Factors determining the host plant range of the phytophagous mite, Tetranychus urticae (Acari: Tetranychidae): a method for quantifying host plant acceptance

The host plant acceptance of the phytophagous mite Tetranychus urticae was experimentally quantified. Host plant acceptance is described as the proportion of adult females settling on the test plant on which they have been placed. On the other hand, the host plant suitability of T. urticae on different plant species is expressed as the mean number of eggs produced by the females within 5 days (hereafter 'fecundity'). An inbred T. urticae line was tested with regard to host plant acceptance and fecundity on 11 potential host plants. These two variables were positively correlated across host plants; host plant species on which the fecundity was low were also those on which females settled less readily compared to host plants with high fecundity. The characteristics of host plant acceptance of the T. urticae are discussed in light of their potential food resource under natural conditions.     

Parasite host range and the evolution of host resistance

Parasite host range plays a pivotal role in the evolution and ecology of hosts and the emergence of infectious disease. Although the factors that promote host range and the epidemiological consequences of variation in host range are relatively well characterized, the effect of parasite host range on host resistance evolution is less well understood. In this study, we tested the impact of parasite host range on host resistance evolution. To do so, we used the host bacterium Pseudomonas fluorescens SBW25 and a diverse suite of coevolved viral parasites (lytic bacteriophage Φ2) with variable host ranges (defined here as the number of host genotypes that can be infected) as our experimental model organisms. Our results show that resistance evolution to coevolved phages occurred at a much lower rate than to ancestral phage (approximately 50% vs. 100%), but the host range of coevolved phages did not influence the likelihood of resistance evolution. We also show that the host range of both single parasites and populations of parasites does not affect the breadth of the resulting resistance range in a naïve host but that hosts that evolve resistance to single parasites are more likely to resist other (genetically) more closely related parasites as a correlated response. These findings have important implications for our understanding of resistance evolution in natural populations of bacteria and viruses and other host–parasite combinations with similar underlying infection genetics, as well as the development of phage therapy.     

The evolution of parasite host range in heterogeneous host populations

Theory on the evolution of niche width argues that resource heterogeneity selects for niche breadth. For parasites, this theory predicts that parasite populations will evolve, or maintain, broader host ranges when selected in genetically diverse host populations relative to homogeneous host populations. To test this prediction, we selected the bacterial parasite Serratia marcescens to kill Caenorhabditis elegans in populations that were genetically heterogeneous (50% mix of two experimental genotypes) or homogeneous (100% of either genotype). After 20 rounds of selection, we compared the host range of selected parasites by measuring parasite fitness (i.e. virulence, the selected fitness trait) on the two focal host genotypes and on a novel host genotype. As predicted, heterogeneous host populations selected for parasites with a broader host range: these parasite populations gained or maintained virulence on all host genotypes. This result contrasted with selection in homogeneous populations of one host genotype. Here, host range contracted, with parasite populations gaining virulence on the focal host genotype and losing virulence on the novel host genotype. This pattern was not, however, repeated with selection in homogeneous populations of the second host genotype: these parasite populations did not gain virulence on the focal host genotype, nor did they lose virulence on the novel host genotype. Our results indicate that host heterogeneity can maintain broader host ranges in parasite populations. Individual host genotypes, however, vary in the degree to which they select for specialization in parasite populations.     

Trade-offs and the evolution of host specialization

Summary Trade-offs in performance on different hosts are thought to promote the evolution of host specificity by blocking host shifts. Yet, in contrast, most experiments using phytophagous insects have shown performance on alternative hosts to be uncorrelated or positively correlated. Recent quantitative genetic models based on mutation—selection balance indicate that underlying constraints on the simultaneous maximization of different components of fitness may not always generate negative genetic correlations. We suggest an alternative or additional explanation for the lack of observed negative genetic correlations. If performance is polygenically controlled and some performance loci possess only antagonistically pleiotropic alleles, then the expression of trade-offs in performance will vary over time in populations. Consequently, a trade-off will be seen only in populations that have adapted to two hosts and are at or close to genetic equilibrium. Therefore, studies testing performance on a novel as compared with a normal host will generally yield non-negative genetic correlations between performance on the two hosts. The results of published studies are consistent with the predictions of this hypothesis.     

