Willow Hybridization Differentially Affects Preference and Performance Of
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Entomologia Experimentalis et Applicata 83: 285±294, 1997. 285 c 1997 Kluwer Academic Publishers. Printed in Belgium. Willow hybridization differentially affects preference and performance of herbivorous beetles ; Colin M. Orians1; , Cynthia H. Huang1, Alexander Wild1, Katherine A. Dorfman1 , Pamela Zee2,MinhTamT.Dao2 & Robert S. Fritz2 1Department of Biology, Vassar College, Poughkeepsie, NY 12603, USA; 2Department of Biology, Williams College, Williamstown, MA 01267, USA; Present address: Department of Biology, Tufts University, Medford, MA 02155, USA; Mt Holyoke College, Department of Geology and Geography, South Hadley, MA 01075, USA Accepted: February 6, 1997 Key words: Salix, hybridization, phenolic glycosides, herbivore preference, herbivore performance Abstract We examined the preferences and performances of ®ve beetle species (four chrysomelids and one scarab) on two species of willows (Salix sericea and S. eriocephala) and their interspeci®c hybrids. Beetle species differed markedly in their responses. In preference assays, two chrysomelid beetle species (Calligrapha multipunctata bigsbyana and Plagiodera versicolora) preferred hybrids, two chrysomelids (Chrysomela scripta and Ch. Knabi) preferred hybrids and S. sericea, and the scarab beetle (Popillia japonica) preferred S. eriocephala. Experiments with puri®ed salicortin indicated that salicortin concentration may contribute to these preferences. The relative performance (growth rate, pupal/adult weight and survivorship) of these beetles on the three willow taxa did not correspond with their feeding preferences. Three species exhibited intermediate performance on hybrid willows (the two Chrysomela spp. and P.japonica); the Chrysomela spp. performed best on S. sericea, while P.japonica performed best on S. eriocephala. One species performed equally well on all three taxa (C. multipunctata bigsbyana). The performance of Pl. versicolora was not tested. Our results support the general pattern that willow taxa with phenolic glycosides are more acceptable to specialist willow herbivores while those taxa without phenolic glycosides are more acceptable to generalist herbivores. We also show that to predict the relative susceptibility of hybrid and parental plants to herbivores, consideration must be given to the inheritance of traits affecting both preference and performance. Introduction There is no reason to expect one single pattern to dominate. For one to dominate, we would have to Hybridization among plants is more common than pre- assume that traits that affect herbivore responses, pref- viously recognized (Arnold, 1992). This realization erence or performance, such as secondary chemistry has led to numerous studies focusing on the ecolo- (Tahvanainen et al., 1985; Lindroth et al., 1988; Roin- gical and evolutionary consequences of hybridization, inen & Tahvanainen, 1989; Schultz, 1989), foliar nutri- especially in the ®eld of plant-herbivore interactions ents (Mattson 1980; Bryant et al., 1987), leaf water (see Strauss, 1994, and references therein). Hybrids content (Horton, 1989), shoot morphology(Price et al., and parental species typically differ in resistance to 1987), plant phenology (Aide, 1988), and plant onto- herbivores. Contrary to Whitham's (1989) original geny (Bryant, 1981; Kearsley & Whitham, 1989) show hypothesis that hybrids are ecological and evolution- similar patterns of inheritance. We would also have ary sinks to pests (because they are more susceptible), to assume that herbivores respond similarly. Because patterns of hybrid resistance are diverse (Boecklen & the inheritance patterns of traits in hybrids are vari- Spellenberg, 1990; Fritz et al., 1994; Strauss, 1994). able (Rieseberg & Ellstrand, 1993) and herbivore spe- cies are known to differ in their responses to intra- GR: 201003416, Pips nr. 135632 BIO2KAP *135632* ento1530.tex; 17/06/1997; 11:56; v.7; p.1 286 and interspeci®c variation in plant quality (Matsuki & Materials and methods MacLean, 1994; Orians & Fritz, 1996; Orians & Floyd, 1997), no single pattern of hybrid susceptibility should The system. This willow system has three distinct be expected. advantages. First, Salix sericea and S. eriocephala Among willow species, much of the variation in are largely sympatric and hybridize naturally (Argus, susceptibility has been attributed to the presence or 1974, 1986). Second, there is a diverse herbivore com- absence of phenolic glycosides (Rowell-Rahier, 1984; munity that feeds on the three willow taxa. Finally, to Tahvanainen et al., 1985; Denno et al., 1990; Kelly & control for confounding environmental effects, these Curry, 1991; Rank, 1992; Kolehmainen et al., 1994). willows can easily be cloned and