Variation in pollen limitation and floral parasitism across a mating system transition in a Pacific coastal dune plant: evolutionary causes or ecological consequences?

Background and Aims Evolutionary transitions from outcrossing to self-fertilization are thought to occur because selfing provides reproductive assurance when pollinators or mates are scarce, but they could also occur via selection to reduce floral vulnerability to herbivores. This study investigated geographic covariation between floral morphology, fruit set, pollen limitation and florivory across the geographic range of Camissoniopsis cheiranthifolia, a Pacific coastal dune endemic that varies strikingly in flower size and mating system.Methods Fruit set was quantified in 75 populations, and in 41 of these floral herbivory by larvae of a specialized moth (Mompha sp.) that consumes anthers in developing buds was also quantified. Experimental pollen supplementation was performed to quantify pollen limitation in three large-flowered, outcrossing and two small-flowered, selfing populations. These parameters were also compared between large- and small-flowered phenotypes within three mixed populations.Key Results Fruit set was much lower in large-flowered populations, and also much lower among large- than small-flowered plants within populations. Pollen supplementation increased per flower seed production in large-flowered but not small-flowered populations, but fruit set was not pollen limited. Hence inadequate pollination cannot account for the low fruit set of large-flowered plants. Floral herbivory was much more frequent in large-flowered populations and correlated negatively with fruit set. However, florivores did not preferentially attack large-flowered plants in three large-flowered populations or in two of three mixed populations.Conclusions Selfing alleviated pollen limitation of seeds per fruit, but florivory better explains the marked variation in fruit set. Although florivory was more frequent in large-flowered populations, large-flowered individuals were not generally more vulnerable within populations. Rather than a causative selective factor, reduced florivory in small-flowered, selfing populations is probably an ecological consequence of mating system differentiation, with potentially significant effects on population demography and biotic interactions.     

Below-ground herbivory limits induction of extrafloral nectar by above-ground herbivores

Background and Aims Many plants produce extrafloral nectar (EFN), and increase production following above-ground herbivory, presumably to attract natural enemies of the herbivores. Below-ground herbivores, alone or in combination with those above ground, may also alter EFN production depending on the specificity of this defence response and the interactions among herbivores mediated through plant defences. To date, however, a lack of manipulative experiments investigating EFN production induced by above- and below-ground herbivory has limited our understanding of how below-ground herbivory mediates indirect plant defences to affect above-ground herbivores and their natural enemies.Methods In a greenhouse experiment, seedlings of tallow tree (Triadica sebifera) were subjected to herbivory by a specialist flea beetle (Bikasha collaris) that naturally co-occurs as foliage-feeding adults and root-feeding larvae. Seedlings were subjected to above-ground adults and/or below-ground larvae herbivory, and EFN production was monitored.Key Results Above- and/or below-ground herbivory significantly increased the percentage of leaves with active nectaries, the volume of EFN and the mass of soluble solids within the nectar. Simultaneous above- and below-ground herbivory induced a higher volume of EFN and mass of soluble solids than below-ground herbivory alone, but highest EFN production was induced by above-ground herbivory when below-ground herbivores were absent.Conclusions The induction of EFN production by below-ground damage suggests that systemic induction underlies some of the EFN response. The strong induction by above-ground herbivory in the absence of below-ground herbivory points to specific induction based on above- and below-ground signals that may be adaptive for this above-ground indirect defence.     

Trichomes as dangerous lollipops: Do lizards also use caterpillar body and frass odor to optimize their foraging?

When attacked by herbivores, plants produce toxic secondary metabolites that function as direct defenses, as well as indirect defenses that attract and reward predators of the offending herbivores. These indirect defenses include both nutritive rewards such as extra floral nectar, as well as informational rewards, such as the production and release of volatile compounds that betray the location of feeding herbivores to predators. Herbivory of Nicotiana attenuata by the tobacco hornworm (Manduca larvae) alters the volatile profiles of both the plant and larval headspace. Herbivory-elicited specific changes in the volatile profiles are detected by arthropod predators of Manduca larvae. The known predators that perceive volatile cues induced by Manduca herbivory of N. attenuata are insects that target Manduca at early developmental stages, when the larvae are still small; large, late-instar larvae may have outgrown these predation risks. However, here we offer evidence that branched chain aliphatic acids derived from the digestion of plant O-acyl sugars from trichomes may betray Manduca larvae to lizard predators during late developmental stages as well.     