Prey preference of the predatory mite Neoseiulus womersleyi Schicha is determined by spider mite webs

Abstract:  We experimentally verified a proximate cause for the preference of prey species by the predatory mite Neoseiulus womersleyi , which prefers the eggs of Amphitetranychus viennensis Zacher to those of Tetranychus urticae Koch. To separate the effects of spider mite eggs and spider mite webs on the prey preference of the predatory mite, we manipulated combinations of eggs and webs of the two prey species. The number of eggs consumed by adult female N. womersleyi was compared with respect to two factors: eggs of the same species under webs of the different species and eggs of different species under webs of the same species. The results revealed that the prey preference of N. womersleyi was determined by the webs, and not by the eggs.     

Effects of pesticides on spin down and webbing production by the two-spotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae)

Spin down in response to pesticides is a known phenomenon. This study aimed at investigating the effects of varying acaricide concentrations and acaricidal combinations on the rate of spin down and thread length, the fate of mites after spinning down and, thus, the implications of spin down in the context of mite control and the relationship between webbing and the efficacy of ovicides. In response to cyhexatin at a concentration of up to 100 mg dm-3 the rate of spin down did not change, but in response to higher concentrations it occurred earlier and ceased before the end of the experiment. The mean thread length did not vary with the concentration of acaricide and, whilst the proportion of mites spinning down increased with a cyhexatin concentration of up to 10 mg dm-3, it did not increase further with higher concentrations thereafter. It is therefore postulated that at any given time only a certain proportion of mites have a propensity to spin down. In combination with cyhexatin, only amitraz (at low concentrations) increased the proportion of mites spinning down, but amitraz (at higher concentrations), flubenzimine and permethrin all inhibited spin down, which may be a useful attribute in pesticide formulations. With regard to the fate of mites after spinning down, they were able to climb back up threads, but the proportion doing so was too small to be of practical importance. Mites which spin down and remain at the end of a thread die, but those which reached another leaf were able to spin down again and were more likely to do so if the leaf bore a cyhexatin residue. Mites reaching another leaf are also able to lay eggs, the number being greater if the leaf which they reach is untreated. Spin down is therefore likely to be of significant advantage to Tetranychus urticae in avoidance of the effects of acaricides. Finally, some eggs were observed to be laid on webbing and not to come into direct contact with clofentezine residues. The use of a fine water spray to simulate dew increased the ovitoxicity of clofentezine, probably by resuspension of the pesticide, but not of amitraz, which inhibits webbing production. The avoidance of acaricides should be added to the several known advantages of webbing production for spider mites. It is concluded that, all other factors being equal, pesticide formulations which inhibit spin down and webbing production are likely to be more effective in control programmes than those which stimulate such behaviour.     

Generalized selection to overcome innate immunity selects for host breadth in an RNA virus

Virus‐host coevolution has selected for generalized host defense against viruses, exemplified by interferon production/signaling and other innate immune function in eukaryotes such as humans. Although cell‐surface binding primarily limits virus infection success, generalized adaptation to counteract innate immunity across disparate hosts may contribute to RNA virus emergence potential. We examined this idea using vesicular stomatitis virus (VSV) populations previously evolved on strictly immune‐deficient (HeLa) cells, strictly immune competent (MDCK) cells, or on alternating deficient/competent cells. By measuring viral fitness in unselected human cancer cells of differing innate immunity, we confirmed that HeLa‐adapted populations were specialized for innate immune‐deficient hosts, whereas MDCK‐adapted populations were relatively more generalized for fitness on hosts of differing innate immune capacity and of different species origin. We also confirmed that HeLa‐evolved populations maintained fitness in immune‐deficient nonhuman primate cells. These results suggest that innate immunity is more prominent than host species in determining viral fitness at the host‐cell level. Finally, our prediction was inexact that selection on alternating deficient/competent hosts should produce innate viral generalists. Rather, fitness differences among alternating host‐evolved VSV populations indicated variable capacities to evade innate immunity. Our results suggest that the evolutionary history of innate immune selection can affect whether RNA viruses evolve greater host‐breadth.     

