Article

Do Options Matter? Settling Behavior, Stylet Sheath Counts, and Oviposition of Aster Leafhoppers (Hemiptera: Cicadellidae) in Two-Choice Bioassays

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Abstract

Polyphagous insects are characterized by a broad diet comprising plant species from different taxonomic groups. Within these insects, migratory species are of particular interest, given that they encounter unpredictable environments, with abrupt spatial and temporal changes in plant availability and density. Aster leafhoppers (Hemiptera: Cicadellidae: Macrosteles quadrilineatus Forbes) arrive in the Canadian Prairies in spring and early summer and are the main vector of a prokaryotic plant pathogen known as Aster Yellows Phytoplasma (AYp) (Candidatus Phytoplasma asteris). Host choice selection behavior of Aster leafhoppers was evaluated through two-choice bioassays, using domesticated and wild plants species commonly found in the Canadian Prairies. Leaf tissues from these plants were collected and stained to quantify the number of stylet sheaths and eggs. To assess possible effects due to insect infection, two-choice bioassays were repeated using leafhoppers infected with AYp and a subset of plant species. When two domesticated or wild plant species were presented together, similar numbers of uninfected Aster leafhoppers were observed on both plant species in most combinations. In domesticated-wild plant bioassays, uninfected Aster leafhoppers preferred to settle on the domesticated species. There was little to no association between settling preferences and stylet sheath and egg counts. These findings provide a better understanding of AY epidemiology and suggest that after domesticated species germination, leafhoppers could move from nearby wild plants into the preferred cereals (Poales: Poaceae) to settle on them, influencing the risk of AYp infection in some of these species.

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... However, polyphagous insects may exhibit different oviposition behavior depending on plant communities they encounter and their host preference. For example, Romero et al. [35] found out that aster leafhoppers (Macrosteles quadrilineatus Forbes) (Hemiptera: Cicadellidae) showed different oviposition behavior when offered multiple host plant species compared to when only one host plant species was given. In a no-choice experiment, no eggs were oviposited on canola by aster leafhoppers [36], but a similar number of eggs was observed on canola as on other more suitable host plant species in a choice experiment. ...
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A technique that allows the staining and observation of whole leafhopper salivary sheaths left behind in plant tissue is described. A derivation of McBride's acid fuchsin stain for fungal hyphae was used, followed by clearing of the plant tissue for observation under a dissecting microscope. Data gathered included the shape and location of sheaths, and the orientation of sheaths to veins. The technique simultaneously stained eggs present ill the plant tissue. Thus, measures of both feeding and oviposition were acquired from the same plant. This method is reliable in recovering known sheaths and is applicable to several hemipteran sheath-feeders. The method should be useful to researchers studying host plant resistance and other aspects of the feeding of haustellate insects.
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The interaction of plasticity, choice, and diet breadth is discussed, and the problems faced by generalists highlighted. The case is made that insects with very broad diets, and hence an ability to choose among foods at some time in their life history, have a problem making decisions that are efficient in terms of speed and quality. Reinterpretations of data from some insect groups, such as flower visitors, are presented in the light of such a neural problem, and explicit tests of the concept that there are neural constraints are reviewed. Possible mechanisms underlying changes in diet breadth are discussed.
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Pathogens and parasites can induce changes in host or vector behavior that enhance their transmission. In plant systems, such effects are largely restricted to vectors, because they are mobile and may exhibit preferences dependent upon plant host infection status. Here we report the first evidence that acquisition of a plant virus directly alters host selection behavior by its insect vector. We show that the aphid Rhopalosiphum padi, after acquiring Barley yellow dwarf virus (BYDV) during in vitro feeding, prefers noninfected wheat plants, while noninfective aphids also fed in vitro prefer BYDV-infected plants. This behavioral change should promote pathogen spread since noninfective vector preference for infected plants will promote acquisition, while infective vector preference for noninfected hosts will promote transmission. We propose the "Vector Manipulation Hypothesis" to explain the evolution of strategies in plant pathogens to enhance their spread to new hosts. Our findings have implications for disease and vector management.
