Exotic weevil invasion increases floral herbivore community density,function, and impact on a native plant

Consumer communities are being re‐arranged through unprecedented rates of human‐mediated invasions and extinctions. Such changes in consumer diversity potentially alter community function and impact on resource populations. Although insect herbivore invasions are increasingly common, the influence of such species additions on native resident herbivore guilds, along with their individual and combined effects on native plant resources, are rarely investigated. Here, we used site‐to‐site and plant‐to‐plant variation in herbivore composition to examine how the addition of an invasive exotic weevil, Rhinocyllus conicus, combines with a guild of native floral herbivores (tephritid flies, pyralid moths) to influence two key components of herbivore community function – aggregate herbivore densities and cumulative levels of seed destruction – on a native thistle, Cirsium canescens. Invasion of a site by R. conicus more than doubled aggregate herbivore density, resulting in increased levels of seed destruction and a halving of seed production by the native thistle. Further, herbivore function was significantly higher on individual plants attacked by R. conicus, compared to plants attacked only by native herbivores. Insect densities and levels of seed destruction on plants attacked by multiple herbivore taxa never exceeded those observed for plants attacked by R. conicus alone, suggesting that increases in herbivore community function with invasion resulted from the inclusion of a functionally dominant insect rather than any complementarity effects. Some evidence for interference between insects emerged, with a trend towards reduced moth and weevil densities in two and three taxon mixtures compared with plants attacked by each taxon alone. However, density compensation was limited so that, overall, the addition of a novel herbivore to the floral guild was associated with a significant increase in herbivore community function and impact on seed production. The results suggest that invasion of a functionally dominant herbivore into an unsaturated recipient community can augment function within a resource guild.     

Regional patch dynamics of Cirsium arvense and possible implications for plant‐animal interactions

Abstract. Plant population biology considers the dynamics of plant modules within stands. However, stands themselves may have considerable regional turnover in space and time. These changes in the number, distribution and size of plant stands generate a dynamic spatial pattern with important implications for the spatial and temporal dynamics of phytophagous insects using these plants as a host. During five successive years we studied the regional distribution and patch dynamics of the creeping thistle Cirsium arvense and the distribution of associated populations of the herbivore Urophora cardui (Diptera: Tephritidae), a specialist stem gall former. The study conducted was in a 15 km² heterogeneous, agricultural area in northeastern Bavaria. The distribution of the number of plants per patch was skewed with many more small C. arvense patches than large ones. During the five years of study, there was a 50% increase in the number of C. arvense patches, and a decrease in the mean number of plants per patch (= patch size) to less than half the patch size of the first year. Whilst patch size was randomly distributed in space, patch density showed a consistent, non‐random spatial pattern. Patch density was spatially auto‐correlated, with areas of high or low patch density having a characteristic dimension of ca. 1 km. Patch size was predictable in time and appeared to be regulated by size dependent processes, with the extinction probability of a patch being negatively correlated with its size. Correlated with the decline of C. arvense patch size during the study, the occupancy and total numbers of the herbivore U. cardui had a marked decrease, suggesting that the regional distribution of the stem gall former is not only influenced by patch number but more importantly by the mean patch size. With decreasing patch sizes, U. cardui was faced with an increasingly dynamic landscape due to higher extinction rates of small patches, although the mean distance between host plant patches decreased.     

A comparison of the responses of two tropical specialist herbivores to host plant patch size

Summary The effects of host plant patch size on the abundances of two specialist herbivores (the chrysomelid beetle, Acalymma innubum and the pentatomid bug, Piezosternum subulatum) were investigated in a natural forest community in the Virgin Islands. Abundances were compared early and late in the season in different sized patches of the cucurbit host plant (Cayaponia americana) growing in open habitat (with no surrounding plant community) and forest habitat (with diverse surrounding plant community). For both herbivore species, adult abundances per patch were positively correlated with patch leaf area, but there was a significant patch size effect (i.e., correlation between herbivore density per unit plant and patch leaf area) only for beetles in the forest habitat. Both herbivore species were significantly affected by surrounding plant diversity, but in opposite ways: beetles were more abundant in open patches whereas bugs were more abundant in forest patches. Relationships between abundance and patch size in open and forest patches changed through the season for both herbivore species. These changing abundance patterns are discussed with respect to (1) increases in the diversity of the plant community surrounding host plant patches, and (2) differences in herbivore movement patterns.     

