Plant Module Size and Attack by the Goldenrod Spindle-Gall Moth

406 Plant module size and attack by the goldenrod spindle-gall moth Stephen B. Heard,1 Graham H. Cox Department of Biology, University of New Brunswick, Fredericton, New Brunswick, Canada E3B 1R3 Abstract—Larvae of the gall-inducing moth Gnorimoschema gallaesolidaginis (Riley) (Lepidoptera: Gelechiidae) attack ramets of Solidago altissima L. and S. gigantea Aiton (Asteraceae), initiating stem galls early in ramet growth. We examined the relationship between ramet size (as an indicator of plant vigour) and galling rate over 3 years at a field site in Toronto, Ontario, Canada. We marked Solidago ramets along line transects, measured their stem diameter, and recorded their fate (galled or ungalled) during the season. For S. altissima, galls were numerous enough for analysis in 2 years, and the frequency of galling increased monotonically with ramet stem diameter in both years. For S. gigantea, galls were numerous enough for analysis in all 3 years, but attack rate – stem diameter relationships were complex. In 2004 the galling frequency peaked at intermediate stem diameter, but in 2005 the galling frequency increased monotonically with stem diameter (and in 2006 the nonsignificant trend was similar). Overall, our data are most consistent with the plant-vigour hypothesis, but the 2004 data for S. gigantea lend some support to the suggestion that herbivore attack might sometimes be most intense on intermediate-sized modules. Résumé—Les larves du papillon de nuit gallicole Gnorimoschema gallaesolidaginis (Riley) (Lepidoptera: Gelechiidae) attaquent les ramilles de Solidago altissima L. et de S. gigantea Aiton (Asteraceae), ce qui provoque la formation de galles sur la tige tôt dans la croissance des ramilles. Nous avons examiné la relation entre la taille de la ramille (comme indicateur de la vigueur de la plante) et le taux de formation des galles pendant trois années à un site de terrain de To- ronto, Canada. Nous avons marqué des ramilles de Solidago le long de lignes de transect, mesuré le diamètre des tiges et déterminé leur sort (avec ou sans galles) au cours de la saison. Chez S. altissima, les galles étaient assez abondantes durant deux des années pour permettre l’analyse; la fréquence de formation des galles s’est accrue de façon monotone en fonction du diamètre des tiges pendant les deux années. Chez S. gigantea, les galles étaient assez abondantes durant les trois années, mais les relations entre le taux d’attaque et le diamètre de la tige étaient complexes. En 2004, le taux de formation des galles a atteint un maximum aux tailles intermédiaires des tiges, mais en 2005 le taux de formation de galles a augmenté de manière monotone en fonction du diamètre de la tige (et en 2006 la tendance était semblable mais non significative). Dans leur ensemble, nos données s’accordent avec l’hypothèse de la vigueur de la plante; cependant, les données de 2004 chez S. gigantea apportent un certain appui aux propositions selon lesquelles l’attaque des herbivores peut quelquefois être plus intense sur les modules de taille intermédiaire. [Traduit par la Rédaction] Heard414 and Cox both preference and performance) is often Introduction highly variable among individual plants within Feeding by phytophagous insects on their host populations (e.g., Wellings 1987; Eber 2004) plants has major ecological, evolutionary, and and among modules (repeated morphological economic consequences (e.g., McConnachie et units such as leaves, branchlets, or ramets) al. 2003; Halpern and Underwood 2006; Maron within individual plants (e.g., Gripenberg et al. and Crone 2006). Herbivory (which reflects 2007; Santos et al. 2008). This variation can Received 31 December 2008. Accepted 17 February 2009. 1Corresponding author (e-mail: [email protected]). doi: 10.4039/n09-016 Can. Entomol. 141: 406–414 (2009) © 2009 Entomological Society of Canada Heard and Cox 407 have important consequences for the impact of Empirical data on module size – attack rate herbivory on plants at the individual and popu- relationships are mixed, with some insect herbi- lation levels (e.g., Heard and Remer 2008). De- vores showing increased herbivory on larger (or spite a wealth of studies examining herbivore faster growing) plants/modules, whereas others load as a function of ecological, environmental, show the opposite pattern (Price 2003; Price et and spatial attributes of plants or modules, there al. 2004; Quiring et al. 2006). Still others show remains considerable uncertainty about the gen- parabolic relationships between plant/module erality of patterns in, and the ecological mecha- size and herbivory (Quiring et al. 2006). The nisms behind, herbivore-load variation. latter possibility is particularly interesting and One interesting aspect of herbivore-load