Growth of coyote willow and the attack and survival of a mid-rib galling sawfly,Euura sp.

We studied the relationship between variation in age and shoot characteristics of the host plant Salix exigua Nuttall (coyote or sandbar willow) and the attack and survival of Euura sp. (an unnamed leaf-midrib galling sawfly). Variation in shoot characteristics resulted from reduced growth as willow ramets aged. Mean shoot length per ramet and mean longest leaf length per shoot decreased by 95% and 50% respectively between 1- and 9-year-old willow ramets. All measured shoot characteristics-shoot length, longest leaf length, number of leaves per shoot, and mean internode length-were significantly negatively correlated with ramet age (r ² ranged from –0.23 to –0.41). Correlations between shoot characteristics were highly positive, indicating that plants also grew in a strongly integrated fashion (r ² ranged from 0.54 to 0.85). Four hypotheses were examined to explain sawfly attack patterns. The host-plant hypothesis was supported in explaining enhanced larval sawfly survival through reduced plant resistance. As willow ramets aged, the probability of Euura sp. attack decreased over 10-fold, from 0.315 on 1-year-old ramets to 0.024 on 2- to 9-year-old ramets. As shoot length increased, the probability of sawfly attack increased over 100-fold, from 0.007 on shoots <100 mm, to 0.800 on shoots in the 1001–1100 mm shoot length class. These attack patterns occurred even though 1-year-old ramets and shoots >500 mm each represented less than 2% of the total shoots available for oviposition. Host plant induced mortality of the egg/early instar stage decreased by 50% on longer leaves and was the most important factor determining survival differences between vigorous and non-vigorous hosts. Sawfly attack was not determined by the resource distribution hypothesis. Although shoots <200 mm contained 82% of the total leaves available, they contained only 43% of the galls initiated. The attack pattern also was not explained by the gall volume hypothesis. Although gall volume increased on longer shoots, there was no significant variation in mid or late instar mortality over shoot length, as would be expected if food resources within smaller galls were limited. The natural enemy attack hypothesis could not explain the pattern of oviposition since predation was greater on longer shoots and leaves. In addition, larval survival was related to oviposition behavior. Due to a 69% reduction in late instar death and an 83% reduction in parasitism, survival of progeny in galls initiated close to the petiole base was 2.8 times greater than in galls initiated near the leaf tip. A 75% reduction in gall volume over this range of gall positions may account for the observed increases in late instar mortality and parasitism.     

Plant stress and larval performance of a dipterous gall former

According to the plant vigour hypothesis, galling insects should respond positively and perform better on vigorous plants or plant parts, the opposite of the predictions of the plant stress hypothesis. I carried out field experiments to analyse the effects of sustained abiotic stress on the interactions between the common reed (Phragmites australis) and a gall-forming fly (Lipara lucens). The reed shoot diameter (a measure of plant vigour) is strongly affected by environmental conditions, where dry and/or nutrient-poor habitats produce thinner (stressed) shoots. L. lucens gall density is negatively correlated with shoot diameter. In a survival experiment with a wide range of shoot diameters, larval mortality was also highly correlated with shoot quality. Gall formation was higher on thinner, stressed shoots. An analysis of the gall tissues revealed that galls induced by L. lucens contain a high amount of a nutrient-rich feeding tissue. The impact of L. lucens is higher on thinner shoots. The results of this study showed that L. lucens performs better on stressed hosts, which contradicts the plant vigour hypothesis for galling insects. The low nutrient availability in the stressed shoots can be compensated by the production of galls with a nutrient-rich feeding tissue.     

The plant vigour hypothesis revisited – how is browsing by ungulates and elephant related to woody species growth rate?