Indirect interaction between two native thistles mediated by an invasive exotic floral herbivore

Spatial and temporal variation in insect floral herbivory is common and often important. Yet, the determinants of such variation remain incompletely understood. Using 12 years of flowering data and 4 years of biweekly insect counts, we evaluated four hypotheses to explain variation in damage by the Eurasian flower head weevil, Rhinocyllus conicus, to the native North American wavyleaf thistle, Cirsium undulatum. The four factors hypothesized to influence weevil impact were variations in climate, weevil abundance, phenological synchrony, and number of flower heads available, either on wavyleaf thistle or on the other cooccurring, acquired native host plant (Platte thistle, Cirsium canescens), or on both. Climate did not contribute significantly to an explanation of variation in R. conicus damage to wavyleaf thistle. However, climate did influence weevil synchrony with wavyleaf flower head initiation, and phenological synchrony was important in determining R. conicus oviposition levels on wavyleaf thistle. The earlier R. conicus was active, the less it oviposited on wavyleaf thistle, even when weevils were abundant. Neither weevil abundance nor availability of wavyleaf flower heads predicted R. conicus egg load. Instead, the strongest predictor of R. conicus egg load on wavyleaf thistle was the availability of flower heads on Platte thistle, the more common, earlier flowering native thistle in the sand prairie. Egg load on wavyleaf thistle decreased as the number of Platte thistle flower heads at a site increased. Thus, wavyleaf thistle experienced associational defense in the presence of flowering by its now declining native congener, Platte thistle. These results demonstrate that prediction of damage to a native plant by an exotic insect may require knowledge of both likely phenological synchrony and total resource availability to the herbivore, including resources provided by other nontarget native species.     

The pattern,rate, and range of within‐patch movement of a stem‐galling fly

1. Information on the movement of insects is critical to understanding the spatial spread, dynamics, and genetic structure of their populations, as well as their interactions with other species. With this in mind, the movement behaviour of the stem‐galling fly Eurosta solidaginis Fitch (Diptera: Tephritidae) was investigated. 2. Fluorescent‐marked adults were released at a single location within pure patches of the host plant, tall goldenrod Solidago altissima, and their distributions censused repeatedly throughout the day. 3. Following their release, male and female flies redistributed themselves in a manner that was well described by a simple‐diffusion model. The diffusion rate was independent of fly density and time since flies were released. 4. Female flies dispersed at a significantly faster rate, and therefore farther on average, than males. Based on the diffusion model, it was estimated that at 2.5–3.0 h post release, males and females had a median dispersal distance of only 2.0 and 2.5 m respectively. Furthermore, 95% of the males were estimated to have dispersed no more than 5.9 m, and females no more than 6.4 m. 5. Post‐release censuses suggested that flies were most active during mid morning, disappeared from the site at a rate of 10–15% per hour (most likely due to mortality), and survived for less than 2 days. Based on the rate of spread, diel activity, and liberal estimates of longevity in the field, 50% of the ovipositing females were predicted to have had a maximum lifetime range of movement within a patch of hosts of ≤ 51 m (95% were expected to have been limited to ≤ 130 m). 6. These data are used to assess whether the absence of a positive correlation between host‐plant preference and offspring performance in this system could be due to the limited scale of dispersal of this species relative to the spatial scale at which its oviposition behaviour has been studied.     

