Equilibrium populations of heterostylous plants

A general relation is derived which must be satisfied by the equilibrium frequencies of the incompatibility types occurring in populations of heterostylous plants. Specific assumptions are made about the underlying genetic mechanism, but these are fulfilled in practically all relevant cases. With usual mating systems corresponding to pollen or zygote elimination and no fitness differences, it follows that all incompatibility types must be equally frequent. Given different fitness values, the general relation leads to more complex equations determining the frequencies. These equations are used for estimating fitness values of the Long, Mid and Short types in Lythrum salicaria. Expressions for the equilibrium frequencies are also found for a special mating system in Oxalis alpina.     

STOCHASTIC LOSS OF STYLE MORPHS FROM POPULATIONS OF TRISTYLOUS LYTHRUM SALICARIA AND DECODON VERTICILLATUS (LYTHRACEAE)

Despite the theoretical importance of stochastic processes in evolution there have been few empirical studies of the interaction between genetic drift and selection on the maintenance of polymorphisms in plant populations. We used computer models to investigate the interaction between drift and frequency-dependent selection in affecting style morph frequencies in populations of tristylous species. Drift produces a distinct pattern of morph frequency variation involving: 1) the loss of the S morph and, to a lesser extent, the M morph; 2) no consistent bias in frequencies within populations; 3) a restricted pattern of variation involving a deficiency of one morph and equal excesses of the other two. Morph frequencies were surveyed in 137 populations of Lythrum salicaria from both its native range in Europe (N = 35) and recent adventive range in Ontario (N = 102), and 133 populations of Decodon verticillatus from four regions in eastern North America with different glacial histories to assess these theoretical predictions. There was a negative relationship between morph loss and population size in both species; the relationship was weaker in D. verticillatus than in L. salicaria. Morph loss was more frequent in the adventive than native range of L. salicaria, and in populations of D. verticillatus from glaciated northern regions compared with the unglaciated southern portion of its range. Simulations incorporating variation in life history, regeneration strategy and mating patterns revealed that the degree of morph loss was strongly influenced by year to year survival, clonal propagation, self-fertilization and departures from disassortative mating. Comparing the pattern of morph frequency variation between species supported these predictions. Morph loss was lower in self-incompatible L. salicaria (0% in Europe; 23% in Ontario), which reproduces through seed compared to self-compatible, clonal D. verticillatus (52%). A stochastic model provides the most parsimonious explanation for observed patterns of morph frequency variation in both species.     

Native or Exotic? Double or Single? Evaluating Plants for Pollinator-friendly Gardens

In a series of dawn-to-dusk studies, we examined the natureand accessibility of nectar rewards for pollinating insectsby monitoring insect visits and the secretion rate and standingcrop of nectar in the British native plant species Salvia pratensis,Stachys palustris, S. officinalis, Lythrum salicaria, Linariavulgaris, the non-native Calendula officinalis, Petunia x hybrida,Salvia splendens, and the possibly introduced Saponaria officinalis.We also compared single with double variants ofLotus corniculatus, Saponaria officinalis, Petunia x hybrida andCalendula officinalis. All the British species studied are nectar-rich and are recommendedfor pollinator-friendly gardens. They showed maximal secretionrates of about 10–90 µg sugar per flower h^-1, andmost had mean standing crops of about 5–60 µg sugarper flower. In all British species studied, the corolla wasdeep enough for the relatively long-tongued bumblebee Bombuspascuorum, but the shallower flowers of Lythrum salicaria werealso much visited by shorter-tongued bees and hoverflies, aswell as by butterflies. The exotic Salvia splendens, presumablycoevolved with hummingbirds in the Neotropics, has such deepflowers that British bees cannot reach the nectar except bycrawling down the corolla. With a secretion rate approaching300 µg sugar per flower h^-1and little depletion by insects,S. splendens accumulated high standing crops of nectar. S. splendens,and single and double flowers of the two probably moth-pollinatedspecies Petunia x hybrida and Saponaria officinalis, receivedfew daytime visits despite abundant nectar but Calendula waswell visited by hoverflies and bees. We compared single anddouble variants of Lotus corniculatus,Petunia x hybrida andCalendula officinalis, and also Saponaria officinalis, the lastbeing probably introduced in Britain (Stace, 1997 New floraof the British Isles. 2nd edn. Cambridge: Cambridge UniversityPress). In Petunia, Saponaria and Lotus, double flowers secretedlittle or no nectar. In Calendula, where doubling involved achange in the proportion of disc and ray florets rather thanmodification of individual flower structure, double and singlecapitula had similar standing crops of nectar. Except inCalendula, exotic or double flowers were little exploited by insect visitors.In the exotics, this was probably due to the absence or scarcityof coevolved pollinators, coupled, in double flowers, with theabsence of nectar. Copyright 2001 Annals of Botany Company Salvia pratensis, Salvia splendens, Stachys palustris, Stachys officinalis,Lythrum salicaria , Linaria vulgaris, Lotus corniculatus, Saponaria officinalis,Petunia x hybrida, Calendula officinalis, wild flowers, double flowers, gardens, nectar, secretion rate, standing crop, pollinators, bumblebees, Bombus, honeybees, Apis, hoverflies, butterflies,Anthidium manicatum     