grown in a com- Phenolic glycosides stimulate feeding by some herb- mon garden (Paige & Capman, 1993; HanhimakiÈ et ivores but deter feeding by others (Matsuda & Mat- al., 1994; Christensen et al., 1995). Clearly this is an suo, 1985; Tahvanainen et al., 1985; Rank, 1992; excellent system in which to investigate the effects of Kolehmainen et al., 1994). Phenolic glycosides are hybridization on the responses of various herbivores. also known to affect the growth of herbivores (Denno et al., 1990; Kelly & Curry, 1991; Matsuki & MacLean, Plants. In 1992 we used leaves from ®eld plants at 1994). In our study system, Salix sericea Marshall and our study site near Milford, NY. This is an area that is S. eriocephala Michx. and their hybrids differ dramat- dominated by S. sericea but also contains S. erioceph- ically in their secondary chemistry (Orians & Fritz, ala and their hybrids. In 1993 and 1994 we used leaves 1995). S. eriocephala foliage contains high concentra- from cuttings grown in a common garden at Williams tions of condensed tannins and no phenolic glycosides, College in Williamstown, MA. In 1992 and 1993 we while S. sericea foliage contains high concentrations of used a single genotype of each taxon. In 1994 we used phenolic glycosides, especially salicortin, but low con- four genotypes of each willow taxon (with 5 replic- centrations of condensed tannins. The concentration ate clones per genotype). To ensure that taxon effects of salicortin in S. sericea often exceeds 10% dry leaf were not confounded by plant gender effects (willows weight (Nichols-Orians et al., 1992, 1993). Hybrids are dioecious), three of the four genotypes of each contain intermediate concentrations of both classes of taxon were female. However, beetle responses to the chemicals (Orians & Fritz, 1995). Given the differ- male genotype were not distinct from their responses ences in phenolic glycoside chemistry in this willow to the female genotypes (data not shown). This result system, we hypothesized that the responses of herbi- is consistent with previous results (R. Fritz, unpubl. vores to parental and hybrid willows would be variable data). The identity of the parental and hybrid willows and that chemistry would be an important determinant was con®rmed using RAPD analysis, and the hybrids of those responses. selected for this study contain intermediate concentra- We have been quantifying the abundance of numer- tions of phenolic glycosides and appear to be F-1 and ous insect herbivores on three willow taxa, S. sericea, F-2 type hybrids (Fritz et al., 1994; Orians & Fritz, S. eriocephala and their hybrids. From our censuses 1995; S. Brunsfeld, pers. comm.) of ®eld plants we have found a diversity of herbi- In the spring of 1993, we collected lea¯ess bud- vore responses (Fritz et al., 1994). We have not until sticks (cuttings) from plants at our study site in NY. now explored potential mechanisms behind the differ- 20 cm cuttings were dipped in rootone to stimulate ent responses. Here we report on the feeding prefer- root growth, and planted into Agway's Potting Soil ences and performance of ®ve beetle species: Calli- Mix. Cuttings were watered daily and fertilized once grapha multipunctata bigsbyana, Plagiodera versicol- with 20-20-20 Agway fertilizer. In May, they were ora, Chrysomela scripta and Chrysomela knabi,and potted into 8 liter pots containing loam, peatmoss and Popillia japonica to S. sericea, S. eriocephala,and vermiculite (3:1:1) and fertilized weekly with 140 ml of their interspeci®c hybrids. By painting salicortin onto 20-20-20 soluble fertilizer (3.2 g/l).The plants were cut leaves, we further determined whether salicortin may back in the late fall of 1993 and allowed to regenerate contribute to the variable responses. in the spring of 1994. In the spring of 1994 they were transplanted into 16 liter pots, fertilized with 15 ml of 10-10-10 insoluble fertilizer (Agway) and watered as needed. To avoid the use of pesticides, we removed herbivores by hand. ento1530.tex; 17/06/1997; 11:56; v.7; p.2 287 Beetles. The responses of four chrysomelids, a wil- Beetles were collected from S. sericea at our New York low leaf beetle (Calligrapha multipunctata bigsby- ®eld site in July. The beetles were brought to Hartwick ana (Kirby) ± henceforth referred to as C. multi- College, in Oneonta NY and maintained on S. sericea punctata), the imported willow leaf beetle (Plagiodera leaves until the choice tests were performed. Choice versicolora (Laicharting)), and two Chrysomela wil- tests were terminated after 4±6 h and the remaining low leaf beetles (Chrysomela knabi Brown & Chryso- leaf area of each disc was measured. (Because using mela scripta Fabricius) were tested. In addition, one beetles reared on S. sericea may bias choice tests, 1993 scarab, the imported Japanese beetle (Popilliajaponica