Host recognition by Rhinocyllus conicus of floral scents from invasive and threatened thistles

One of the main obstacles of classical biological control is that biological control organisms cannot be recalled once they are released in nature. It is particularly true for the flowerhead weevil, Rhinocyllus conicus Frölich, which was released as a biological control organism for the invasive musk thistle, Carduus nutans L. (MT). While weevils successfully suppressed introduced populations of musk thistles and other invasive thistle species, non-target attacks have been reported on multiple native thistles including federally listed threatened and endangered (T&E) thistle species. To investigate the foraging behavior of female weevils on invasive and native thistles, we examined volatile organic compounds (VOCs) emitted from MT and a T&E plant species, Sacramento Mountains thistle, Cirsium vinaceum Wooton & Standley (SMT) in the Lincoln National Forest, New Mexico. We used a dynamic headspace volatile collection system and gas chromatography-mass spectrometry to compare volatile profiles between MT and SMT. Female weevils reacted to 7 electrophysiologically active chemical compounds in the blends based on gas chromatography-electroantennography. The behavioral response of female weevils was indifferent when VOCs from both thistles were offered in y-tube olfactometry experiments. Yet, they preferred VOCs collected from MT to purified air. The searching time of female weevils was longer to VOCs collected from SMT over controls. Investigating signals during the initial host recognition of released biological control organisms may open new opportunities to reduce non-target attacks on T&E plant species.

     

Effect of plant patch shape on the distribution and abundance of three lepidopteran species associated with Brassica oleracea

• 1 Plant patch shape may affect the abundance of herbivorous insects. Patches of the same size but longer or irregular have a higher perimeter/area relationship (P/A) than square or regular ones, which may determine the immigration, emigration and abundance of individuals in the patch.
• 2 Only specialist species should be affected by plant patch shape. Those species that are more abundant in smaller patches should be more abundant in patches with higher P/A, whereas those that are more abundant in larger patches should be more abundant in patches with lower P/A.
• 3 We studied the density of eggs, larvae and pupae of Pieris brassicae, Plutella xylostella and Trichoplusia ni in square (low P/A) and I‐shaped (high P/A) patches of 144 plants of Brassica oleracea. We also estimated their immigration to these patches, and the final plant weight.
• 4 Plant patch shape affected the abundance, but not the distribution, of the two specialist species. Whereas P. brassicae was denser in I‐shaped patches, P. xylostella was more abundant in square patches. The generalist T. ni was not affected by patch shape. Immigration of P. brassicae was higher in I‐shaped patches, but immigration of P. xylostella and T. ni was not affected by patch shape. Plants were heavier in the centre of square patches.
• 5 Our results suggest that plant patch shape affects the density of herbivorous insects and should be considered independently from other plant patch variables when studying the population dynamics of these organisms.

     

Movement of the Apollo butterfly Parnassius apollo related to host plant and nectar plant patches

1. The movement of adults of the endangered Apollo butterfly, Parnassius apollo, was studied using mark–recapture data, within a population consisting of discrete patches of the species’ host plant (n = 43), which were segregated spatially from patches of the species’ main nectar plants (n = 14). 2. The Apollo routinely moved large distances (median 260 m, maximum 1840 m), and moved frequently between the two types of patches. Only 27% (28/105) of the recaptures were made on the same host plant patch as the release. 3. The population acts as a patchy population where the adults mix over the whole area, but successful reproduction can only take place in the discrete host plant patches. 4. Occurrence on a host plant patch was restricted by the area size of the host plant patch and the spatial configuration of nectar plant patches. Thus, although the Apollo is a good flyer, its movement over the patches is still constrained by the segregation of adult and larval resources.     