Factors affecting regulation of maternal investment in an indeterminate flowering plant (Cercidium microphyllum; Fabaceae)

On palo verde trees, nearly 80% of potential offspring were lost during flower bud development. Flower buds at the base of racemes developed earliest and were more likely to survive depredations of gelechiid larvae or abortion from putative limited resources. Herbivory accounted for greater cumulative losses than putative resources; however, an herbivore exclusion experiment suggested that buds damaged by herbivores would have aborted anyway without damage. In a natural experiment, plants that received water runoff had significantly higher densities of racemes, with only slight increases in numbers of buds, flowers, or pods per raceme. Similarly, thinning racemes experimentally to assess limited resources among potentially competing racemes increased the number of viable flower buds per raceme only slightly, with negligible herbivore damage. The production of more racemes rather than more reproductive structures per raceme may be a mechanism to allocate limited resources more efficiently. Alternatively, in studies with animals, similar patterns in the production of broods are thought to be mechanisms to avoid nest predation. Thus, while putative limited resources and resource allocation patterns reduced the proximate effects of larvae, herbivory must be considered as a possible ultimate factor in the patterns observed here because limited resources may be allocated in ways to reduce herbivory.     

Are tetraploids more successful? Floral signals,reproductive success and floral isolation in mixed-ploidy populations of a terrestrial orchid

Background and Aims Polyploidization, the doubling of chromosome sets, is common in angiosperms and has a range of evolutionary consequences. Newly formed polyploid lineages are reproductively isolated from their diploid progenitors due to triploid sterility, but also prone to extinction because compatible mating partners are rare. Models have suggested that assortative mating and increased reproductive fitness play a key role in the successful establishment and persistence of polyploids. However, little is known about these factors in natural mixed-ploidy populations. This study investigated floral traits that can affect pollinator attraction and efficiency, as well as reproductive success in diploid and tetraploid Gymnadenia conopsea (Orchidaceae) plants in two natural, mixed-ploidy populations.Methods Ploidy levels were determined using flow cytometry, and flowering phenology and herbivory were also assessed. Reproductive success was determined by counting fruits and viable seeds of marked plants. Pollinator-mediated floral isolation was measured using experimental arrays, with pollen flow tracked by means of staining pollinia with histological dye.Key Results Tetraploids had larger floral displays and different floral scent bouquets than diploids, but cytotypes differed only slightly in floral colour. Significant floral isolation was found between the two cytotypes. Flowering phenology of the two cytotypes greatly overlapped, and herbivory did not differ between cytotypes or was lower in tetraploids. In addition, tetraploids had higher reproductive success compared with diploids.Conclusions The results suggest that floral isolation and increased reproductive success of polyploids may help to explain their successful persistence in mixed-ploidy populations. These factors might even initiate transformation of populations from pure diploid to pure tetraploid.     

Antiherbivore defenses alter natural selection on plant reproductive traits

While many studies demonstrate that herbivores alter selection on plant reproductive traits, little is known about whether antiherbivore defenses affect selection on these traits. We hypothesized that antiherbivore defenses could alter selection on reproductive traits by altering trait expression through allocation trade‐offs, or by altering interactions with mutualists and/or antagonists. To test our hypothesis, we used white clover, Trifolium repens, which has a Mendelian polymorphism for the production of hydrogen cyanide—a potent antiherbivore defense. We conducted a common garden experiment with 185 clonal families of T. repens that included cyanogenic and acyanogenic genotypes. We quantified resistance to herbivores, and selection on six floral traits and phenology via male and female fitness. Cyanogenesis reduced herbivory but did not alter the expression of reproductive traits through allocation trade‐offs. However, the presence of cyanogenic defenses altered natural selection on petal morphology and the number of flowers within inflorescences via female fitness. Herbivory influenced selection on flowers and phenology via female fitness independently of cyanogenesis. Our results demonstrate that both herbivory and antiherbivore defenses alter natural selection on plant reproductive traits. We discuss the significance of these results for understanding how antiherbivore defenses interact with herbivores and pollinators to shape floral evolution.     