Development of two spotted spider mite (Acari: Tetranychidae) populations on strawberry and raspberry cultivars

Five strawberry (Fragaria sp.) and five raspberry (Rubus ideaus L.) cultivars were evaluated for resistance to two spotted spider mite (Tetranychus urticae Koch.). Two methods of assessing the development of two spotted mite populations using detached leaves were compared. The number of eggs laid and mites which developed were compared. The strawberry cvs Hapil and Pegasus had significantly greater development of two spotted mite populations than the cvs Rhapsody, Symphony and Elsanta. The raspberry cv. Joan Squires had higher populations of two spotted mite whilst the raspberry cv. Leo the least, when compared with cvs Glen Clova, Glen Moy and Glen Prosen. Differences were observed in oviposition sites and mite distribution when comparing raspberries with strawberries. The method of assessing the populations development of two spotted mite which involved maintaining the cut leaf stem in water may be of potential use for studying population dynamics of both two spotted mite and possible predators over extended periods of time.     

Acaricidal potential of some essential oils and their monoterpenoids against the two-spotted spider mite Tetranychus urticae (Koch.)

Abstract

The acaricidal potential of fourteen essential oils and fourteen of their major monoterpenoids were tested against the Two-spotted spider mite Tetranychus urticae. Two different time intervals of 24 and 48 hrs were used in evaluation. The assays after 24 hrs showed that mattercary, fennel, caraway, garlic, cinnamon, chenopodium and eucalyptus were highly potent. As for monoterpenoids, chlorothymol was found to be the most effective against T. urticae followed by thymol, carvacrol and cinnamaldehyde.     

Ecological factors driving the long-term evolution of influenza's host range

The evolution of a pathogen''s host range is shaped by the ecology of its hosts and by the physiological traits that determine host specificity. For many pathogen traits, there is a trade-off: a phenotype suitable for infecting one set of hosts poorly infects another. Introducing and analysing a simple evo-epidemiological model, here we study how such a trade-off is expected to affect evolution of the host ranges of influenza viruses. We examine a quantitative trait underlying host specificity, given by an influenza virus''s degree of adaptation to certain conformations of sialic acid receptors, and investigate how this receptor preference evolves in a minimal network of host species, including humans, that differ in life history and receptor physiology. Using adaptive dynamics theory, we establish thresholds in interspecific transmission rates and host population sizes that govern the emergence and persistence of human-adapted viruses. These ecological thresholds turn out to be largely independent of the strength of the evolutionary trade-off, underscoring the importance of ecological conditions in determining a disease''s host range.     

Deep phylogeographical structure and parallel host range evolution in the leaf beetle Agelasa nigriceps

To understand the mechanisms behind the diversification of herbivorous insects through insect–plant interactions, it is important to know how the insects change their diet breadth in response to environmental changes. In this study, we investigated the phylogeographical pattern of the leaf beetle Agelasa nigriceps to infer the evolutionary history of its host range. While this beetle commonly uses Actinidia arguta (Actinidiaceae) as a host plant, it has been recorded recently on Pterostyrax hispidus (Styracaceae), which is now increasing in abundance at some localities in Japan due to the indirect effects of high population size of a mammalian herbivore. Considerable variation among populations in the ability of Ag. nigriceps to use P. hispidus suggests that P. hispidus is a newly acquired host plant for this beetle. Phylogenetic analyses using mitochondrial DNA sequences and amplified fragment length polymorphism (AFLP) revealed a high degree of phylogeographical structure in Ag. nigriceps throughout Japan, which is consistent with the hypothesis that several glacial refugia existed in the Japanese archipelago. In contrast, no genetic structure associated with the host plants was detected. Both the mitochondrial DNA and AFLP analyses showed that populations that can use P. hispidus are polyphyletic. These results and geographical variation in host use suggest that the host range expansion to a novel host, P. hispidus, is a very recent and possibly ongoing phenomenon and has occurred independently in several regions. Our study illustrates that the host range of herbivorous insects can evolve repeatedly in response to similar environmental changes.     