Intraspecific variation in herbivore‐induced plant volatiles influences the spatial range of plant–parasitoid interactions

Chemical information influences the behaviour of many animals, thus affecting species interactions. Many animals forage for resources that are heterogeneously distributed in space and time, and have evolved foraging behaviour that utilizes information related to these resources. Herbivore‐induced plant volatiles (HIPVs), emitted by plants upon herbivore attack, provide information on herbivory to various animal species, including parasitoids. Little is known about the spatial scale at which plants attract parasitoids via HIPVs under field conditions and how intraspecific variation in HIPV emission affects this spatial scale. Here, we investigated the spatial scale of parasitoid attraction to two cabbage accessions that differ in relative preference of the parasitoid Cotesia glomerata when plants were damaged by Pieris brassicae caterpillars. Parasitoids were released in a field experiment with plants at distances of up to 60 m from the release site using intervals between plants of 10 or 20 m to assess parasitism rates over time and distance. Additionally, we observed host‐location behaviour of parasitoids in detail in a semi‐field tent experiment with plant spacing up to 8 m. Plant accession strongly affected successful host location in field set‐ups with 10 or 20 m intervals between plants. In the semi‐field set‐up, plant finding success by parasitoids decreased with increasing plant spacing, differed between plant accessions, and was higher for host‐infested plants than for uninfested plants. We demonstrate that parasitoids can be attracted to herbivore‐infested plants over large distances (10 m or 20 m) in the field, and that stronger plant attractiveness via HIPVs increases this distance (up to at least 20 m). Our study indicates that variation in plant traits can affect attraction distance, movement patterns of parasitoids, and ultimately spatial patterns of plant–insect interactions. It is therefore important to consider plant‐trait variation in HIPVs when studying animal foraging behaviour and multi‐trophic interactions in a spatial context.     

Effect of plant barriers and citrus leaf age on dispersal of Diaphorina citri (Hemiptera: Liviidae)

Studies designed to measure dispersal capacity of Diaphorina citri Kuwayama (Hemiptera: Liviidae) are needed to provide the epidemiological knowledge necessary to improve management of citrus huanglongbing. In this study, a mark–release–recapture technique was used to investigate whether 1) host or non‐host plants of D. citri can act as barriers for dispersing insects and 2) presence or absence of young citrus leaves influence movement of D. citri towards citrus plants. The experimental field consisted of four circular and adjacent areas containing citrus trees, Citrus sinensis (L.) Osbeck cv. ‘Hamlin’, planted in concentric circles at 18, 24 and 30 m from the release centre. Insect activity was monitored by recapturing at each distance using yellow stick traps. Dense plantings of tall non‐host plants of D. citri such as corn had no effect on insect dispersal towards citrus plants when compared to a shorter cover crop such as grass. In contrast, suitable host plants acted as traps decreasing movement of D. citri. Diaphorina citri dispersed at greater speeds in the absence of young leaves reaching 140 m within 6 hours after release, whereas in the presence of young leaves, individuals reached at most 60 m at 1 day after release. Results suggest that D. citri control measures may be more efficient during periods of highest vegetative activity when insects are less active. Moreover, the use of suitable host plants for D. citri as trap plants may be a potential tactic to prevent movement of insects into the crop.     

Classical Biological Control of Nodding and Plumeless Thistles

Nodding (musk) thistle (Carduus thoermeri Weinmann in the Carduus nutans L. group) and plumeless thistle (Carduus acanthoides L.) are introduced noxious weeds of Eurasian origin. Both weeds are problematic in pastures, rangelands, and croplands and along state highways in many parts of the United States. The success of both species of thistles is largely due to their prolific seed production, seed longevity, competitive ability, and lack of natural enemies. Classical biological control of nodding thistle in Virginia has been achieved with three exotic thistle herbivores, Rhinocyllus conicus Froelich (Coleoptera: Curculionidae), Trichosirocalus horridus (Panzer) (Coleoptera: Curculionidae), and Cassida rubiginosa Müller (Coleoptera: Chrysomelidae). T. horridus also effectively controls plumeless thistle. These insect herbivores complement each other. Nodding thistle biological control is achieved in about 5–6 years in Virginia, Missouri, and Montana. In addition, a rust fungus (Puccinia carduorum Jacky) (Uredinales: Pucciniaceae) has been introduced and established for control of nodding thistle in Virginia. Development and reproduction of the three thistle herbivores are not adversely affected by the rust. The rust hastens plant senescence and reduces seed production. Control of plumeless thistle with R. conicus and T. horridus takes approximately twice as long as control of nodding thistle.     

