Floral Herbivory Increases with Inflorescence Size and Local Plant

acta oecologica 34 (2008) 21–25

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Original article Floral herbivory increases with inﬂorescence size and local plant density in Digitalis purpurea

Nina Sletvold*, John M. Grindeland1

Department of Biology, University of Oslo, P.O. Box 1066 Blindern, N-0316 Oslo, Norway article info abstract

Article history: Insect herbivores search for their host plants in heterogeneous environments, and the ef- Received 15 January 2007 ficiency of host location may be influenced by plant architecture and abundance. In this Accepted 7 March 2008 study, we ask how plant and habitat characteristics traditionally thought to attract pollina- Published online 29 April 2008 tors are related to attack rates by floral herbivores. Patterns of floral herbivory by the moth larva Eupithecia pulchellata were studied in relation to inflorescence size and local plant den- Keywords: sity in two years in a natural population of the facultative biennial Digitalis purpurea. Over- Density-dependence all levels of herbivory were low, 84% of the infested plants lost less than 10% of their Insect herbivory flowers. Only 9% of the plants lost more than 20% of their flowers. Probability of herbivory Monocarpic perennial at the plant level increased strongly with inflorescence height, and it was considerably Plant–animal interactions higher in dense patches compared to sparse ones. There was no effect of local plant density Plant size on the functional relationship between inflorescence size and probability of herbivory. Both number and proportion of damaged flowers per plant increased with inflorescence height. The results suggest that E. pulchellata is attracted to dense patches and large individuals of D. purpurea, and that negative effects of herbivory increase with plant size. This implies diminishing returns for investment in more flowers in D. purpurea, and indi- cates that herbivory may select for smaller flowering size and flower number in this monocarpic species. ª 2008 Elsevier Masson SAS. All rights reserved.

1. Introduction it is increasingly realized that plant traits usually interpreted in terms of pollinator attraction may affect herbivores and The efficiency of host location is an important fitness determi- seed predators as well (Herrera, 2000; Ehrleń et al., 2002; nant for insect herbivores. Insect herbivores search for their Go´ mez, 2003). One plant trait that may have an effect on host plants in habitats where plant architecture, size and den- both mutualistic and antagonistic interactions is inflores- sity typically vary on a local scale, and herbivores are expected cence size. to evolve strategies that deal with high levels of habitat com- The fact that large floral displays attract more pollinators is plexity (Cook and Holt, 2006). Host-plant ‘apparency’ in the well established (reviewed by Ohashi and Yahara, 1999), habitat should influence location rates (Courtney, 1985), and though usually accompanied by a decrease in the proportion

* Corresponding author. Present address: NTNU, Museum of Archaeology and Natural History, Section of Natural History, N-7491 Trondheim, Norway. E-mail address: [email protected] (N. Sletvold). 1 Present address: Sør-Trøndelag University College, Rotvoll Alle´ 1, N-7004 Trondheim, Norway. 1146-609X/$ – see front matter ª 2008 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.actao.2008.03.002 22 acta oecologica 34 (2008) 21–25