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Plant domestication and agronomic selection for increased yield may have an associated effect of reducing plant defence against herbivorous insects. This hypothesis is based on evidence for a metabolic cost associated with defence, and on evidence that increases in yield generally come from the re-partitioning of photoassimilates rather than from fundamental increases in photosynthetic rates. We propose that for plants in which domestication and crop development constitute strong selection for increased growth and reproduction, reallocation of resources may result in lower defence against insects. We examine this hypothesis by means of comparative studies of growth, reproduction and resistance in a complex of maizes and closely related wild taxa, the teosintes. The results of these studies are consistent with assumptions of differential investment in growth and reproduction between wild and domesticated plants. A wild perennial grew slowest and had lowest grain production, while a modern cultivar grew fastest and had the highest grain yield. A wild annual and a land-race cultivar were intermediate. Damage from a diverse assemblage of folivorous insects, and from a specialist stemboring lepidopteran larva, fit the defence predictions closely. A gradient of attack levels suggests that the wild perennial is most defended, followed in descending order by the wild annual, the land-race cultivar and the modern high-yielding variety. Alternative hypotheses for this pattern are consistent with some, but not all, of our data.
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Vector-borne plant pathogenic bacteria can induce changes in infected plants favoring the insect vector behavior and biology. The study aimed to determine the effect of maize bushy stunt phytoplasma (MBSP) postinoculation period on the host plant preference and transmission efficiency by the corn leafhopper, Dalbulus maidis DeLong & Wolcott, 1923 (Hemiptera: Cicadellidae). In a series of choice tests, D. maidis preference was measured as settling and oviposition on healthy maize plants versus infected maize plants showing early disease symptoms, advanced symptoms, or no symptoms. Finally, transmission efficiency of D. maidis was measured when the vector previously acquired the phytoplasma from asymptomatic source plants at different postinoculation periods. D. maidis adults preferred to settle and to oviposit on healthy than on symptomatic infected plants with advanced disease symptoms, and preferred asymptomatic plants over symptomatic ones. MBSP transmission by D. maidis was positively correlated with the postinoculation period of the source plant. Results suggest an MBSP modulation for D. maidis preference on asymptomatic infected maize plants in the early stages of the crop, allowing the pathogen an undetected transmission.
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We hypothesize that the evolution of an ecologically important character, the host associations of specialized phytophagous insects, has been influenced by limitations on genetic variation. Using as a historical framework a phylogenetic reconstruction of the history of host associations in the beetle genus Ophraella (Chrysomelidae), we have employed quantitative-genetic methods to screen four species for genetic variation in larval survival, oviposition (in one species only), and feeding responses to their congeners' host plants, in the Asteraceae. We here report results of studies of one species and evaluate the results from all four. Analysis of half-sib/full-sib families and of progenies of wild females of O. notulata, a specialist on Iva (Ambrosiinae), provided evidence of genetic variation in larval consumption of five of six test plants and in adult consumption of four of six. Larval mortality was complete on five plants; only on Ambrosia, a close relative of the natural host, was there appreciable, and genetically variable, survival. Oviposition on Ambrosia showed marginally significant evidence of genetic variation; a more distantly related plant elicited no oviposition at all. In compiling results from four Ophraella species, reported in this and two other papers, we found no evidence of genetic variation in 18 of 39 tests of feeding responses and 14 of 16 tests of larval survival on congeners' hosts. This result is consistent with the hypothesis that absence or paucity of genetic variation may constrain or at least bias the evolution of host associations. The lower incidence of genetic variation in survival than in feeding behavior may imply, according to recent models, that avoidance is a more common evolutionary response to novel plants than adaptation. The usually great disparity between mean performance on congeners' hosts and the species' natural hosts, and an almost complete lack of evidence for negative genetic correlations, argue against the likelihood that speciation has occurred by sympatric host shift. The presence versus apparent absence of genetic variation in consumption was correlated with the propinquity of relationship between the beetle species tested and the species that normally feeds on the test plant, suggesting that the history of host shifts in Ophraella has been guided in part by restrictions on genetic variation. It was also correlated with the propinquity of relationship between a test plant and the beetle's natural host. The contributions of plant relationships and insect relationships, themselves correlated in part, to the pattern of genetic variation, are not readily distinguishable, but together accord with phylogenetic evidence that these and other phytophagous insects adapt most readily to related plants. In this instance, therefore, the macroevolution of an ecologically important character appears to have been influenced by genetic constraints. We hypothesize that absence of the structural prerequisites for genetic variation in complex characters may affect genetic variation and the trajectory of evolution.