Herbivory by a Phloem-feeding insect inhibits floral volatile production

There is extensive knowledge on the effects of insect herbivory on volatile emission from vegetative tissue, but little is known about its impact on floral volatiles. We show that herbivory by phloem-feeding aphids inhibits floral volatile emission in white mustard Sinapis alba measured by gas chromatographic analysis of headspace volatiles. The effect of the Brassica specialist aphid Lipaphis erysimi was stronger than the generalist aphid Myzus persicae and feeding by chewing larvae of the moth Plutella xylostella caused no reduction in floral volatile emission. Field observations showed no effect of L. erysimi-mediated floral volatile emission on the total number of flower visits by pollinators. Olfactory bioassays suggested that although two aphid natural enemies could detect aphid inhibition of floral volatiles, their olfactory orientation to infested plants was not disrupted. This is the first demonstration that phloem-feeding herbivory can affect floral volatile emission, and that the outcome of interaction between herbivory and floral chemistry may differ depending on the herbivore's feeding mode and degree of specialisation. The findings provide new insights into interactions between insect herbivores and plant chemistry.     

Reciprocal feeding facilitation between above- and below-ground herbivores

Interspecific interactions between insect herbivores predominantly involve asymmetric competition. By contrast, facilitation, whereby herbivory by one insect benefits another via induced plant susceptibility, is uncommon. Positive reciprocal interactions between insect herbivores are even rarer. Here, we reveal a novel case of reciprocal feeding facilitation between above-ground aphids (Amphorophora idaei) and root-feeding vine weevil larvae (Otiorhynchus sulcatus), attacking red raspberry (Rubus idaeus). Using two raspberry cultivars with varying resistance to these herbivores, we further demonstrate that feeding facilitation occurred regardless of host plant resistance. This positive reciprocal interaction operates via an, as yet, unreported mechanism. Specifically, the aphid induces compensatory growth, possibly as a prelude to greater resistance/tolerance, whereas the root herbivore causes the plant to abandon this strategy. Both herbivores may ultimately benefit from this facilitative interaction.     

Natural and artificial floral damage induces resistance in Nemophila menziesii (Hydrophyllaceae) flowers

Resistance to leaf herbivory is well-documented in plants. In contrast, resistance to herbivory in flowers has received very little attention, even though reproductive tissues are often essential for plant reproduction. Plants may protect reproductive tissues with a range of defenses from constitutive to induced, although ecological costs associated with constitutive defense or resistance are expected to be higher than costs associated with induced responses. Induced responses in flowers may be effective against floral herbivores while minimizing the negative impacts of resistance on pollinators. This study examines induced responses in Nemophila menziesii (Hydrophyllaceae), a plant that frequently receives high levels of floral herbivory. I report that natural caterpillar herbivory increased levels of resistance against caterpillars later in the season. Similarly, artificial clipping to flowers consistently reduced natural damage to flowers vs unclipped controls over two years. Neither whole-plant nor individual seed set was affected by the reduction of floral damage. Induced resistance in reproductive tissues may benefit plants that are exposed to both floral herbivory and pollinator activity and can be an important link between plant antagonists and plant mutualists.     

Geographic variation in oviposition preference for male and female host plants in a geometrid moth: implications for evolution of host choice

Several dioecious plant species exhibit sexual dimorphisms in defensive traits. However, the effects of sexual dimorphism on defense against herbivores remain poorly understood. Eurya japonica (Thunb.) (Theaceae) is a dioecious shrub that shows sexual dimorphism in the chemical defense of flower buds. Female calyces contain higher concentrations of total phenolics and condensed tannins than do male calyces. Male flower buds are edible for a florivore moth, Chloroclystis excisa (Butler) (Lepidoptera: Geometridae), whereas the female flower buds are lethal to the moth larvae. The moths prefer to oviposit on male over female E. japonica flower buds. As the moths also occur in areas lacking E. japonica, we tested whether the oviposition preference for E. japonica flower sex differed between moths sympatric and allopatric with E. japonica. The moths sympatric with E. japonica showed a stronger preference for male E. japonica than the moths allopatric with E. japonica. Our phylogeographic study using mitochondrial cytochrome oxidase subunit I gene sequences revealed little genetic differentiation between moth populations sympatric and allopatric with E. japonica. These results suggest that the adaptive oviposition preference for flower sex of E. japonica has evolved rapidly in C. excisa.     