Effect of a Light-break on the Growth and Development of Epilobium hirsutum and Lythrum salicaria in Short Photoperiods

Plants of Epilobium hirsutumand Lythrum salicaria were grownin light for 9 h per day with the following adjustments of lightand dark periods. (a) Photoperiod of 9 h and an uninterrupteddark period of 15 h, (b) photoperiod of 8 h and 1 h of lightof the same intensity in the middle of the dark period. Plants of both the species receiving the 9 h continuous photoperiodremained dwarfed in habit. In the 8 + 1 h light-break treatment,Epilobium showed no response but Lythrum responded by a completedevelopmental shift from a short-day to a long-day form. Leafexpansion, stem elongation, dry matter production and flowerinitiation were promoted. In Epilobium, the pattern of total dry weight distribution toleaves, stem and roots remained similar in the two treatmentsbut in Lythrum light-break treatment caused more dry matterto be diverted to stems at the expense of leaves. These changesin morphogenetic pattern of differentiation were accompaniedby physiological changes as the chlorophyll content and therespiration rates were also higher.     

Genetic incompatibility and offspring quality in the tristylous plant Lythrum salicaria (Lythraceae)

The purpose of this study was to determine the incompatibility relationships among the three different morphs of the tristylous plant Lythrum salicaria, and to assess whether there are fitness consequences to breakdown in incompatibility. Twenty-four different types of pollinations were performed using all possible combinations of anthers and stigmas. These can be grouped as legitimate pollinations (pollinations from the appropriate anther level of a compatible morph), illegitimate intermorph pollinations (pollinations from the inappropriate anther level of a compatible morph), intramorph pollinations (pollinations between individuals of the same morph), and self pollinations. Legitimate pollinations produced significantly more seed than illegitimate intermorph pollinations, intramorph pollinations, and self pollinations. The difference between legitimate and illegitimate intermorph pollination success is particularly interesting because pollen for these crosses came from the same genetic individual. Pollination types also differed in offspring quality. In the rare examples where progeny were produced by illegitimate intermorph pollinations, the progeny did not have significantly lower values for seedling growth traits compared to legitimate progeny. Seedlings produced by self pollinations had significantly lower values for four out of six seedling growth traits measured. Comparisons of legitimate and self progeny traits indicate that inbreeding depression for most traits is close to or above 0.5. The finding that some seed were produced from illegitimate pollinations suggests that there is variation among individuals in incompatibility. There was a significant effect of parental morph type on the probability of breakdown in incompatibility with the midstyled maternal parents setting more seed from illegitimate pollinations.     

Impacts of major predators on tropical agroforest arthropods: comparisons within and across taxa

Non-native plants can have adverse effects on ecosystem structure and processes by invading and out-competing native plants. I examined the hypothesis that mature plants of non-native and native species exert differential effects on the growth of conspecific and heterospecific seedlings by testing predictions that (1) invasive vegetation has a stronger suppressive effect on seedlings than does native vegetation, (2) seedlings of invasive species are better able to grow in established vegetation than are native seedlings, and (3) invasive species facilitate conspecific and inhibit heterospecific seedling growth. I measured growth rates and interaction intensities for seedlings of four species that were transplanted into five wetland monoculture types: invasive Lythrum salicaria; native L. alatum, Typha angustifolia, T. latifolia; unvegetated control. Invasive L. salicaria had the strongest suppressive effect on actual and per-individual bases, but not on a per-gram basis. Seedlings of T. latifolia were better able to grow in established vegetation than were those of L. salicaria and T. angustifolia. These results suggest that L. salicaria is not a good invader of established vegetation, but once established, it is fairly resistant to invasion. Thus, it is likely that disturbance of established vegetation facilitates invasion by L. salicaria, allowing it to compete with other species in even-aged stands where its high growth rate and consequent production of aboveground biomass confer a competitive advantage.     