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Models and empirical studies on host selection in plant-insect, algae-amphipod, host-parasite and prey-predator systems assume that oviposition preference is determined by the quality of the oviposition site for offspring development. According to the oviposition-preference-offspring-performance hypothesis, oviposition-preference hierarchy should correspond to host suitability for offspring development because females maximize their fitness by optimizing offspring performance. We show, we believe for the first time, that adult feeding site and related adult performance may explain most of the variation in adult feeding and oviposition site selection of an oligophagous grass miner, Chromatomyia nigra (Diptera). This study advances our understanding of the complex interactions between plants and herbivores because it shows that bust-preference patterns are not only shaped by the optimization of offspring performance, as previously assumed, but also by the optimization of adult performance.
Article
1. Enemy-free space (EFS) was defined by Jeffries & Lawton (1984) as 'ways of living that reduce or eliminate a species' vulnerability to one or more species of natural enemies'. EFS has emerged in the literature as a significant niche-moulding factor. However, the lack of consistency among the empirical studies as to how EFS should be defined, and what hypotheses should be tested in order to evaluate its relative importance, prompted us to review the literature and to propose a working definition that results in a general set of testable hypotheses. 2. To test the relative importance of EFS in structuring the communities of organisms, we propose a set of three falsifiable null hypotheses that must be tested sequentially and rejected. Ho1: The fitness of the organism in an original habit (e.g. on an original host plant) in the presence of natural enemies is equal to the fitness of the organism in that habit in the absence of natural enemies. Acceptance of the alternative hypothesis that the fitness of the organism in the presence of natural enemies is less than in the absence of natural enemies is necessary to demonstrate the importance of natural enemies. Ho2: The fitness of the organism in an alternative habit with natural enemies is equal to the fitness of the organism in the original habit with natural enemies. Acceptance of the alternative hypothesis that the fitness of the organism in the alternative habit with natural enemies is greater than that in the original habit with natural enemies is necessary to demonstrate that the alternative habit provides EFS. Ho3: The fitness of the organism in an alternative habit without natural enemies equals the fitness of the organism in the original habit without natural enemies. Acceptance of the alternative hypothesis that the fitness of the organism in an alternative habit without natural enemies is less than in the original habit without natural enemies is necessary to demonstrate the relative importance of EFS compared with other co-occurring niche-moulding factors such as competition or host nutritional quality. 3. We searched the literature and evaluated fifty-three references (nineteen references to seventeen different terrestrial systems and thirty-four references to twenty-four different freshwater systems) to test our hypotheses. 4. Of the forty-one systems examined, nineteen (46%) tested only for differences in vulnerability of the prey or host species between EFS and non-EFS options (our Ho2); sixteen (39%) tested for the importance of natural enemies and the effectiveness of the alternative habit in providing EFS (our Ho1 and Ho2); and only ten systems (24%) tested for Ho1, Ho2 and the relative importance of EFS in the system as' measured by fitness (our Ho3). 5. Of the systems that tested for EFS, sixteen of nineteen (84%), thirteen of sixteen (81%) and seven of ten (70%) showed evidence in support of the existence of EFS according to hypothesis Ho2 only, hypotheses Ho1 and Ho2, and our three working hypotheses, respectively. 6. These results indicate that very few studies have actually tested for the existence of EFS. Nevertheless, results from this Limited number of natural systems suggest that EFS may be important in moulding the niches of arthropods. Because of the large number of claims for EFS in systems where none of the basic hypotheses were investigated, we suggest that authors test for EFS experimentally, be judicious in selecting articles to cite in support of EFS, and exert care in attributing it as a selective force in the evolution of arthropods in specific systems.