vari- suggests the possibility of a trade-off between ation has been the relationship between herbi- vigour and stress, sink competition, or gall in- vore attack and plant (or module) size. This duction that creates an optimum module size relationship has important implications for the for herbivore attack. impact of herbivory on plant populations be- Gall inducers are particularly good model cause larger plants or modules often contribute organisms for the study of herbivore–host inter- disproportionately to population-level reproduc- actions because they have very intimate relation — making attack on larger plants or mod- tionships with their host plants and because ules likely to have more severe consequences at each individual is unambiguously associated with the population level. Furthermore, several hy- a single host-plant module. Price (1991) has argued potheses accounting for herbivore preference for that gall inducers are particularly likely to con- (and performance on) individual plants or mod- form to the plant-vigour hypothesis. However, ules lead to predictions about the effects of module because sink-competition and induction-stimulus size on herbivore attack rates. Of these, the most mechanisms are specific to gall inducers among obvious is the plant-vigour hypothesis (Price 1991), herbivores, and should lower realized attack on which suggests that herbivores should prefer, and large modules, one could argue instead that gall (or) perform better on, plants or modules that inducers are particularly unlikely to conform to through their vigorous growth represent a rich the plant-vigour hypothesis. Resolution of this source of resources. Such vigorously growing conflict will require empirical data for a large modules should, all else being equal, have grown and diverse set of gall inducer – host combina- larger than their less vigorous neighbours and tions, but unfortunately the set of studied gall so, according to the plant-vigour hypothesis, the attack rate should increase with module size. inducers remains small and weighted heavily The attack-rate pattern would be similar if toward members of a few insect families larger modules were simply more apparent to (Quiring et al. 2006). Notably, few data exist searching herbivores. However, several hypoth- for lepidopteran gall inducers (just two entries eses suggest roughly the opposite. For instance, in the compilation of Quiring et al. 2006). We the plant-stress hypothesis posits that plants or examined the relationship between module size modules under stress are more suitable for and attack for larvae of the goldenrod spin- attack because they are often nutritionally supe- dle-gall moth, Gnorimoschema gallae- rior and less able to mount anti-herbivore defences solidaginis (Riley) (Lepidoptera: Gelechiidae), on (Rhoades 1979; White 1984). Modules experi- its two goldenrod hosts, late goldenrod, Solida- encing stress during growth are, of course, go altissima L., and tall goldenrod, S. gigantea likely to be smaller than their unstressed neigh- Aiton (Asteraceae). (Moths on S. gigantea have bours. For gall inducers (but not necessarily been treated as G. jocelynae Miller, although other herbivores), two other mechanisms might their status as a host race versus a cryptic spe- lead to better performance on smaller plant cies remains unclear; for simplicity we use the modules: vigorously growing modules may be name G. gallaesolidaginis throughout.) Our unsuitable for attack because their tissues repre- study does not directly test the mechanistic basis sent strong nutrient sinks that act in competition of size-related patterns of attack and thus does with gall growth and insect feeding (e.g., not include direct tests of the plant-vigour and Larson and Whitham 1997) or because gall in- plant-stress hypotheses. However, we do docu- duction on such a module requires a larger dose ment the effect of module size on attack in of the gall-inducing stimulus than an individual unmanipulated field populations of the two host insect can provide (Bjorkman 1998; McKinnon species in 3 years and we provide a direct test et al. 1999). of the optimal-module-size hypothesis. © 2009 Entomological Society of Canada 408 Can. Entomol. Vol. 141, 2009 Materials and methods Gall surveys We surveyed goldenrod populations for the G. gallaesolidaginis Study organisms and study sites presence of galls through the gall-initiation season in 2004, 2005, and 2006. Solidago gigantea and S. altissima are clonal In early spring of each year we laid line transects perennials, regrowing each spring from under- through mixed-species stands of Solidago and ground rhizomes. Many individual shoots (= then marked with a numbered metal tag every ramets) generally arise from one genetic indi- Solidago

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