The way herbivores select what to eat is of considerable practical and theoretical interest, and has given rise to different theories and hypotheses. The plant vigour hypothesis predicts that herbivores feed preferentially on vigorous, i.e., large and/or fast-growing plants or plant parts. These predictions have previously primarily been tested on variation within plant species. Here we test whether differences in vigour among plant species in the same environment can explain differences in herbivore attack. We studied variation in browsing pressure by a guild of large herbivores on different woody species in an African savanna ecosystem. Shoot growth rate, annual shoot length, basal shoot diameter and annual shoot volume of 14 woody plant species were measured in the field. Plant species’ shoot vigour represented by the first PCA axis scores generated from the four shoot variables were then related to browsing pressure (% utilisation) on each of the species by native ungulates and elephant. Nutrient and fibre concentrations and tannin activity were also determined for the 14 woody plant species. We found ungulate browsing pressure to show a unimodal relationship with plant species’ shoot vigour. The heaviest browsing pressure was on plant species with shoots of intermediate vigour. We suggest that species with less vigorous shoots had low nutrient and high fibre concentrations and offered small bite sizes, whereas species with vigorous shoots had high nutrient concentrations but larger shoot diameters than the bite diameters of browsing ungulates. Elephant browsing pressure was not related to plant species’ shoot vigour.     

Host plant effects on the development and survivorship of the galling insect Neopelma baccharidis (Homoptera: Psyllidae)

In this study, the mortality factors acting upon the galling psyllid Neopelma baccharidis Burckhardt (Homoptera) caused by its host plant, Baccharis dracunculifolia De Candole (Asteraceae) were analysed. In March 1999, 982 galls of the same cohort were randomly marked on 109 individuals of B. dracunculifolia in the field. Galls were censused each month during their development, from April to August, and dead galls were collected and analysed for mortality factors. Gall dehiscence rates were calculated for each month. The major mortality source of N. baccharidis was gall dropping (13.2% of the original cohort), which is probably a normal outcome of previous mortality caused by the other factors observed in this study. Unknown factors killed 11.7% of this gall population and were ascribed to plant resistance during gall development. Empty galls represented 7.7% of the observed mortality and may be a consequence of egg retention or egg mortality/abortion related to variations in plant quality. Shoot mortality was high during the dry season and killed 7.5% of the galls, but this impact was minimized after the third month from gall formation due to the ability of nymphs to accelerate development and emerge from galls on dying shoots. However, the size of dehisced galls on dead shoots tended to be smaller, possibly affecting adult performance. Mortality of N. baccharidis attributed to B. dracunculifolia strongly controlled the galling insect population, killing 40.7% of the original cohort of galls. Plant‐mediated mortality was caused by often neglected factors acting predominantly during the first 3 months of development, which are critical to gall survivorship. These results reinforce the importance of bottom‐up forces in plant‐insect systems.     

The interplay between plant traits and herbivore attack: a study of a stem galling midge in the neotropics

1. The relationship between plant traits and the frequency of attack by a stem galling midge, Neolasioptera sp. (Diptera: Cecidomyiidae), on Eremanthus erythropappus (Asteraceae) was studied. The morphological changes of the host after a galler attack and the potential effects of these changes on attacks by the next generation of gallers were analysed. The study was conducted in the Serra do Japi, São Paulo, south-eastern Brazil. 2. Galled branches were significantly longer, thicker, and had more leaves than ungalled branches. Accordingly, gall establishment was higher in the longer and more foliose branches. Hence, it is suggested that ovipositing females are maximizing their performance by selecting larger branches. 3. Galled branches were larger than ungalled branches of the same age. Two hypotheses, not necessarily exclusive, can explain this pattern: (1) the plant vigour hypothesis that the females are choosing the more vigorous, fast-growing branches, which still remain more vigorous after galling; or (2) the resource regulation hypothesis that galling increases branch growth rates, thus increasing resource quality for forthcoming conspecifics. 4. Co-occurrence frequencies of current and past generation galls showed that the likelihood of a branch being galled increased when it, or the branch from which it stemmed, had been galled before. The data indicated that this preference was conditioned by the number of previous attacks. Heavier attack intensities, such as one gall in the same branch and another in the branch from which it stemmed, decreased the probability of further galling. 5. The suggested links between herbivore attack and plant traits indicate that studies on host selection by phytophagous insects must take into account that herbivory itself may change the plant traits that are postulated to be selected by the insects.     