Congruent population genetic structures and divergence histories in anther‐smut fungi and their host plants Silene italica and the Silene nutans species complex

Study of the congruence of population genetic structure between hosts and pathogens gives important insights into their shared phylogeographical and coevolutionary histories. We studied the population genetic structure of castrating anther‐smut fungi (genus Microbotryum) and of their host plants, the Silene nutans species complex, and the morphologically and genetically closely related Silene italica, which can be found in sympatry. Phylogeographical population genetic structure related to persistence in separate glacial refugia has been recently revealed in the S. nutans plant species complex across Western Europe, identifying several distinct lineages. We genotyped 171 associated plant–pathogen pairs of anther‐smut fungi and their host plant individuals using microsatellite markers and plant chloroplastic single nucleotide polymorphisms. We found clear differentiation between fungal populations parasitizing S. nutans and S. italica plants. The population genetic structure of fungal strains parasitizing the S. nutans plant species complex mirrored the host plant genetic structure, suggesting that the pathogen was isolated in glacial refugia together with its host and/or that it has specialized on the plant genetic lineages. Using random forest approximate Bayesian computation (ABC‐RF), we found that the divergence history of the fungal lineages on S. nutans was congruent with that previously inferred for the host plant and probably occurred with ancient but no recent gene flow. Genome sequences confirmed the genetic structure and the absence of recent gene flow between fungal genetic lineages. Our analyses of individual host–pathogen pairs contribute to a better understanding of co‐evolutionary histories between hosts and pathogens in natural ecosystems, in which such studies remain scarce.     

A plant pathogen causes extensive mortality in an invasive insect herbivore

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Root volatiles in plant–plant interactions II: Root volatiles alter root chemistry and plant–herbivore interactions of neighbouring plants

Volatile organic compounds (VOCs) emitted by plant roots can influence the germination and growth of neighbouring plants. However, little is known about the effects of root VOCs on plant–herbivore interactions of neighbouring plants. The spotted knapweed (Centaurea stoebe) constitutively releases high amounts of sesquiterpenes into the rhizosphere. Here, we examine the impact of C. stoebe root VOCs on the primary and secondary metabolites of sympatric Taraxacum officinale plants and the resulting plant‐mediated effects on a generalist root herbivore, the white grub Melolontha melolontha. We show that exposure of T. officinale to C.stoebe root VOCs does not affect the accumulation of defensive secondary metabolites but modulates carbohydrate and total protein levels in T. officinale roots. Furthermore, VOC exposure increases M. melolontha growth on T. officinale plants. Exposure of T. officinale to a major C. stoebe root VOC, the sesquiterpene (E)‐β‐caryophyllene, partially mimics the effect of the full root VOC blend on M. melolontha growth. Thus, releasing root VOCs can modify plant–herbivore interactions of neighbouring plants. The release of VOCs to increase the susceptibility of other plants may be a form of plant offense.     