Adaptive learning of host preference in a herbivorous arthropod

Although many publications deal with the effects of experience on behaviour, adaptive learning (i.e. behavioural change with experience resulting in improved reproductive success) is poorly documented. We present direct evidence that learning of host preference improves fitness in the herbivorous mite, Tetranychus urticae . Individual mites from two strains were repeatedly given a choice between two host plants, tomato and cucumber, and then subjected to a performance test on each. For both strains, food experience affected the subsequent choice: individual mites learned to prefer cucumber over tomato. The performance test showed this effect to be adaptive, as the food plant the mites learned to prefer (cucumber) allowed for increased oviposition, survival and development. These findings have important implications for the interpretation of the preference–performance relationship among herbivorous arthropods. The frequently reported absence of such a relationship may be due to experience-dependent preference and/or performance.     

Within-plant and within-leaf dispersion pattern of two-spotted spider mite,Tetranychus urticae Koch (Acari: Tetranychidae) on okra

Tetranychus urticae distribution on spatial scale both within leaf and within plant was assessed by the index of dispersion (Id), Lloyd's mean crowding index (Imc), Lloyd's patchiness index (Ip) and Morisita's index (I _δ). Id values in all leaves and leaf parts promised aggregated distribution regardless of leaf position and leaf areas. The values of Imc estimated were all larger than the mean. The value of Ip and I _δ  also indicated increased degree of aggregation and clumping in all leaves and leaf areas. While pattern of distribution is same in all the leaves and leaf parts, population estimated is high in young fully opened top leaves than the grown-up middle and bottom leaves. Within the young leaf, top area of upper side of the leaf housed relatively increased number of T. urticae than bottom area of upper side of the leaf. Because young fully opened leaves consistently contained major proportion of mites (48.33%), particularly the top leaf area (79.62%), these leaves and leaf areas can be used as sampling unit for population estimation to minimise the time spent on sampling. However, it warrants future research for predictive models to associate a number of mites of top leaf to other leaves and top area to other areas. Further, releasing predators in young fully opened leaves may increase predator efficiency if supplementary studies on predator–prey relationship on spatial scale are triggered.     

Geographical range size and host specificity in ectoparasites: a case study with Amphipsylla fleas and rodent hosts

Aim  To identify the factors that determine the geographical range sizes of ectoparasites with different degrees of host specificity.
Location  The study used data on the distributions of fleas of the genus Amphipsylla and their rodent hosts across the Holarctic.
Methods  All known points of occurrence of 32 flea species and 51 species of their rodent hosts were mapped. The shape and size of the geographical range of each species were estimated using a combination of the minimal convex polygon technique and modelling with the garp algorithm. Factors determining the geographical range sizes of the fleas were identified using stepwise multiple regression analysis.
Results  The geographical range size of fleas that are strongly host-specific across their entire ranges correlated positively with the geographical range size of the fleas' principal hosts, and negatively with the geographical range size of the fleas' potential competitors. The geographical range sizes of both (1) fleas that are locally host-specific but that shift their host preferences geographically, and (2) host-opportunistic fleas were positively correlated only with the area of the geographical ranges of their principal hosts. Strongly host-specific fleas occupied 0.2–80.0% of the geographical range of their principal hosts, whereas this figure was 0.9–83.7% in locally host-specific fleas and 16.6–63.7% in host-opportunistic fleas.
Main conclusions  The main determinant of the geographical range size of a flea species is the size of the geographical range of its hosts. The role of potential competitors in determining the geographical range size is stronger in host-specific than in host-opportunistic fleas. Cases in which the geographical range of a parasite is smaller than the geographical range(s) of its host(s) owing to narrower parasite environmental tolerances are much more frequent in host-opportunistic than in host-specific fleas.     