Herbivore‐induced plant responses in Brassica oleracea prevail over effects of constitutive resistance and result in enhanced herbivore attack

1. Plant responses to herbivore attack may have community‐wide effects on the composition of the plant‐associated insect community. Thereby, plant responses to an early‐season herbivore may have profound consequences for the amount and type of future attack. 2. Here we studied the effect of early‐season herbivory by caterpillars of Pieris rapae on the composition of the insect herbivore community on domesticated Brassica oleracea plants. We compared the effect of herbivory on two cultivars that differ in the degree of susceptibility to herbivores to analyse whether induced plant responses supersede differences caused by constitutive resistance. 3. Early‐season herbivory affected the herbivore community, having contrasting effects on different herbivore species, while these effects were similar on the two cultivars. Generalist insect herbivores avoided plants that had been induced, whereas these plants were colonised preferentially by specialist herbivores belonging to both leaf‐chewing and sap‐sucking guilds. 4. Our results show that community‐wide effects of early‐season herbivory may prevail over effects of constitutive plant resistance. Induced responses triggered by prior herbivory may lead to an increase in susceptibility to the dominant specialists in the herbivorous insect community. The outcome of the balance between contrasting responses of herbivorous community members to induced plants therefore determines whether induced plant responses result in enhanced plant resistance.     

Increased competitive ability and herbivory tolerance in the invasive plant <Emphasis Type="Italic">Sapium sebiferum</Emphasis>

The evolution of increased competitive ability (EICA) hypothesis predicts that release from natural enemies in the introduced range favors exotic plants evolving to have greater competitive ability and lower herbivore resistance than conspecifics from the native range. We tested the EICA hypothesis in a common garden experiment with Sapium sebiferum in which seedlings from native (China) and invasive (USA) populations were grown in all pairwise combinations in the native range (China) in the presence of herbivores. When paired seedlings were from the same continent, shoot mass and leaf damage per seedling were significantly greater for plants from invasive populations than those from native populations. Despite more damage from herbivores, plants from invasive populations still outperformed those from native populations when they were grown together. Increased competitive ability and higher herbivory damage of invasive populations relative to native populations of S. sebiferum support the EICA hypothesis. Regression of biomass against percent leaf damage showed that plants from invasive populations tolerated herbivory more effectively than those from native populations. The results of this study suggest that S. sebiferum has become a faster-growing, less herbivore-resistant, and more herbivore-tolerant plant in the introduced range. This implies that increased competitive ability of exotic plants may be associated with evolutionary changes in both resistance and tolerance to herbivory in the introduced range. Understanding these evolutionary changes has important implications for biological control strategies targeted at problematic invaders.     

Induced defences in the neotropical tree Bauhinia brevipes (Vog.) to herbivory: effects of damage-induced changes on leaf quality and insect attack

Induced defences to herbivory are physical, nutritional, and allelochemical traits that change in plants following damage or stress, and that reduce the performance and/or preference of herbivores. The aim of this study was to verify the occurrence and effect of induced responses in Bauhinia brevipes (Vog.) (Leguminosae) which defend it against herbivores, through the manipulation of its leaves, and their effects on herbivore foraging behaviour. We selected 15 plants in the field, and three shoots per plant were subjected to one of three treatments: (1) damaged shoots (simulation of the main types of foliar herbivory and insect exclusion); (2) damaged control shoots (insect exclusion); and (3) control shoots (not manipulated). Water and nitrogen content, tannin concentration, levels of herbivory, and shoot growth rates were compared among treatments. Leaf quality varied among treatments. Damaged leaves showed higher tannin concentration, and lower water and nitrogen content compared to undamaged leaves. On the other hand, they experienced higher rates of herbivory than leaves on control shoots. Moreover, shoots that were experimentally induced showed a higher increase in final shoot length. These results suggest that simulated herbivory on B. brevipes reduced the nutritional quality of its leaves and increased the amount of secondary compounds, therefore altering insect herbivore attack and increasing shoot performance.     