Article
Natural enemies have been proposed as important agents of natural selection on herbivorous insects that may facilitate host plant shifts and increases in diet breadth. However, there is little experimental field work to support claims of host-shifting via escape from natural enemies, i.e., to enemy-free space. In this study, we took the unique approach of experimentally creating a host shift for a specialized leafmining fly, Liriomyza helianthi (Diptera: Agromyzidae). We manually transferred leafminer larvae from their normal host plant, Helianthus annuus (Asteraceae), to a variety of novel plants: Helianthus maximilianii, Ambrosia artemisiifolia, Taraxacum officinale, and Centaurea solstitialis. By exposing transferred larvae on normal and novel plants to natural enemy attack under field conditions, we were able to examine whether host-shifting can provide an herbivore with enemy-free space. Our data show that enemy-free space does exist for L. helianthi immediately following a host shift, as mortality in novel plants averaged 17% less than in the normal host. Nevertheless, there was significant within- and between-year heterogeneity in results over the 3-yr period of the study. We found that escape from natural enemies was related to annual variation in the diversity and abundance of parasitoid species. In years when parasitoid assemblages were dominated by endoparasitoids, mortality of larvae averaged 22% lower in novel hosts. However, when generalist ectoparasitoids, Diglyphus spp. (Hymenoptera: Eulophidae), were present, there was no advantage of developing in novel plants, a result that could be explained by the less discriminating nature of the ectoparasitoids. When overall levels of mortality from natural enemies were high, the benefit of novel plants was also reduced. This pattern suggests that, as available larval hosts become scarce, parasitoids may be more likely to forage on novel host plants in search for prey, thus diminishing the opportunity for enemy-free space. Nevertheless, our study showed that enemy-free space can exist for an herbivorous insect utilizing a novel host plant, and that natural enemies may, in some cases, offset physiological fitness costs often associated with developing in novel plants. If all else is equal, the balance of these factors may facilitate the inclusion of novel host plants into the feeding repertoire of an herbivore.
Chapter
This chapter presents a phytoplasma taxonomic scheme that is based on the analysis of two evolutionary markers: the 16S ribosomal ribonucleic acid (rRNA) gene and the spacer region that separates the 16S from the 23S rRNA genes. This chapter provides an outline of the procedures that are available for phylogenetically classifying an unknown phytoplasma strain. Information presented in this chapter includes phytoplasma phylogenetic relationships based on restriction fragment length polymorphisms (RFLP) analysis and sequence analysis of the 16/23S spacer regions. Phylogenetic analyses have provided a coherent framework for the classification of diverse taxa, including the Mollicutes. Another very attractive aspect of this type of analysis is the ability to analyze these phylogenetic markers from non-culturable prokaryotes, such as plant pathogenic mycoplasma-like organisms (MLOs). There are advantages and disadvantages associated with both RFLP and sequence analysis of rDNA for classifying phytoplasmas. RFLP analysis of the polymerase chain reaction (PCR)-amplified 16S rRNA gene is a rapid method to assess the potential affinity of an unknown phytoplasma.