Two-dimensional patterns of soluble proteins including three hydrolytic enzymes of mature pollen of tristylous <Emphasis Type="Italic">Lythrum salicaria</Emphasis>

Lythrum salicaria, now a widespread invasive species, exhibits tristyly, a form of heteromorphic selfincompatibility. In tristyly, each plant exhibits one (and only one) of three morphologically different floral forms. Moreover, each flower produces two types of stamens, and these two exhibit different incompatibility reactions. Differences between stamens of a single flower must be the result of epigenetic phenomena and for that reason, we performed two-dimensional gel electrophoresis (2-DE) to analyze fractions of soluble proteins derived from the pollen coat and protoplast including three hydrolytic enzymes from the six different stamen types (two from each of three floral forms). There were significant differences in the 2-D protein profiles both between pollen from the same flower and between the same type of pollen from two different flowers, in the pollen coat as well as in the protoplast extracts. In five of the six samples of pollen fractions, characteristic peptides were found. Quantitative differences between pollen from the same flower were observed in case of esterases. Furthermore, analysis of proteases and acid phosphatases revealed also qualitative differences between these enzymes in pollen from the same flower.     

Fluctuating asymmetry as an ecological indicator of heavy metal stress in Lythrum salicaria

Fluctuating asymmetry (FA), a measure of random deviation of organismal traits from perfect bilateral symmetry, can be induced by environmental and/or genetic stress. We have conducted a replicated experiment to test the effect of a locally abundant heavy metal pollutant, lead, on the FA of a common wetland invasive species, Lythrum salicaria (purple loosestrife). The exposure to lead significantly reduced the shoot length, number and length of leaves as well as the biomass, but increased FA of L. salicaria leaves. The investigation suggests that FA in L. salicaria may be used as an ecological indicator to identify environmental stress caused by certain heavy metal pollutants. However, more studies are needed for the use of FA in L. salicaria as a comprehensive tool for biomonitoring of a wider array of environmental pollutants.     

ASSORTATIVE POLLINATION IN LYTHRUM

The pollination patterns of bees and butterflies were observed in an assemblage composed of 57 plants of Lythrum alatum and 30 plants of L. salicaria. Lythrum salicaria was preferred by both groups of pollinators as indicated by the greater number of flights from this species as compared to flights from L. alatum. The degree of assortative pollination was measured in both groups of pollinators and was greater in bees. Fidelity in bees appeared to be in response to species attractiveness, whereas in butterflies it appeared to be in response to species balance.     

Is the increased vigour of invasive weeds explained by a trade-off between growth and herbivore resistance?

Blossey and Nötzold (1995) recently hypothesised that the increased vigour of certain invasive plant species has been at the expense of defences against natural enemies. A prediction of their evolution of increased competitive ability (EICA) hypothesis is that invasive genotypes are relatively poorly defended. We tested this prediction with herbivore bioassays and with direct quantification of plant secondary metabolites comparing non-indigenous genotypes of Lythrum salicaria L. (purple loosestrife) with indigenous forms. The herbivore bioassays revealed no significant intra-specific variation in herbivore resistance between indigenous and non-indigenous hosts. The phenolic content of L. salicaria leaves was significantly higher in indigenous genotypes, as predicted by the EICA hypothesis. The average phenolic content of leaves (regardless of their origin) was, however, low, implying that the role of plant phenolics in purple loosestrife anti-herbivore defence is probably limited. It is suggested that the EICA hypothesis, as tested in the current study, does not explain the increased vigour of L. salicaria in non-indigenous habitats.     

Can Gas-Exchange Characteristics help Explain the Invasive Success of Lythrum salicaria?