Priority resource access mediates competitive intensity between an invasive weevil and native floral herbivores

Mechanisms underlying invasive species impacts remain incompletely understood. We tested the hypothesis that priority resource access by an invasive biocontrol weevil, Rhinocyllus conicus, intensifies and alters the outcome of competition with native floral herbivores over flower head resources of the non-target, native host plant Cirsium canescens, specifically with the predominant, synchronous tephritid fly Paracantha culta. Four main results emerged. First, we documented strong, asymmetric competition, with R. conicus out-competing P. culta. Second, weevil priority access to floral resources accelerated competitive suppression of P. culta. Evidence for competitive suppression with increased weevil priority included decreases in both the numbers and the total biomass of native flies, plus decreases in individual P. culta fly mass and, so, potential fitness. Third, we found evidence for three concurrent mechanisms underlying the competitive suppression of P. culta by R. conicus. Prior use of a flower head by R. conicus interfered with P. culta pre-oviposition behavior. Once oviposition occurred, the weevil also reduced fly post-oviposition performance. Preemptive resource exploitation occurred, shown by the significant effect of flower head size on the total number of insects developing and in the magnitude of R. conicus effects on P. culta. Interference also occurred, shown by a spatial shift of surviving P. culta individuals away from the preferred receptacle resources as R. conicus priority increased. Finally, fourth, using an individual-based model (IBM), we found that the competitive interactions documented have the potential for imposing demographic consequences, causing a reduction in P. culta population sizes. Thus, priority resource access by an invasive insect increased competitive impact on the predominant native insect in the invaded floral guild. This study also provides the first experimental evidence for non-target effects of a weed biological control agent on an associated native insect herbivore.     

Preference–performance relationship in the gall midge Rabdophaga rosaria: insights from a common‐garden experiment with nine willow clones

1. Oviposition preferences of herbivorous insects are predicted to match offspring performance on different host taxa or on conspecific host genotypes. In gall‐inducing insects, host‐plant properties such as growth rate and gall size, which are determined by plant genotype and growing conditions, may have a significant impact on offspring performance and, hence, should influence oviposition site selection. 2. The present study investigated host preference of the European rosette willow gall midge Rabdophaga rosaria (Loew) in relation to offspring success on seven clones of Salix myrsinifolia Salisb. and two naturally hybridised S. myrsinifolia × phylicifolia L. clones growing in a replicated design in an experimental field under two fertilisation regimes. For each clone, the average growth rate, number of shoot tips, and leaf and gall size were determined, and their effects on midge preference and larval survival were examined. 3. Main shoot height, number of shoot tips, and gall size were significantly affected by clone. The midges clearly preferred certain clones over the others, but preferences were not related to willow growth traits or to gall size. Survival probability was higher in large than in small galls, but females did not prefer large‐leaved clones that produced the biggest rosette galls. Midge oviposition was also uncorrelated with prior rates of leaf‐rust infection and with feeding preferences of voles and folivorous insects. 4. The weak preference–performance relationship of R. rosaria within S. myrsinifolia is probably explained by evolutionary constraints that prevent generalist insects from achieving an ability to discriminate among conspecific hosts of variable quality.     

What shapes local density? The importance of migration rates and local growth for density‐patch size relationships in two Cionus weevils

1. The relative effect of migration and local growth on the spatio‐temporal density‐distribution of two co‐existing herbivorous weevils, Cionus scrophulariae L. and C. tuberculosus Scop., in 32 host plant Scrophularia nodosa L. patches of varying sizes was investigated. 2. Predictions of the temporal development of the slope in the density‐patch size relationships were derived from a basic population model with scale‐dependent migration rates. The model indicated that the slopes in the density‐patch size relationships during the early season should be reflected by the net scaling of immigration and emigration rates, whereas the slopes during the later season should increase as a result of local growth. 3. Emigration rates of the weevils were estimated in a field experiment, were the weevils coexisted in space and time. These results were then combined with a previous estimate of immigration rates in order to determine the net scaling of migration rates. 4. The emigration rate differed between species, caused by different movement rates in small patches, which could explain differences in the general slope of the density‐patch size relationships of the weevils in the natural figwort patches throughout the summer. The slopes in the relationships in the early season were largely predicted by the net scaling of migration rates. The slope also increased in the later season for C. tuberculosus, whereas the slope decreased for C. scrophulariae. 5. It was concluded that the understanding of both inter‐ and intra‐specific variations in density‐patch size relationships of insect herbivores can be improved using population models incorporating scale‐dependent migration and local growth.     