Data
Plant anti-herbivore defenses are known to be affected by life-history evolution, as well as by domesti-cation and breeding in the case of crop species. A suite of plants from the maize genus Zea (Poaceae) and the specialist herbivore Dalbulus maidis (DeLong & Wolcott) (Hemiptera: Cicadellidae) were used to test the hypothesis that anti-herbivore defenses are affected by plant life-history evolution and human intervention through domestication and breeding for high yield. The suite of plants included a maize (Zea mays ssp. mays L.) commercial hybrid, a maize landrace, two populations of the annual Balsas teosinte (Z. mays ssp. parviglumis Iltis & Doebley), and perennial teosinte (Z. di-ploperennis Iltis, Doebley & Guzman). Leaf toughness, pubescence, and oviposition preference were compared among the suite of host plants looking for effects of transitions in life history (i.e., from perennial to annual life cycle), domestication (i.e., from wild annual to domesticated annual), and breeding (i.e., from landrace to hybrid maize) on defense against D. maidis. Results on leaf toughness suggested that the life-history and domestication transitions weakened the plant's resistance to pene-tration by the mouthparts and ovipositor of D. maidis, whereas results on pubescence suggested that this putative defense was strengthened with the breeding transition, contrary to expectations. Results on oviposition preference of D. maidis coincided with the expectation that life-history and domesti-cation transitions would lead to preference for Balsas teosinte over perennial teosinte, and of landrace maize over Balsas teosinte. Also, a negative correlation suggested that oviposition preference is signif-icantly influenced by leaf toughness. Overall, the results suggested that Zea defenses against the spe-cialist herbivore D. maidis were variably affected by plant life-history evolution, domestication, and breeding, and that chemical defense may play a role in Zea defense against D. maidis because leaf toughness and pubescence only partially explained its host preferences.
Article
(1) The holly leaf-miner (Phytomyza ilicis) is a monophagous leaf-mining agromyzid that attacks holly (Ilex aquifolium). (2) Twenty-five individual holly bushes maintained six- to eightfold differences in levels of infestation over 3 years (three generations of mines), despite being in close proximity. Differences in levels of infestation were due to differential oviposition by adult flies. (3) Infestation levels were negatively correlated (nominal P < 0.05) with nitrogen levels in young foliage at the time of oviposition in one generation of flies (1988); however, this result is difficult to interpret because several significance tests were carried out. No other characteristics of the bushes were correlated with levels of infestation. (4) Contrary to expectation, larval performance and host-plant selection by adult female were not correlated. There were no differences in mine size, larval growth rate, or size of adult females at emergence on heavily infested or lightly infested bushes. Percentage successful emergence was low, and did not differ significantly with mine density. (5) The heaviest source of miner mortality (miscellaneous larval deaths) was density-independent. Mortality of larvae surviving miscellaneous larval deaths was significantly spatially density dependent (revealed by k-factor analysis), due to parasitism by Chrysocharis gemma. Spatially density-dependent larval parasitism, and miscellaneous larval deaths were inversely correlated. (6) We discuss these results in the context of the ideal free distribution (Fretwell 1972), speculating that high densities of animals reduce the quality of intrinsically superior habitat patches via spatially density-dependent larval parasitism, leading to equal miner performance on all bushes.
Article
Effects of host and nonhost substrates on probing and preoviposition activity were monitored for adult summerform pear psylla, Cacopsylla pyricola Foerster. Tendency to initiate probing activity during a 15-min observation period was similar on a susceptible pear (Pyrus communis L. `Bartlett'), a presumed nonsusceptible pear (P. calleryana Decne.), and apple, but was reduced on quince. Depriving pear psylla access to pear for 4-6 h or 14-16 h before the observation period increased the percentage of psylla that probed during the observation period; effects of host deprivation on tendency to probe were independent of substrate. Tendency to initiate probing on P. communis was greater on lower leaf surfaces than upper leaf surfaces, which may suggest that probing is initiated in response to cues received at the leaf surface. The percentage of pear psylla that probed during the observation period was not affected by age of P. communis foliage. Mean probe duration and mean percentage of the 15-min observation period spent probing were unaffected by substrate; both measures increased linearly with amount of time that pear psylla were deprived of pear before the observation period. Mean percentage of the observation period spent in contact with the leaf surface was higher for P. communis and P. calleryana than non-Pyrus species. Despite extensive probing activity on apple, very little preoviposition behavior was noted; similarly, age of P. communis foliage did not affect tendency to initiate probing behavior, but did affect preoviposition activity. These two results suggest that probing and preoviposition activities were released by different cues.