of open flowers probed (Mitchell et al., 2004; Grindeland et al., delay of flowering for several seasons is common (Sletvold 2005). In contrast, little data exist on the relationship between and Grindeland, 2007). The pink to purple, protandrous herbivore damage intensity and plant size (Klinkhamer et al., flowers are produced on terminal racemes from mid June. 1997). Several studies have found pre-dispersal seed predation Flowers open sequentially from bottom and upwards. The rate to increase with inflorescence size (Traveset, 1994; Ehrleń, length of a population’s flowering period is typically 3–4 1996; Brody and Mitchell, 1997), indicating that herbivores weeks, but large among-individual differences are found due may select for smaller display size. However, although large to variation in total number of flowers (Sletvold, 2002). Plants plants attract more herbivores, the proportion of flowers or are self-compatible and primarily visited by bumblebees fruits eaten may be a constant (Sperens, 1997) or decreasing (Grindeland et al., 2005). (Brody and Mitchell, 1997) function of plant size, indicating Plants experience herbivory by larvae of the monophagous selection for large display size. species E. pulchellata Stephens (Geometridae, Lepidoptera), the Herbivore response to variation in host-plant size may also foxglove pug. This species is restricted to the coastal areas of be influenced by local plant density. According to optimal for- south-western Norway, following the core distribution of its aging theory, host searching time is expected to decrease with host plant (Aarvik et al., 2000). Little specific data exist on higher host-plant density, and the probability of herbivory in the field ecology of this species. The moth flies during night dense populations should increase (Stephens and Krebs, in spring and early summer (May–June), and is supposed to 1986). Empirical results have, however, been mixed (Kunin, be most abundant early in the flowering season of its host 1999; Masumoto et al., 2000; Sheppard and Vitou, 2000; Raghu plant. Oviposition takes place on immature buds and the larva et al., 2005), and a few studies have reported negative density- feeds inside the flowers, consuming the stamens and ovaries dependence (Shea et al., 2000; Bishop, 2002). At present there (Skou, 1984). The presence of the larva inhibits flower opening, is little consensus on how insects respond to variation in although the ‘‘bud’’ grows to a size well beyond the point host density at the patch level (reviewed by Cook and Holt, where it would normally open. There is never more than 2006). Density may also change the shape of the relationship one larva per flower (J.M. Grindeland and N. Sletvold, unpub- between herbivory and inflorescence size. Rausher (1983) lished data), suggesting that only one egg is laid on each flower demonstrated that butterflies may alter oviposition behaviour bud. The seed set of damaged flowers is zero. in response to plant density variation, but whether this is a common pattern is unknown. A few, recent studies have found density-dependent response curves in plant–pollinator interactions (Ohashi and Yahara, 2002; Grindeland et al., 2.2. Field work 2005), but this is largely unexplored in plant–herbivore systems. The study was conducted in a pasture at Ulvik (Hordaland This study examines pre-pollination floral herbivory by the County) in south-western Norway. The pasture consisted of geometrid moth Eupithecia pulchellata (Geometridae, Lepidop- a mosaic of areas with varying focal plant density; some areas tera) in the facultative biennial Digitalis purpurea L. (Plantagi- are disturbed (by grazing/small screes) with high D. purpurea naceae). The moth oviposits on flower buds, and the larva density, whereas other areas are closed grassland with low feeds on the anthers and ovaries inside the developing flower. D. purpurea density (see Sletvold and Rydgren, 2007 for further Attacked flowers set no seeds, and flower damage is equiva- details). In 1997 all individuals within an area of approxi- lent to fruit loss. The main objective is to establish how her- mately 100 5 m on a roadside inside this pasture were se- bivory by E. pulchellata is related to variation in inflorescence lected. Inflorescence height, total number of flowers and size in D. purpurea. In addition we explore whether local plant presence/absence of E. pulchellata were recorded for a total of density affects damage patterns. The more specific questions 394 individuals. On plants where the herbivore was present addressed are: (1) does probability of floral herbivory increase the number of damaged flowers was determined. This was with inflorescence size, and does plant density at the patch done during three consecutive days late in the flowering sea- level influence this relationship? (2) Does the proportion of son (mid July). The roadside included areas of both dense and damaged flowers decrease with inflorescence size? sparse character, but no data on local density were collected. In 1999, focal plants were chosen within patches of uni- form local density, where patch boundaries roughly followed 2. Material and methods local distribution of D. purpurea. From the approximately 450 flowering plants present in the pasture, 14 patches including 2.1. Study system a total of 133 plants were randomly selected and assigned to two different plant density categories: seven dense patches Foxglove, D. purpurea L., is a facultative biennial herb which with plants less than 0.5 m apart, and seven sparse patches grows on naturally disturbed soil at places like river banks, with plants more than 1 m apart. Focal plants were haphaz- heaths, screes and tree uprootings, or on soil disturbed by hu- ardly selected within each patch according to the number man activities, as roadsides, wood-clearings and pastures. available. In dense patches plant number varied from 6 to 14 The species has a persistent seed bank from which abundant with a total of 65, and in sparse patches from 8 to 11 with a to- germination takes place after soil disturbance (van Baalen, tal of 68. Inter-patch distances varied from 5 to 20 m. Plant 1982). Seeds mainly germinate in spring, and individuals height, total number of flowers and presence/absence of form basal rosettes that remain vegetative during the first E. pulchellata were recorded for all individuals. Last recording summer. Flowering may occur in the second summer, but date was July 8th. acta oecologica 34 (2008) 21–25 23