Sex‐related spatial patterns of Poa ligularis in relation to shrub patch occurrence in northern Patagonia

Abstract. Poa ligularis is a dioecious species and a valuable forage plant which is widespread in the arid steppe of northern Patagonia (Argentina). The vegetation in these areas consists of a system of perennial plant patches alternating with bare soil areas defining contrasting micro‐environments. We hypothesized that (1) male and female individuals of P. ligularis are spatially segregated in different micro‐environments, (2) the intensity of spatial segregation of sexes depends on plant structure and (3) spatial segregation of sexes is enhanced by competitive interactions between the sexes within the vegetation patches. We analysed the spatial distribution of female and male individuals in relation to the spatial pattern of vegetation in two areas differing in their vegetation structure. The location of P. ligularis within patches where either male, female or both sexes occurred was also analysed. The results indicate that different patterns of spatial distribution of sexes of P. ligularis may be found at the community level depending on the dominant life forms and geometric structure of plant patches. Where patches are of a lower height, with a high internal patch cover, individuals of both sexes are concentrated within patch canopies. In sites characterized by large, tall patches and less internal patch cover suitable microsites for female and male P. ligularis occur both within and outside the patch with males located at further distances from the patch edge. Where the patch is large and tall enough to allow the establishment of males and females at relatively high numbers, males occupy the patch periphery or even colonize the interpatch bare soil. These spatial patterns are consistent with selective traits in which females better tolerate intraspecific competition than males, while males tolerate wider fluctuations in the physical environment (soil moisture, nitrogen availability, wind intensity, etc.).     

Interpatch movement of the red milkweed beetle,Tetraopes tetraophthalmus: individual responses to patch size and isolation

Individual movement patterns and the effects of host plant patch size and isolation on patch occupancy were examined for red milkweed beetles, Tetraopes tetraophthalmus, residing in a heterogeneous landscape. Male beetles were found to move both more often and farther between host plant patches than female beetles, and this difference affected the patterns of patch occupancy observed. Overall, unoccupied milkweed patches were smaller and more isolated than patches occupied by beetles. Patches uninhabited by females tended to be more isolated, but not necessarily smaller, than patches with female beetles, indicating that females may be affected more by patch isolation than patch size. Presence of male beetles on patches showed a stronger response to patch size than to patch isolation. Differences in movement between males and females illustrate the need for demographically based dispersal data. Comparisons of Tetraopes interpatch movement patterns between landscapes composed of patches of different size revealed that landscapes with overall smaller patches may have greater rates of interpatch movement.     

Comparison of invertebrate herbivores on native and non‐native Senecio species: Implications for the enemy release hypothesis

The enemy release hypothesis posits that non‐native plant species may gain a competitive advantage over their native counterparts because they are liberated from co‐evolved natural enemies from their native area. The phylogenetic relationship between a non‐native plant and the native community may be important for understanding the success of some non‐native plants, because host switching by insect herbivores is more likely to occur between closely related species. We tested the enemy release hypothesis by comparing leaf damage and herbivorous insect assemblages on the invasive species Senecio madagascariensis Poir. to that on nine congeneric species, of which five are native to the study area, and four are non‐native but considered non‐invasive. Non‐native species had less leaf damage than natives overall, but we found no significant differences in the abundance, richness and Shannon diversity of herbivores between native and non‐native Senecio L. species. The herbivore assemblage and percentage abundance of herbivore guilds differed among all Senecio species, but patterns were not related to whether the species was native or not. Species‐level differences indicate that S. madagascariensis may have a greater proportion of generalist insect damage (represented by phytophagous leaf chewers) than the other Senecio species. Within a plant genus, escape from natural enemies may not be a sufficient explanation for why some non‐native species become more invasive than others.