Effects of plant quality and ant defence on herbivory rates in a neotropical ant‐plant

1. Understanding the degree to which populations and communities are limited by both bottom‐up and top‐down effects is still a major challenge for ecologists, and manipulation of plant quality, for example, can alter herbivory rates in plants. In addition, biotic defence by ants can directly influence the populations of herbivores, as demonstrated by increased rates of herbivory or increased herbivore density after ant exclusion. The aim of this study was to evaluate bottom‐up and top‐down effects on herbivory rates in a mutualistic ant‐plant. 2. In this study, the role of Azteca alfari ants as biotic defence in individuals of Cecropia pachystachya was investigated experimentally with a simultaneous manipulation of both bottom‐up (fertilisation) and top‐down (ant exclusion) factors. Four treatments were used in a fully factorial design, with 15 replicates for each treatment: (i) control plants, without manipulation; (ii) fertilised plants, ants not manipulated; (iii) unfertilised plants and excluded ants and (iv) fertilised plants and ants excluded. 3. Fertilisation increased the availability of foliar nitrogen in C. pachystachya, and herbivory rates by chewing insects were significantly higher in fertilised plants with ants excluded. 4. Herbivory, however, was more influenced by bottom‐up effects – such as the quality of the host plant – than by top‐down effects caused by ants as biotic defences, reinforcing the crucial role of leaf nutritional quality for herbivory levels experienced by plants. Conditionality in ant defence under increased nutritional quality of leaves through fertilisation might explain increased levels of herbivory in plants with higher leaf nitrogen.     

Effects of simulated shoot and leaf herbivory on vegetative growth and plant defense in Acacia drepanolobium

Plants have considerable ability to respond to herbivory, both with (above-ground) regrowth and with increased defense. We simulated both leaf and shoot herbivory in controlled, replicated experiments on individuals of Acacia drepanolobium in Laikipia, Kenya. These experiments were carried out on individuals that had experienced different, experimentally controlled histories of large mammalian herbivory. Both forms of simulated herbivory were associated with compensatory regrowth. Branches whose shoots had been removed grew significantly more over the next year than paired control branches, fully compensating for the lost shoot length. Branches whose leaves were removed both grew faster and had more leaves one year later than did control branches. Shoot removal, but not leaf removal, increased the production of side shoots. However, because past herbivore pressure was negatively associated with net shoot growth, there may be a long-term cost of herbivory even when plants appear to fully compensate for herbivory in the short term. In contrast to the effects on growth, simulated herbivory did not significantly increase physical (spines) or chemical (tannins) defenses, and there were no significant negative correlations between compensatory growth and plant defense.     

Effects of insect attack to stems on plant survival,growth, reproduction and photosynthesis

Studies of insect herbivory have mostly focused on leaf‐feeding even though most woody plant biomass is stem tissue. Attack to stems has the potential to be more detrimental to plant performance than attack to leaves. Here we asked how severe is the impact of insect stem herbivory on plant performance. We quantify the effect of insect stem herbivory via a meta‐analysis of 119 papers in 100 studies (papers by the same authors were treated as the same study). These studies involved 92 plant species and 70 species of insect herbivore (including simulated herbivory). Attack to plant stems reduced plant performance by an average of approximately 22%. Stem herbivory had greatest impacts on plant and branch survival, which was reduced by 63%. Measures of plant reproduction and vegetative biomass were reduced by 33% and 16% respectively, while measurements of photosynthetic rate were not significantly different between plants with and without stem herbivore attack. Stem herbivory led to a decline in leader performance but an increase in performance of laterals, highlighting the importance of plant compensation. Juvenile plants were more severely affected by stem herbivory than adult plants, and studies conducted in greenhouses found more severe effects than studies conducted in the field. Stem herbivory did not have a significant effects on any of the non‐performance responses measured (defence compounds, SLA, root:shoot, phenology and plant carbon and nitrogen). We compare our results with results from various meta‐analyses considering herbivory on other plant parts. The impact of insect herbivory to stems on plant performance appears at least as severe as insect herbivory to roots and leaves, if not more.     