Table 1 – Generalized linear model analyses of the effects of inflorescence height (1997) and inflorescence height and density (1999) on the probability of herbivory and proportion of damaged flowers (binomial errors, logit link), and on the number of damaged flowers (Poisson errors, log link) Source of variation Probability of herbivory Number damaged Proportion damaged

df c2 P df c2 P df c2 P

1997 (n ¼ 394) Height 1 22.15 <0.0001 1 108.9 <0.0001 1 9.21 0.0024

1999 (n ¼ 133) Height 1 12.71 0.0004 Density 1 20.27 <0.0001

2.3. Data analysis (Table 1, Fig. 2a and b). In total, 84% of the damaged plants lost less than 10% of the flowers to the herbivore, whereas Due to differences in sampling strategy and type of data col- only 9% of the plants had as high losses between 20 and 30% lected, data from the two years were analysed separately. of the flowers (Fig. 3). Damaged flowers were usually Probability of herbivory and proportion of damaged flowers consecutive. (damaged flowers/total flowers) were analysed by logit models, i.e. binomial response distribution and logit link function (Agresti, 1996). Number of damaged flowers (a relatively 4. Discussion small integer count) was analysed by log-linear models, i.e. Poisson response distribution and log-link function (Crawley, Both larger inflorescence size and higher local plant density 1993; Agresti, 1996). Inflorescence height and total flower increased the probability of herbivory by the moth larva E. pul- number was highly correlated in both study years chellata in this population of D. purpurea. Moreover, the propor- (rs1997 ¼ 0.87, P < 0.001, rs1999 ¼ 0.91, P < 0.001), and we report tion of damaged flowers increased with inflorescence size, results using the former as our size estimate (analyses con- suggesting herbivore-mediated selection for smaller flowering ducted with flower number yielded nearly identical results). size and flower number. Inflorescence height was always included as a continuous Higher host-plant density has been found to increase ovi- covariate, and in 1999 plant density was included as a fixed position rate in various insects (Rausher, 1983; Kunin, 1999), categorical factor. In all analyses backward selection and but the opposite relation has also been reported (Shea et al., Type III tests (retain criterion P < 0.05) were used to find the 2000). In the present study, the probability of floral herbivory most appropriate model. The goodness of fit of logit and log- in D. purpurea was around twice as high in dense patches com- linear models was assessed by Pearson Chi-square statistics. pared to sparse ones, but no effect of density was found on the All analyses were performed with the Insight module in SAS 8.02 (SAS Institute Inc., 2001).

1.0 3. Results 0.8 3.1. Probability of herbivory 0.6 The abundance of E. pulchellata varied between years, in 1997 mean per plant probability of herbivory was 0.22, and in 0.4 1999, 0.52. Probability of floral herbivory at the plant level increased strongly with inflorescence height in both years, and 0.2 Probability of herbivory it was considerably higher in dense patches compared to sparse ones in 1999 (Table 1, Fig. 1). Effects were strictly addi- 0.0 60 80 100 120 140 160 tive, i.e. there was no significant interaction between inflores- Inflorescence height (cm) cence height and local density (c2 ¼ 0.51, P ¼ 0.47, n ¼ 133). 1997 Low 1999 High 1999 3.2. Magnitude and pattern of herbivory Fig. 1 – Probability of herbivory as a function of The number of damaged flowers in an attacked individual inflorescence height and local plant density. Symbols are ranged from 1 to 18 (mean sd ¼ 3.4 3.2), and the proportion observed means (±SE), calculated for 10-cm size classes of damaged flowers in attacked individuals ranged from 0.012 (61–70 cm, 71–80 cm etc.). Lines are fitted from the models: to 0.30 (mean sd ¼ 0.068 0.067). Both number and (1997) logit (herbivory) [ 0.0235 3 height L 3.40, n [ 394; proportion of damaged flowers increased moderately with in- (1999) logit (herbivory) [ 0.0304 3 height D 1.87 3 florescence height when all individuals were considered density L 2.03, n [ 133. 24 acta oecologica 34 (2008) 21–25

a b 0.35 20 Observed 18 Observed 0.30 Fitted line 16 Fitted line 0.25 14 12 0.20 10 0.15 8 6 0.10

Number damaged 4 Proportion damaged 0.05 2 0 0.00 30 50 70 90 110 130 150 30 50 70 90 110 130 150 Inflorescence height (cm) Inflorescence height (cm)