Effects of floral herbivory on foraging behaviour of bumblebees and female reproductive success in Pedicularis gruina (Orobanchaceae)

The effects of floral herbivores on floral traits may result in alterations in pollinator foraging behaviour and subsequently influence plant reproductive success. Fed-upon plants may have evolved mechanisms to compensate for herbivore-related decreased fecundity. We conducted a series of field experiments to determine the relative contribution of floral herbivores and pollinators to female reproductive success in an alpine herb, Pedicularis gruina, in two natural populations over two consecutive years. Experimental manipulations included bagging, hand supplemental, geitonogamous pollination, and simulated floral herbivory. Bumblebees not only avoided damaged flowers and plants but also decreased successive visits of flowers in damaged plants, and the latter may reduce the level of geitonogamy. Although seed set per fruit within damaged plants was higher than that in intact plants, total seed number in damaged plants was less than that in intact plants, since floral herbivory-mediated pollinator limitation led to a sharp reduction of fruit set. Overall, the results suggest that resource reallocation within inflorescences of damaged plants may partially compensate for a reduction in seed production. Additionally, a novel finding was the decrease in successive within-plant bumblebee visits following floral herbivory. This may increase seed quantity and quality of P. gruina since self-compatible species exhibit inbreeding depression. The patterns of compensation of herbivory and its consequences reported in this study give an insight into the combined effects of interactions between floral herbivory and pollination on plant reproductive fitness.     

Does Plant Origin Influence the Fitness Impact of Flower Damage? A Meta-Analysis

Herbivory has been long considered an important component of plant-animal interactions that influences the success of invasive species in novel habitats. One of the most important hypotheses linking herbivory and invasion processes is the enemy-release hypothesis, in which exotic plants are hypothesized to suffer less herbivory and fitness-costs in their novel ranges as they leave behind their enemies in the original range. Most evidence, however, comes from studies on leaf herbivory, and the importance of flower herbivory for the invasion process remains largely unknown. Here we present the results of a meta-analysis of the impact of flower herbivory on plant reproductive success, using as moderators the type of damage caused by floral herbivores and the residence status of the plant species. We found 51 papers that fulfilled our criteria. We also included 60 records from unpublished data of the laboratory, gathering a total of 143 case studies. The effects of florivory and nectar robbing were both negative on plant fitness. The methodology employed in studies of flower herbivory influenced substantially the outcome of flower damage. Experiments using natural herbivory imposed a higher fitness cost than simulated herbivory, such as clipping and petal removal, indicating that studies using artificial herbivory as surrogates of natural herbivory underestimate the real fitness impact of flower herbivory. Although the fitness cost of floral herbivory was high both in native and exotic plant species, floral herbivores had a three-fold stronger fitness impact on exotic than native plants, contravening a critical element of the enemy-release hypothesis. Our results suggest a critical but largely unrecognized role of floral herbivores in preventing the spread of introduced species into newly colonized areas.     

Does persistence of showy calyces in Limonium leptolobum enhance pollinator attraction?

Aims The calyx, the outermost whorl of a flower (usually green), has been considered to function to protect flowers. In some species, however, calyces are colorful and retained during seed development. Limonium species have been exploited as cut flower crops because the calyces persist for several months after the corolla has closed. To explore the adaptive significance of the persistent calyx in a desert plant Limonium leptolobum, we ask whether persistence of calyces can enhance pollinator attraction by enlarging floral displays, increasing reproductive success in this self-incompatible species.Methods The yellow flower of L. leptolobum lasted 1–2 days but its white, membranous calyx extended fully after the corolla closed, and persisted for over 2 months in the field, making hundreds of 'showy flowers' on one individual. To examine the ecological function of calyces, we test the pollinator attraction hypothesis. In an experimental population, we compared the difference in visit frequency and visitor behavior between intact inflorescences and inflorescences with their calyces removed on the same individual plants.Important findings In four experimental plots four types of floral visitors were observed including bees, butterflies, syrphid flies and day-flying moths. No significant preference was observed between calyx-free and intact inflorescences for both first arrivals and total visit frequency of all types of floral visitors, indicating that the persistence of calyces did not make plants more attractive to potential pollinators. The pollinator attraction hypothesis for the showy calyces was not supported by the current data. Whether the calyx in this desert plant helps seed development or has other functions needs further study.     

