wageningen University Laboratory of Entomology Plant genotype affects the herbivore community of evening primrose (Oenothera biennis ). Camille Ponzio Msc Plant Science April- August 2010 Report no. 010.22 Thesis ENT-80424 1st examiner : Marcel Dicke 2nd examiner : Jennifer Thaler Summary Intraspecific variation within a single plant species plays a key role in determining its associated arthropod diversity and community composition. In a field experiment I replicated 6 genotypes of evening primrose ( Oenothera biennis ) on which 5 treatments were equally applied (including herbivory from two leaf chewers) and looked at the abundance of the 10 most common insect species. While the effects of genotype on the herbivore community associated to evening primrose had already been in part documented in previous studies, the novelty of this study is that it showed that genotypic variation in evening primrose had differential effects on the generalist and specialist herbivores. Generalists tended to prefer genotypes which had been previously shown to be highly attractive to herbivory to the generalist Japanese beetle ( Popillia.japonica ) Specialists on the other hand avoided these genotypes and were highly abundant on the genotypes which are usually avoided by Japanese beetles. Variation in several plant traits could in part explain the variation in abundance, notably for the specialists. The phenolic compound Oenothein B, nitrogen content, water content and trichome characteristics accounted for 73 to 93% of the variation in the abundance of specialists. While no significant effects could be seen on the generalists, several trends indicated that the plant traits may correlate in the opposite direction with generalists. While the original goal of this study was to investigate the effects of herbivore induced plant responses on the insect community, remarkably the treatments had very little significant effects on a majority of the insects, with only the specialist primrose weevil (Acanthoscelidius acephalus ) showing a preference for plants damaged by conspecifics. Table of contents Introduction .............................................................................................................................. 1 Research aims and questions ................................................................................................... 3 Materials and methods ............................................................................................................. 4 Study System .......................................................................................................................... 4 Common garden experiment .................................................................................................. 5 Measurements ......................................................................................................................... 6 Statistics ................................................................................................................................. 8 Results ..................................................................................................................................... 11 Discussion : ............................................................................................................................... 15 Individual and community -level effects of genetic variation .............................................. 15 Treatment effects .................................................................................................................. 16 Future research: .................................................................................................................... 17 Conclusions ............................................................................................................................. 18 Acknowledgements ................................................................................................................. 19 References ............................................................................................................................... 20 Introduction Plants frequently display genotypic variation that can influence the host plant preference and abundance of individual arthropod species (Karban 1992, Underwood and Rausher 2000) and ultimately regulate the structure of the herbivore community (Maddox and Root 1987, Fritz and Price 1988, Dungey et al. 2000, Tovar-Sánchez and Oyama 2006, Whitham et al. 2006, Poelman et al. 2008). Several studies have shown that not only can herbivorous insects discriminate between hybrid plants and the parent species that compose them (Dungey et al. 2000, Fritz et al. 2003, Hochwender and Fritz 2004), they can also differentiate between different genotypes of a single host plant species (Fritz and Price 1988, Maddox and Root 1990, Shuster et al. 2006). The role of plant genotype in structuring arthropod communities is such that high heritability has been found in several model systems. It has been that it estimated that 50-63% of the variation in arthropod community composition in a single cottonwood species could be explained by plant genotypic variation (Shuster et al. 2006). Likewise, in evening primrose ( Oenothera biennis ) differences among the genotypes accounted for up to 41% of the variation in arthropod diversity and abundance (Johnson and Agrawal 2005). The notion that insect communities will strongly vary according to the genotypic differences in their host plant is not surprising in view of the wide array of host plant traits, herbivore induced or not, that can affect arthropods. Intraspecific genotypic variation has been found in an array of plant traits, such as morphological traits, (Inbar and Gerling 2008) phenological traits (Hunter et al. 1997), trichomes (Lambert et al. 1995, Luo et al 2010), plant chemistry (Dungey et al. 2000, Walling 2000, Osier and Lindroth 2001) and plant secondary chemical defense compounds (Hamilton et al. 2002), all of which have been shown to have effects on arthropods. Further work on evening primrose by Johnson et al. (2009) showed that a large proportion of variation (73%) in herbivory by Japanese beetles, a common and abundant herbivore, could be explained by genetic variation in several secondary compounds and life history traits. Genetic variation in plant resistance and induced responses can strongly affect the population of individual herbivore species (Underwood and Rausher 2000, Poelman et al 2009) and the structure of the arthropod community (Fritz and Price 1988, Maddox and Root 1990). These induced defenses are known to differentially affect arthropods according to their host specificity. While generalists herbivores are generally negatively affected or deterred by chemical defenses (van Dam et al. 1995, Lankau 2007), many studies show that secondary compounds may make a plant highly attractive to specialist herbivores that have become adapted to these specific compounds (van Dam et al. 1995, Siemens and Mitchell- Olds 1996). Much evidence indicates that specialists use the unique secondary compounds as cues to locate host plants and to assess the suitability of the plant for feeding and oviposition, and thus will have a preference for host plants having higher concentrations of the compound (Da Costa and Jones 1971, Raybould and Moyes 2001, Macel and Vrieling 2003). Differing levels of secondary compounds between plant genotypes could be expected to lead to differences the ratio between generalists and specialists on a given genotype. 1 While herbivores of the same feeding guild elicit the same general type of responses in the host plant, it is often the case that these induced defenses will show some degree of variation from one herbivore to another (Viswanathan et al. 2005), as the elicited responses can largely depend on the nature of damage inflicted and the biochemical composition of the attacker’s saliva. (Musser et al. 2002) The effects triggered in the plant affect not only the herbivore and its conspecifics ( Denno et al . 1995), but also other herbivore species both across space and time. Indeed, an increasing number of studies are demonstrating the specificity of induced responses, in that the identity of the initial damaging herbivore can differentially affect the host plant preferences of subsequent herbivores and modify the community structure throughout the season (Zandt and Agrawal 2004, Viswanathan et al. 2005, 2007, Poelman et al. 2008). 2 Research aims and questions The main objective of this research was to discover if arthropod abundance and diversity varied among the different genotypes of evening primrose ( Oenothera biennis ). A second aim was to determine the effects of early season herbivory on the same insect community. More specifically, this research addresses the following questions: 1) Does plant genotype affect individual arthropod species and their community structure? 2) Are generalists and specialist herbivores equally affected by plant genotype? If so, can differences in effects be explained by variation in certain plant traits? 3) Does early season herbivory by the generalist Japanese beetle ( Popillia japonica ) or the specialist primrose weevil (Acanthoscelidius acephalus ) similarly affect the nature of the subsequent colonizers as well as the arthropod community structure and diversity? As previous studies on evening primrose have shown that evening primrose genotypes strongly varied in their associated herbivore abundance and richness, this study first