Release of phylogenetic constraints through low resource heterogeneity: the case of gall-inducing sawflies

Abstract. 1. A group of six unusual sawfly species, which do not conform to the phylogenetic constraints hypothesis as it has been applied to sawflies, was examined in natural populations. All species were in the genus Pontania (Hymenoptera: Tenthredinidae), which induce galls on leaves of willow species (Salicaceae). An understanding of these non‐conformist species was important as a test of the validity of the general hypothesis. 2. The six species of sawfly, Pontania mandshurica, P. cf. arcticornis, P. aestiva, P. arcticornis, P. pacifica, and P. nr. pacifica, showed no oviposition preference for long, vigorous shoots, in contrast to 37 documented tenthredinid species that have demonstrated such a preference. Rather, the non‐conformist species attacked the shortest shoot length classes more frequently and larval establishment in galls was successful. 3. The evident escape from the phylogenetic constraint, which commonly limits sawfly attack to the most vigorous shoots in a willow population, resulted from low apparent heterogeneity of the resources exploited by these Pontania species. At the time of female oviposition, shoots and leaves were too uniform to allow discrimination by females among shoot length classes, resulting in random, or near random attack of shoots. 4. The unusual relative uniformity of resources to which sawflies were exposed resulted from several characteristics. (1) Females emerged early relative to shoot growth phenology, making discrimination among shoot length and vigour difficult or impossible. (2) Low heterogeneity in leaf length resulted in resource similarity independent of shoot length. (3) Abscission of leaves occurred after emergence of larvae from leaf galls so that differential abscission of leaves in relation to shoot length became irrelevant. (4) In some cases, low variance in shoot lengths was evident in old ramets lacking long, vigorous shoots. Probably as a result of low resource heterogeneity, larvae survived well across all shoot length classes, revealing no ovipositional preference and larval performance linkage related to the exploitation of the longest shoot length classes in a population of willows, as in the conformist species. Therefore, larval survival did not provide positive feedback on female preferential behaviour for long shoots, as in the conformist species studied.     

Preference and performance linkage in aPhyllocolpa sawfly on the willow,Salix miyabeana,on Hokkaido

A leaf-folding sawfly in the genusPhyllocolpa (Hymenoptera: Tenthredinidae) attackingSalix miyabeana (Salicaceae) was studied near Sapporo, Hokkaido, along the Ishikari River in 1993. Host plant individuals were young trees 4–7 years old which were growing rapidly, producing some long shoots with large leaves. On a gradient of shoot length classes from 0–5 cm long to over 80 cm long, shoots were much more abundant in the shorter shoot length classes. However, attacks by ovipositing females increased as shoot length increase from 0 attacks on the shortest shoots to 5.17 attacks per shoot on the longest shoots. The frequency of attack per leaf increased from 0 to 0.13 over the same range of shoot lengths. This pattern of attack resulted in a high frequency of larval establishment in feeding sites, between 0.96 and 1.00, in all attacked shoot length categories. However, probability of survival to a late instar larva increased with shoot length and corresponded to the attack pattern, indicating a preference-performance linkage between female ovipositional decisions and larval survival. The patterns found for thisPhyllocolpa species are similar for galling sawflies in North America and Europe, especially in the genusEuura, members of which make stem, bud and leaf midrib galls. Extending the pattern to aPhyllocolpa species broadens identification of pattern and ultimately the generality of the emerging theory on populations of galling sawflies.     

Testing a new hypothesis: plant vigor and phylloxera distribution on wild grape in Arizona