Fig. 2 – The number (a) and proportion (b) of damaged flowers in relation to inflorescence height in 1997. Models: log (damaged flowers) [ 0.023 3 height L 2.5, n [ 394; logit (damaged/total flowers) [ 0.0069 3 height L 4.8, n [ 394.

moth’s actual response curve to plant height. This suggests flowers in large plants may be the result of an increased num- that E. pulchellata is attracted to dense patches, but that behav- ber of oviposition events. The fact that damaged flowers are iour within the patch is independent of local host density. found in a row may then be due to a restricted flying time of Similar results were obtained in an earlier study of pollinators the moths; they may be present in the population for a minor within this population; plant visitation rate by bumblebees part of the flowering period of a large individual. Also, the were higher in dense patches, but the relationship with plant moth may actively discard plants already infested. Observa- size was unaffected by density (Grindeland et al., 2005). These tions of moth behaviour are necessary to separate between observations confirm that beneficial and damaging visitors these explanations. may respond to the same habitat characteristics, as e.g. local The average proportion of flowers damaged by the herbi- plant density. vore also increased with inflorescence size. Though magni- The probability of floral herbivory increased with inflores- tudes were low, proportional herbivory was more than cence height in both seasons, suggesting that increased height doubled in the largest D. purpurea inflorescences compared means increased attractiveness to the herbivore. The number to the smallest ones, indicating herbivore-mediated selection of attacked flowers also increased with height. Damaged for smaller reproductive size and flower number. If herbivore flowers were usually placed consecutively along the inflores- population size reaches levels where competition for flower cence, indicating that oviposition sequences were longer on buds become important, the impact of this interaction should large plants. This could reflect that the number of available increase. buds in the correct stage for oviposition will increase with In the present study losses due to herbivory were small plant size, and the moth may accordingly utilize more flowers compared to observations in other systems (reviewed in in large plants. Alternatively, the higher number of damaged Crawley, 1992). For most plants in our study less than 10% of the flowers were destroyed by the herbivore, and the highest amount observed was 30%. This magnitude is well within 50 46 the limits of what has earlier been found to be fully compen- 45 sated in terms of further flower (Ehrleń, 1996) or seed (Lowen- berg, 1994; Garcia and Ehrleń, 2002) production, indicating 40 that selection exerted by the herbivore interaction will be 35 weak. At the end of the flowering period, plants with tiny buds that never develop into flowers can be found in the field, 30 28 and these may represent a surplus flower production provid-

25 ing a buffer against unpredictable flower damage. Still, the ‘decision’ to bolt and flower is made sometime during autumn 20 Frequency the season before actual flowering in D. purpurea, and flower

15 number may to a large degree be set at the time bolting is ini- tiated. Possibilities of adjustments as a response to environ- 10 mental variation during flowering should thus be much 5 smaller than in iteroparous perennials. 5 44 1 To conclude, probability of floral herbivory increased 0 strongly with both inflorescence size and local plant density 0-4 5-9 10-14 15-19 20-24 25- in D. purpurea, and effects were strictly additive. Proportion Percentage fruit loss of damaged flowers also increased with size, suggesting that Fig. 3 – (a) Distribution of percentage fruit loss in plants E. pulchellata may impact the relationship between plant size experiencing herbivory in 1997, n [ 88. and fitness in D. purpurea, and contribute to selection for acta oecologica 34 (2008) 21–25 25

smaller display size. In monocarpic plants, flowering is highly a natural population of Digitalis purpurea. Funct. Ecol. 19, size-dependent (Metcalf et al., 2003; Sletvold and Grindeland, 383–390. 2007), and herbivores that alter the relationship between Herrera, C.M., 2000. Measuring the effects of pollinators and herbivores: evidence for non-additivity in a perennial herb. size and fitness may influence optimal timing of flowering Ecology 81, 2170–2176. and life history evolution (Rose et al., 2005). We know that in Klinkhamer, P.G.L., Kubo, T., Iwasa, Y., 1997. Herbivores and the D. purpurea, seedling number increases more than proportion- evolution of the semelparous perennial life-history of plants. ally with plant size (Sletvold, 2002), and we are currently ex- J. Evol. Biol. 10, 529–550. ploring what levels of herbivory it would take to counteract Kunin, W.E., 1999. 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