Since its introduction to North America, Lythrum salicaria (L.) (purple loosestrife) has become invasive in marshy and riparian habitats. We compared gas-exchange responses to external CO₂ partial pressure and light, as well as related leaf structural and biochemical characteristics, of L. salicaria with those of co-occurring native Asclepias syriaca (common milkweed) and Solidago graminifolia (lance-leaved goldenrod) along a pond bank in the Black Rock Forest, Cornwall, New York, USA to examine if the invasive success of L. salicaria may be influenced by robust leaf gas-exchange characteristics, including relatively high rates of photosynthesis and low rates of respiration, compared with those of less successful co-occurring native plant species. Neither the mean rate of net photosynthesis measured at ambient CO₂ and saturating photon flux density (A) nor the mean dark respiration rate (R_D) differed significantly between L. salicaria and either of the native species, while both the mean maximum rate of photosynthesis at saturating CO₂ concentration and photon flux density (A _max) and the mean rate of respiration measured in light (R_L) were significantly higher in L. salicaria than A. syriaca, but no different between L. salicaria and S. graminifolia. Likewise, photosynthetic nitrogen-use efficiency was greater in L. salicaria than A. syriaca only, while photosynthetic water-use efficiency was significantly less in both L. salicaria and S. graminifolia than in A. syriaca. Despite limited interspecific differences in leaf photosynthesis, respiration, and resource-use efficiency, particularly between L. salicaria and S. graminifolia, we found that L. salicaria assimilated 208% more carbon per unit of energy invested in leaf biomass than either of the co-occurring native species, suggesting that increased photosynthetic energy-use efficiency may influence its observed invasive success. This revised version was published online in July 2006 with corrections to the Cover Date.     

Fecundity and fitness in cross-compatible pollinations of tristylous North American Lythrum salicaria populations

As heterostyly and self incompatibility are linked in tristylous L. salicaria, all cross-compatible pollinations – those between anthers and styles of the same level – should produce viable seed. The rigor of this theory was tested using seed set and germination of cross-compatible pollinations in 18 naturalized L. salicaria populations in Minnesota/Wisconsin (n=342 genotypes; n=86 shorts, n=127 mids, and n=129 longs). Seed set for short-styled genotypes ranged from zero to 135 (36+38); mids–0–156, (53+39), and longs–0–151 (59+39). Mean seed set per capsule was not significantly different for mids and longs, but both were significantly greater than that of shorts (F=14, P<0.0001). Zero seed set frequently occurred in most compatible crosses, in contrast with theoretical expectations. The high percentage of populations deviating from normality must be due to incompatibility. An incompatibility system independent of heterostyly could cause this, where failed crosses result from matches of incompatibility specificities in pollen and pistil. This independence is questionable, however, given the non-significant difference in failed outcrosses for pooled χ² comparing within and between populations (χ²=0.395, P>0.5). A sporophytic incompatibility model is proposed with a minimum of three specificities. Zero seed set in compatible crosses is due to the addition of alleles from L. alatum, a distylous species that forms introgressive hybrids with L. salicaria. Reduced fecundity could increase the deficiency of shorts, and significantly greater seed germination of shorts could explain the continued presence of short-styled individuals in L. salicaria populations. Received: 3 February 2000 / Accepted: 25 March 2000     

Evidence for a shift in life-history strategy during the secondary phase of a plant invasion

We investigated the correlated response of several key traits of Lythrum salicaria L. to water availability gradients in introduced (Iowa, USA) and native (Switzerland, Europe) populations. This was done to investigate whether plants exhibit a shift in life-history strategy during expansion into more stressful habitats during the secondary phase of invasion, as has recently been hypothesized by Dietz and Edwards (Ecology 87(6):1359, 2006). Plants in invaded habitats exhibited a correlated increase in longevity and decrease in overall size in the transition into more stressful mesic habitats. In contrast, plants in the native range only exhibited a decrease in height. Our findings are consistent with the hypothesis that secondary invasion is taking place in L. salicaria, allowing it to be more successful under the more stressful mesic conditions in the invaded range. If this trend continues, L. salicaria may become a more problematic species in the future.     

Limits and effects of invasion by the nonindigenous wetland plant <Emphasis Type="Italic">Lythrum salicaria</Emphasis> (purple loosestrife): a seed bank analysis

We used seed bank analyses to investigate the role of dispersal in limiting invasion by Eurasian Lythrum salicaria within and among North American wetlands, and the changes in seed bank diversity associated with this invader. We compared the number and species composition of seedlings emerging from soil sampled in 11 uninvaded wetlands and paired uninvaded and invaded sites within 10 invaded wetlands under both seedling competition and noncompetitive conditions. Almost no L. salicaria emerged in samples from uninvaded wetlands, indicating dispersal limitation despite prodigious seed production in nearby wetlands. However L. salicaria emerged in all samples from uninvaded sites in invaded wetlands, suggesting environmental limits on establishment within invaded wetlands. Conditions that provided opportunities for seedlings to compete reduced survival of Typha spp. but not L. salicaria seedlings. However, this was due to species-specific differences in post-emergence mortality rather than response to competition. Competition did reduce seedling mass, but this effect did not differ among species. Species richness of emerging seedlings was lower for invaded than uninvaded wetlands. Lower seed bank richness may be a cause or consequence of L. salicaria invasion. Efforts to reduce seed dispersal to uninvaded wetlands would likely slow the spread of this invader.     