Summary Longer, meaning more vigorous, shoots of a wild grape clone (Vitis arizonica) were more susceptible to attack by second and third generations of leaf-galling grape phylloxera,Daktulopsphaira vitifoliae, as the growing season progressed. Although there was no significant difference in mean shoot length between attacked and unattacked shoots within a clone at the beginning of shoot elongation, attacked shoots were significantly longer than unattacked shoots when elongation had ceased (P<0.01). Also, long attacked shoots had a significantly greater population of phylloxera galls than short attacked shoots (P<0.01) as the season progressed. The phylloxera population on long shoots increased rapidly while the population on short shoots remained the same. Longer shoots also produced significantly more axillary shoots than shorter shoots as the season progressed (P<0.001), and the number of axillary shoots accounted for 66 percent of the variance in number of attacked leaves on a shoot. Experimental evidence showed that there was a significantly greater percentage of available leaves attacked on long shoots than on short shoots (P<0.05) and the leaves on long shoots generally had a greater number of galls per leaf. The relationship between shoot length and probability of attack was also tested by comparing shoots lengths of 10 attacked clones and 10 unattacked clones at a second location. Mean shoot lengths of attacked clones were significantly longer than mean shoot lengths of unattacked clones (P<0.05), and mean shoot lengths of attacked shoots within a clone were significantly longer than unattacked shoots (P<0.001). Longer shoot length accounted for 81 percent of the variance in probability of attack. The reason for this pattern of attack was that long shoots produced newly expanding leaves over a longer time during the growing season and multivoltine phylloxera require undifferentiated tissue to initiate gall formation. Patterns of attack within a shoot were characterized by an uneven distribution of galls among leaves. This was due to development time between generations and the current availability of undifferentiated tissue at times of colonization. This study supports the hypothesis that some herbivore species are favored more by vigorous plants than by stressed plants.     

Influence of tree growth rate, shoot size and foliar chemistry on the abundance and performance of a galling adelgid

1. The abundance and performance of the Eastern Spruce Gall Adelgid, Adelges abietis , was evaluated on young, open-grown trees of White Spruce, Picea glauca , whose growth rates had been increased through fertilization or decreased through root-pruning.
2. In general, gall densities were highest on control trees and on mid-crown branches. Reduced galling success on fertilized trees was largely due to higher overwintering mortality of first-generation nymphs. Foliar magnesium, total tannin and total phenol contents were positively, and nitrogen and total monoterpene contents negatively, related to gall density and/or galling success.
3. Although short shoots were more abundant, shoot length was parabolically related to gall density. There was a non-significant parabolic trend between shoot size and galling success and volume. The number and average size of A. abietis females emerging from galls were positively related to gall volume, and realized fecundity was positively correlated to female size. Consequently, fitness was approximately twice as high for nymphs colonizing intermediate-sized than small or large shoots.
4. These results do not support the plant vigour or plant stress hypotheses. The results do, however, agree with predictions of the modified plant stress hypothesis for sucking insects. It is speculated that Adelges abietis lacks the necessary resources for successful gall formation on small shoots and is unable to produce a stimulus large enough to induce gall formation on large shoots.     

Nematode‐induced galls in Miconia albicans: effect of host plant density and correlations with performance

The study of nematodes parasitizing native plants plays a crucial role in understanding plant–pathogen interactions. In the present study we describe the patterns of attack by an undescribed species of Ditylenchus occurring in Miconia albicans (Melastomataceae), a widespread, native shrub from the Brazilian cerrado. We also tested the hypothesis that nematode‐induced leaf galls negatively correlate to host plant performance and that gall density is a function of host plant density. We collected paired healthy and attacked shoots from 28 individuals of M. albicans and estimated the leaf area lost to nematode‐induced galls in up to 10 leaves per shoot. We analyzed the relationships between leaf area lost to nematode galls and reproductive traits. Nematode attack levels were also compared to the spatial distribution of the host plant. Inflorescence length and fruit production were significantly reduced in attacked shoots compared with healthy shoots. Seeds from attacked shoots showed no significant reduction in germinability or germination time when compared with seeds collected from healthy shoots. Gall density was positively correlated with host density. Despite being seldom studied in tropical ecosystems, nematodes may play an important role in plant fitness and in structuring tropical communities.     

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.     