Article
During the years 1958-1962 studies were conducted to determine the origin of populations of the six-spotted leafhopper, Macrosteles fascifrons (Stal) in a 100-squaremile area in Anoka County, Minnesota, Two sources of infestation were found: eggs produced locally, and adults entering from outlying areas, Some eggs laid in the fall overwintered, while those produced in spring and summer had short incubation periods. In 1961 there was evidence of 3 complete generations and a partial fourth. The host plants on which the leafhopper was found to reproduce arc listed, It is suggested that dispersal flights arc primarily long distance, covering large areas at a time. Local dispersal flights are probably of secondary importance. Leafhoppers originating outside the study area appeared to be more important as initial sources of inoculum than populations produced locally.
Article
Settling of male and female potato leafhoppers, Empoasca fabae (Harris), were compared over a diel regime (16:8 [L:D] h) on alfalfa, Medicago sativa L. cultivar `Ranger', in the laboratory. On average, >80% of the females and 50% of the males were recorded on stems. Leaves were used more frequently by males than by females, but little use was made of the upper leaf surface by either sex. The proportions of both males and females on stems declined over time, whereas over time the proportion of males increased on leaves. Based on stem and leaf surface area available to leafhoppers, both sexes preferred stems and avoided leaves. Scotophase did not appear to alter the dynamics of leafhopper distribution. Females more frequently caused expression of injury symptoms in the alfalfa stems than did males in a 36-h access period. Thus, use of females in electronic monitoring and other feeding studies of adults for assessing injury to plants is supported for studies that are 36 h or shorter and where gender is not germane.
Article
We analyzed the probing (stylet penetration) behaviors of the sharpshooter leafhopper Graphocephala atropunctata (Signoret) on grape with an alternating current (AC) electrical penetration graph (EPG) monitor. We characterized waveforms likely to represent stylet penetration pathway phase and xylem ingestion. The total probing duration of the cohort represented 68% of all 20-h monitoring periods for all insects, yet only a small proportion of that probing time was spent in high amplitude/pathway activities. Few changes in behavior occurred once a probe had started. This was shown by the low number of waveform events (i.e., uninterrupted occurrences of a behavior) per probe for each waveform type, which varied from a mean of 1–2.43. Conditional probability analysis supported that hypothesis, because insects usually terminated a probe and began a new one after ingestion-related events, rather than repeating in the same probe the previously performed waveforms. The size of grape leaves used for the assays directly influenced the amount of time insects ingested from xylem or performed other low-amplitude waveforms. Information from this work establishes benchmarks for future research addressing the mechanisms of Xylella fastidiosa Wells et al. transmission and sharpshooter ecology.
Article
Abstract 1. Few entomological studies include soil-dwelling insects in mainstream ecological theory, for example the preference–performance debate. The preference–performance hypothesis predicts that when insect herbivores have offspring with limited capacity to relocate in relation to a host plant, there is a strong selection pressure for the adult to oviposit on plants that will maximise offspring performance. 2. This paper discusses the proposition that insect herbivores that live above ground, but have soil-dwelling offspring, should be included in the preference–performance debate. Twelve relevant studies were reviewed to assess the potential for including soil insects in this framework, before presenting a preliminary case study using the clover root weevil (Sitona lepidus) and its host plant, white clover (Trifolium repens). 3. Maternal S. lepidus preferentially oviposited on T. repens plants that had rhizobial root nodules (which enhance offspring performance) rather than T. repens plants without nodules, despite plants having similar foliar nutritional quality. This suggests that adult behaviour above ground was influenced by below-ground host-plant quality. 4. A conceptual model is presented to describe how information about the suitability for offspring below ground could underpin oviposition behaviour of parental insects living above ground, via plant- and soil-mediated semiochemicals. These interactions between genetically related, but spatially separated, insect herbivores raise important evolutionary questions such as how induced plant responses above ground affect offspring living below ground and vice versa.