Differential Effects of Typha Litter and Plants on Invasive Lythrum Salicaria Seedling Survival and Growth

Invasive species are a problem because of their detrimental ecological and economic effects. Increased disturbance caused by human impacts is hypothesized as a primary factor promoting the spread of invaders. Plants and plant litter can have important effects on plant colonization and community composition by affecting seedling survival and growth. I examined the hypothesis that invasion of non-native Lythrum salicaria in Typha-dominated marshes is disturbance-dependent. If so, the removal of Typha plants or litter would increase the survival and growth of L. salicaria seedlings. Additionally, the removal of both plants and litter could result in an additive or synergistic effect on the establishment of L. salicaria. Alternatively, L. salicaria may be a successful invader because it has a high capacity to establish and grow regardless of neighbours. In this case, L. salicaria would be expected to perform well even in plants and litter. Strategies for managing L. salicaria will depend on which factors promote invasion. I measured the differential effects of plants and litter, alone and in combination, on the survival and growth of L. salicaria seedlings transplanted into marshes. The presence of plants and litter did not affect seedling survival in relatively dry wetland sites, indicating that L. salicaria seedlings have the capacity to persist in the presence of neighbouring Typha spp. competitors. However, removal of both plants and litter allowed increased growth of L. salicaria seedlings in drier wetlands. Therefore, growth was facilitated by disturbance that removed all vegetation. Small disturbances (0.6 m²) decreased competitive suppression by native Typha spp. neighbours and resulted in significant increases in growth. Disturbance of wetlands at risk of invasion by L. salicaria should be avoided.     

Increased relative abundance of an invasive competitor for pollination, Lythrum salicaria, reduces seed number in Mimulus ringens

When exotic plant species share pollinators with native species, competition for pollination may lower the reproductive success of natives by reducing the frequency and/or quality of visits they receive. Exotic species often become numerically dominant in plant communities, and the relative abundance of these potential competitors for pollination may be an important determinant of their effects on the pollination and reproductive success of co-occurring native species. Our study experimentally tests whether the presence and abundance of an invasive exotic, Lythrum salicaria L. (Lythraceae), influences reproductive success of a co-flowering native species, Mimulus ringens L. (Phrymaceae). We also examine the mechanisms of competition for pollination and how they may be altered by changes in competitor abundance. We found that the presence of Lythrum salicaria lowered mean seed number in Mimulus ringens fruits. This effect was most pronounced when the invasive competitor was highly abundant, decreasing the number of seeds per fruit by 40% in 2006 and 33% in 2007. Reductions in the number of seeds per fruit were likely due to reduced visit quality resulting from Mimulus pollen loss when bees foraged on neighboring Lythrum plants. This study suggests that visit quality to natives may be influenced by the presence and abundance of invasive flowering plants.     

Plasticity of Lythrum salicaria and Phragmites australis growth characteristics across a European geographical gradient

Plants of Lythrum salicaria and Phragmites australis originating from localities across the European north–south geographical gradient were cultivated in parallel in an outdoor tub experiment. A strong correlation was found between growth and morphometric characteristics related to plant size (plant height, basal diameter, aboveground- and belowground plant biomass, etc.) and the position of the respective populations along the north–south gradient. Plants of both L. salicaria and P. australis from the southern localities grew taller and more vigorously and flowered later than plants from relatively more northern localities. From this point of view, the plants originating from south European populations were comparable to invasive North American plants. Our study indicates that explanation of the competitive success of populations invading new geographical areas may involve the role of geographic gradients within the species native range.     