Effects of simulated root herbivory and fertilizer application on growth and biomass allocation in the clonal perennialSolidago canadensis

Summary Compensatory growth in response to simulated belowground herbivory was studied in the old-field clonal perennialSolidago canadensis. We grew rootpruned plants and plants with intact root systems in soil with or without fertilizer. For individual current shoots (aerial shoot with rhizome and roots) and for whole clones the following predictions were tested: a) root removal is compensated by increased root growth, b) fertilizer application leads to increased allocation to aboveground plant organs and increased leaf turnover, c) effects of fertilizer application are reduced in rootpruned plants. When most roots (90%) were removed current shoots quickly restored equilibrium between above-and belowground parts by compensatory belowground growth whereas the whole clone responded with reduced aboveground growth. This suggests that parts of a clone which are shared by actively growing shoots act as a buffer that can be used as source of material for compensatory growth in response to herbivory. Current shoots increased aboveground mass and whole clones reduced belowground mass in response to fertilizer application, both leading to increased allocation to aboverground parts. Also with fertilizer application both root-pruned and not root-pruned plants increased leaf and shoot turnover. Unfertilized plants, whether rootpruned or not, showed practically no aboveground growth and very little leaf and shoot turnover. Effects of root removal were as severe or more severe under conditions of high as under conditions of low nutrients, suggesting that negative effects of belowground herbivory are not ameliorated by abundant nutrients. Root removal may negate some effects of fertilizer application on the growth of current shoots and whole clones.     

Effects of gall damage by the introduced biocontrol agent Epiblema strenuana (Lep., Tortricidae) on the weed Parthenium hysterophorus (Asteraceae)

Effects of gall damage by the introduced moth Epiblema strenuana on different growth stages of the weed Parthenium hysterophorus was evaluated in a field cage using potted plants with no competition and in naturally regenerated populations with intraspecific competition. Gall damage at early stages of plant growth reduced the plant height, main stem height, flower production, leaf production, and shoot and root biomass. All galled, potted plants with no competition produced flowers irrespective of the growth stage at which the plants were affected by galling, but lesser than in ungalled plants. Gall induction during early growth stages in field plants experiencing competition prevented 30% of the plants reaching flowering. However, 6% of the field plants escaped from gall damage, as their main stems were less vigorous to sustain the development of galls. Flower production per unit total plant biomass was lower in galled plants than in ungalled plants, and the reduction was more intense when gall damage was initiated at early stages of plant growth. In potted plants with no competition, the number of galls increased with the plant vigour, as the gall insects preferred more vigorous plants. But in field plants there were no relationship between gall abundance and plant vigour, as intraspecific competition enhanced the negative effects of galling by reducing the vigour of the weed.     

Latitudinal variation in herbivory: influences of climatic drivers,herbivore identity and natural enemies

Although a number of investigations have concluded that lower latitudes are associated with increases in herbivore abundance and plant damage, the generality of this pattern is still under debate. Multiple factors may explain the lack of consistency in latitude–herbivory relationships. For instance, latitudinal variation in herbivore pressure may be shaped entirely or not by climatic variables, or vary among herbivore guilds with differing life‐history traits. Additionally, the strength of top–down effects from natural enemies on herbivores might also vary geographically and influence latitude–herbivory patterns. We carried out a field study where we investigated the effects of latitude and climate on herbivory by a seed‐eating caterpillar and leaf chewers, as well as parasitism associated to the former across 30 populations of the perennial herb Ruellia nudiflora (Acanthaceae). These populations were distributed along a 5° latitudinal gradient from northern Yucatan (Mexico) to southern Belize, representing one‐third of the species' latitudinal distribution and the entirety and one‐third of the precipitation and temperature gradient of this species' distribution (respectively). We found opposing latitudinal gradients of seed herbivory and leaf herbivory, and this difference appeared to be mediated by contrasting effects of climate on each guild. Specifically, univariate regressions showed that seed herbivory increased at higher latitudes and with colder temperatures, while leaf herbivory increased toward the equator and with wetter conditions. Multiple regressions including temperature, precipitation and latitude only found significant effects of temperature for seed herbivory and latitude for leaf herbivory. Accordingly, that latitudinal variation in seed herbivory appears to be driven predominantly by variation in temperature whereas latitudinal variation in leaf herbivory was apparently driven by other unexplored correlates of latitude. Parasitism did not exhibit variation with latitude or climatic factors. Overall, these findings underscore that the factors driving latitudinal clines in herbivory might vary even among herbivore species coexisting on the same host plant.