Reproductive ecology of distylous Palicourea padifolia (Rubiaceae) in a tropical montane cloud forest. II. Attracting and rewarding mutualistic and antagonistic visitors

By definition, the floral morphs of distylous plants differ in floral architecture. Yet, because cross-pollination is necessary for reproductive success in both morphs, they should not differ in attributes that contribute to attracting and rewarding floral visitors. Floral and vegetative attributes that function in distylous polymorphism in hummingbird-pollinated Palicourea padifolia (Rubiaceae) and the responses of pollinators and insect herbivores to the resources offered by both morphs were investigated. The performance of each morph along multiple stages of the reproductive cycle, from inflorescence and nectar production to fruit production, was surveyed, and pollinator behavior and nectar standing crops were then observed. Costs associated with such attractiveness were also evaluated in terms of herbivore attack and of plant reproductive fitness (female function) as a function of leaf herbivory. The number of inflorescences, floral buds, open flowers, and ripe fruits offered by either floral morph were similar, but short-styled plants almost doubled the number of developing fruits of long-styled plants. Long-styled flowers produced higher nectar volumes and accumulated more nectar over time than short-styled flowers. Measures of nectar standing crop and data on pollinator behavior suggest that hummingbirds respond to this morph-specific scheduling of nectar production. Lastly, long-styled plants suffered a higher herbivore attack and lost more leaf area over time than those with short-styled flowers. Herbivory was negatively correlated with fruit number and fruit mass, and long-styled plants set significantly less fruit mass than short-styled plants. The results suggest that pollinators and herbivores may exert selective pressures on floral and vegetative traits that could also influence gender function.     

Superparasitism in Cotesia glomerata does not benefit the host plant by reduction of herbivory caused by Pieris brassicae

Superparasitism occurs in Cotesia glomerata L. (Hymenoptera: Braconidae), a gregarious endoparasitoid of Pieris spp. (Lepidoptera: Pieridae). The responses of Pieris brassicae L. larvae to superparasitism were examined in order to elucidate the ecological significance of this behaviour. Models of tritrophic interactions often imply that attraction of herbivore natural enemies by the plant constitutes a defence. Parasitoid attack on herbivores is assumed to result in a reduction in herbivory and or an increase in plant fitness. Coupled with the active involvement of the plant in producing signals, this can be seen as an indirect mediation of wound induced defence. The results show that superparasitism of P. brassicae by the parasitoid C. glomerata reduced survivorship but increased food consumption and weight growth in P. brassicae larvae. The duration of host larval development was found prolonged as the number of oviposition increased and superparasitized larvae (three to five time parasitized) grew slower than unparasitized larvae or larvae parasitized one or two times.     

Effect of defoliation by checkerspot caterpillars (Euphydryas phaeton) and sawfly larvae (Macrophya nigra and Tenthredo grandis) on their host plants (Chelone spp.)

Summary The effect of defoliation by herbivores, checkerspot caterpillars (Euphydryas phaeton) and sawfly larvae (Macrophya nigra and Tenthredo grandis), on the reproductive output of turtlehead (Chelone spp.) was examined. Defoliation prior to development of flower buds reduced the number of reproductive stalks, flower buds, flowers and seed capsules. Severe herbivory, after flower buds appeared, decreased the final number of seed capsules and seeds per capsule. The availability of the host plants to the herbivores was a function of prior defoliation and environmental conditions. Sawfly larvae, by defoliating the plants in midsummer, forced prediapause checkerspot caterpillars to wander in search of food plants. Decimation of these perenials by postdiapause checkerspot caterpillars in a dry spring retarded growth of turtlehead and, consequently, most of the plants were not available for egg-laying by sawflies and checkerspots.     

Pollinators cause stronger selection than herbivores on floral traits in Lobelia cardinalis (Lobeliaceae)

Measures of selection on floral traits in flowering plants are often motivated by the assumption that pollinators cause selection. Flowering plants experience selection from other sources, including herbivores, which may enhance or oppose selection by pollinators. Surprisingly, few studies have examined selection from multiple sources on the same traits. We quantified pollinator-mediated selection on six floral traits of Lobelia cardinalis by comparing selection in naturally and supplementally (hand-) pollinated plants. Directional, quadratic and correlational selection gradients as well as total directional and quadratic selection differentials were examined. We used path analysis to examine how three herbivores--slugs, weevils and caterpillars--affected the relationship between floral traits and fitness. We detected stronger total selection on four traits and correlational selection (γ(ij)) on three trait combinations in the natural pollination treatment, indicating that pollinators caused selection on these traits. Weak but statistically significant selection was caused by weevil larvae on stem diameter and anther-nectary distance, and by slugs on median-flower date. In this study, pollinators imposed stronger selection than herbivores on floral traits in L. cardinalis. In general, the degree of pollen limitation and rate of herbivory are expected to influence the relative strength of selection caused by pollinators or herbivores.     