Pollination and reproduction enhance the invasive potential of an early invader: the case of Lythrum salicaria (purple loosetrife) in South Africa

The potential of an alien plant to spread rapidly and colonize new habitat may be related to the mode of reproduction and the ability to attract pollinators. Most studies focus on widespread invasive plants, in which pollinators are rarely limiting. Here, we assess the ability of a recent invader in South Africa, the tristylous Lythrum salicaria to self-reproduce and whether this can explain the delay between introduction and spread. This study was conducted in one of the largest known populations (a total of 7 populations in South Africa) of L. salicaria in the Liesbeek river in the fynbos biome. We assessed the importance of pollinators and autonomous selfing in L. salicaria by comparing seed set between pollinator excluded and naturally pollinated flowers. Overall, 5 pollinators (4 native and 1 alien) were recorded with Cape honeybees and Africa Monarch butterflies the most prominent. Seed and fruit set were significantly higher in open pollinated flowers compared to pollinator excluded flowers. Also, seed and fruit set in pollinator excluded flowers were higher in long and medium morphs compared to short morphs. Germination was high for seeds from pollinator, but also from pollinator excluded treatments. This shows that L. salicaria in South Africa is self-compatible to some extent, but it is frequented by pollinators, significantly increasing seed production. Despite L. salicaria being tristylous, all 3 morphs are present in South Africa and with a huge seed production, this species has the potential to become a major invader of rivers and wetlands in South Africa.

     

Impact and management of purple loosestrife (Lythrum salicaria) in North America

The invasion of non-indigenous plants is considered a primary threat to integrity and function of ecosystems. However, there is little quantitative or experimental evidence for ecosystem impacts of invasive species. Justifications for control are often based on potential, but not presently realized, recognized or quantified, negative impacts. Should lack of scientific certainty about impacts of non-indigenous species result in postponing measures to prevent degradation? Recently, management of purple loosestrife (Lythrum salicaria), has been criticized for (1) lack of evidence demonstrating negative impacts of L. salicaria, and (2) management using biocontrol for lack of evidence documenting the failure of conventional control methods. Although little quantitative evidence on negative impacts on native wetland biota and wetland function was available at the onset of the control program in 1985, recent work has demonstrated that the invasion of purple loosestrife into North American freshwater wetlands alters decomposition rates and nutrient cycling, leads to reductions in wetland plant diversity, reduces pollination and seed output of the native Lythrum alatum, and reduces habitat suitability for specialized wetland bird species such as black terns, least bitterns, pied-billed grebes, and marsh wrens. Conventional methods (physical, mechanical or chemical), have continuously failed to curb the spread of purple loosestrife or to provide satisfactory control. Although a number of generalist insect and bird species utilize purple loosestrife, wetland habitat specialists are excluded by encroachment of L. salicaria. We conclude that (1) negative ecosystem impacts of purple loosestrife in North America justify control of the species and that (2) detrimental effects of purple loosestrife on wetland systems and biota and the potential benefits of control outweigh potential risks associated with the introduction of biocontrol agents. Long-term experiments and monitoring programs that are in place will evaluate the impact of these insects on purple loosestrife, on wetland plant succession and other wetland biota.     

Latitudinal variation in resistance and tolerance to herbivory in the perennial herb Lythrum salicaria is related to intensity of herbivory and plant phenology

Both the length of the growing season and the intensity of herbivory often vary along climatic gradients, which may result in divergent selection on plant phenology, and on resistance and tolerance to herbivory. In Sweden, the length of the growing season and the number of insect herbivore species feeding on the perennial herb Lythrum salicaria decrease from south to north. Previous common‐garden experiments have shown that northern L. salicaria populations develop aboveground shoots earlier in the summer and finish growth before southern populations do. We tested the hypotheses that resistance and tolerance to damage vary with latitude in L. salicaria and are positively related to the intensity of herbivory in natural populations. We quantified resistance and tolerance of populations sampled along a latitudinal gradient by scoring damage from natural herbivores and fitness in a common‐garden experiment in the field and by documenting oviposition and feeding preference by specialist leaf beetles in a glasshouse experiment. Plant resistance decreased with latitude of origin, whereas plant tolerance increased. Oviposition and feeding preference in the glasshouse and leaf damage in the common‐garden experiment were negatively related to damage in the source populations. The latitudinal variation in resistance was thus consistent with reduced selection from herbivores towards the northern range margin of L. salicaria. Variation in tolerance may be related to differences in the timing of damage in relation to the seasonal pattern of plant growth, as northern genotypes have developed further than southern have when herbivores emerge in early summer.