Article
. 1Enemy-free space (EFS) was defined by Jeffries & Lawton (1984) as ‘ways of living that reduce or eliminate a species’ vulnerability to one or more species of natural enemies’. EFS has emerged in the literature as a significant niche-moulding factor. However, the lack of consistency among the empirical studies as to how EFS should be defined, and what hypotheses should be tested in order to evaluate its relative importance, prompted us to review the literature and to propose a working definition that results in a general set of testable hypotheses.2To test the relative importance of EFS in structuring the communities of organisms, we propose a set of three falsifiable null hypotheses that must be tested sequentially and rejected. Ho1: The fitness of the organism in an original habit (e.g. on an original host plant) in the presence of natural enemies is equal to the fitness of the organism in that habit in the absence of natural enemies. Acceptance of the alternative hypothesis that the fitness of the organism in the presence of natural enemies is less than in the absence of natural enemies is necessary to demonstrate the importance of natural enemies. Ho2: The fitness of the organism in an alternative habit with natural enemies is equal to the fitness of the organism in the original habit with natural enemies. Acceptance of the alternative hypothesis that the fitness of the organism in the alternative habit with natural enemies is greater than that in the original habit with natural enemies is necessary to demonstrate that the alternative habit provides EFS. Ho3: The fitness of the organism in an alternative habit without natural enemies equals the fitness of the organism in the original habit without natural enemies. Acceptance of the alternative hypothesis that the fitness of the organism in an alternative habit without natural enemies is less than in the original habit without natural enemies is necessary to demonstrate the relative importance of EFS compared with other co-occurring niche-moulding factors such as competition or host nutritional quality.3We searched the literature and evaluated fifty-three references (nineteen references to seventeen different terrestrial systems and thirty-four references to twenty-four different freshwater systems) to test our hypotheses.4Of the forty-one systems examined, nineteen (46%) tested only for differences in vulnerability of the prey or host species between EFS and non-EFS options (our Ho2); sixteen (39%) tested for the importance of natural enemies and the effectiveness of the alternative habit in providing EFS (our Ho1 and Ho2); and only ten systems (24%) tested for Ho1, Ho2 and the relative importance of EFS in the system as measured by fitness (our Ho3).5Of the systems that tested for EFS, sixteen of nineteen (84%), thirteen of sixteen (81%) and seven of ten (70%) showed evidence in support of the existence of EFS according to hypothesis Ho2 only, hypotheses Ho1 and Ho2, and our three working hypotheses, respectively.6These results indicate that very few studies have actually tested for the existence of EFS. Nevertheless, results from this limited number of natural systems suggest that EFS may be important in moulding the niches of arthropods. Because of the large number of claims for EFS in systems where none of the basic hypotheses were investigated, we suggest that authors test for EFS experimentally, be judicious in selecting articles to cite in support of EFS, and exert care in attributing it as a selective force in the evolution of arthropods in specific systems.
Article
Source areas for Six-Spotted leafhopper migrations into Manitoba are determined by wind trajectory studies and are found to be mainly in South Dakota, Nebraska and Kansas. There is a high enough frequency of southerly winds for the insects to migrate into Manitoba every spring. Attempts to correlate general weather conditions, both in southern Manitoba and in the source areas,with lettuce crop damage in Manitoba, were inconclusive.This leads to the conclusion that the intensity of aster yellows infestation in Manitoba depends on the environment experienced by the lettuce crop. Favourable meteorological conditions occurring for a few days only, at just the right time, can produce heavy infection in any year.