The effects of large herbivores on the landscape dynamics of a perennial herb

Background and Aims

Models assessing the prospects of plant species at the landscape level often focus primarily on the relationship between species dynamics and landscape structure. However, the short-term prospects of species with slow responses to landscape changes depend on the factors affecting local population dynamics. In this study it is hypothesized that large herbivores may be a major factor affecting the short-term prospects of slow-responding species in the European landscape, because large herbivores have increased in number in this region in recent decades and can strongly influence local population dynamics.

Methods

The impact of browsing by large herbivores was simulated on the landscape-level dynamics of the dry grassland perennial polycarpic herb Scorzonera hispanica. A dynamic, spatially explicit model was used that incorporated information on the location of patches suitable for S. hispanica, local population dynamics (matrices including the impact of large herbivores), initial population sizes and dispersal rate of the species. Simulations were performed relating to the prospects of S. hispanica over the next 30 years under different rates of herbivory (browsing intensity) and varying frequencies of population destruction (e.g. by human activity).

Key Results

Although a high rate of herbivory was detected in most populations of S. hispanica, current landscape-level dynamics of S. hispanica were approximately in equilibrium. A decline or increase of over 20 % in the herbivory rate promoted rapid expansion or decline of S. hispanica, respectively. This effect was much stronger in the presence of population destruction.

Conclusions

Browsing by large herbivores can have a dramatic effect on the landscape dynamics of plant species. Changes in the density of large herbivores and the probability of population destruction should be incorporated into models predicting species abundance and distribution.     

Differential Flower Herbivory and Gall Formation on Males and Females of Neea psychotrioides, a Dioecious Tree

The goal of this study was to determine whether there exists intersexual differences in the degree of herbivore attack on a tropical, dioecious tree. Neea pychotrioides is a Costa Rican tree that experiences at least two types of attack on its reproductive structures: flower herbivory and gall formation on inflorescences. Although female trees were more common than males in the two study populations (overall sex ratio = 1.5 females/male), male flowers were more likely to be eaten by lepidopteran larvae. Similarly, inflorescence-borne, fly-induced galls were more frequent on males. Ten families of insects from three orders (Diptera, Hymenoptera, Lepidoptera) occupied the galls with varying levels of parasitism and hyperparasitism occurring within. Although the sexes were not significantly different in vegetative size, males had larger floral displays (more inflorescences and larger flowers) that may have acted as attractants to herbivores and gallmakers. The results are consistent with the hypothesis that flower damage and the energetic costs of producing galls exert a greater expense to males, ultimately resulting in death, and over time, a skewing of the sex ratio.     

Escape from floral herbivory by early flowering in Arabidopsis halleri subsp. gemmifera

Natural selection on flowering phenology has been studied primarily in terms of plant–pollinator interactions and effects of abiotic conditions. Little is known, however, about geographic variation in other biotic factors such as herbivores and its consequence for differential selection on flowering phenology among populations. Here, we examine selection by floral herbivores on the flowering phenology of Arabidopsis halleri subsp. gemmifera using two adjacent populations with contrasting herbivory regimes. Intensive floral herbivory by the leaf beetle Phaedon brassicae occurs in one population, while the beetle is absent in another population. We tested the hypothesis that the two populations experience differential selection on flowering time that is attributable to the presence or absence of floral herbivory. A two-year field study showed that early flowering was favoured in the population under intensive floral herbivory, whereas selection for early flowering was not found in one year in the population where floral herbivory was absent. Selection for early flowering disappeared when the abundance of floral herbivores was artificially decreased in a field experiment. Thus, the heterogeneous distribution of P. brassicae was a major agent for differential selection on flowering time. However, flowering time did not differ between the two populations when plants were grown in the laboratory. The lack of genetic differentiation in flowering time may be explained by ongoing gene flow or recent invasion of P. brassicae. This study illustrates that the role of floral herbivory in shaping geographic variation in selection on flowering phenology may be more important than previously thought.