Adaptation in a spider mite population after long-term evolution on a single host plant

Evolution in a single environment is expected to erode genetic variability, thereby precluding adaptation to novel environments. To test this, a large population of spider mites kept on cucumber for approximately 300 generations was used to establish populations on novel host plants (tomato or pepper), and changes in traits associated to adaptation were measured after 15 generations. Using a half-sib design, we investigated whether trait changes were related to genetic variation in the base population. Juvenile survival and fecundity exhibited genetic variation and increased in experimental populations on novel hosts. Conversely, no variation was detected for host choice and developmental time and these traits did not evolve. Longevity remained unchanged on novel hosts despite the presence of genetic variation, suggesting weak selection for this trait. Hence, patterns of evolutionary changes generally matched those of genetic variation, and changes in some traits were not hindered by long-term evolution in a constant environment.     

No effect of host–parasite co‐evolution on host range expansion

Antagonistic co‐evolution between hosts and parasites (reciprocal selection for resistance and infectivity) is hypothesized to play an important role in host range expansion by selecting for novel infectivity alleles, but tests are lacking. Here, we determine whether experimental co‐evolution between a bacterium (Pseudomonas fluorescens SBW25) and a phage (SBW25Φ2) affects interstrain host range: the ability to infect different strains of P. fluorescens other than SBW25. We identified and tested a genetically and phenotypically diverse suite of co‐evolved phage variants of SBW25Φ2 against both sympatric and allopatric co‐evolving hosts (P. fluorescens SBW25) and a large set of other P. fluorescens strains. Although all co‐evolved phage had a greater host range than the ancestral phage and could differentially infect co‐evolved variants of P. fluorescens SBW25, none could infect any of the alternative P. fluorescens strains. Thus, parasite generalism at one genetic scale does not appear to affect generalism at other scales, suggesting fundamental genetic constraints on parasite adaptation for this virus.     

The evolution of diet breadth: Monophagy and polyphagy in swallowtail butterflies

Much of the world's biodiversity has resulted from specialization of insect populations onto different plant species, partially through evolution of preference in ovipositing females. Here I report an experimental analysis of how an oviposition preference hierarchy has evolved during the evolutionary diversification of an insect group to produce taxa ranging from monophagous to polyphagous. Tests on the Papilio machaon group of swallowtail butterflies show that the preference hierarchy for plant species is evolutionarily dynamic within this species complex, yet constrained among most populations within species, creating a geographic mosaic of populations differing to various degrees in patterns of host preference. The results indicate that different diet breadths can evolve within a group of closely-related species through a combination of conservatism in preference hierarchy among some populations, occasional but rare rearrangements in preference among others, correlations in preference for some plant species, and availability of similarly ranked hosts.     

毒死蜱亚致死剂量对二斑叶螨实验种群动态的影响

【目的】为探讨毒死蜱(Chlorpyrifos)对二斑叶螨Tetranychus urticae Koch种群动态的影响,室内研究了毒死蜱亚致死剂量对二斑叶螨实验种群成螨和若螨的生长发育、存活和繁殖情况的影响。【方法】用浸虫法进行测定,生命表方法分析亚致死效应。【结果】结果表明毒死蜱亚致死剂量LC25处理成螨后,雌成螨寿命、总产卵量、子代孵化率、性比与对照组无显著差异,其净增值率、内禀增长率、周限增长率以及种群加倍时间显著低于对照组。毒死蜱亚致死剂量LC25处理若螨后,总产卵量、净增值率、世代平均周期显著低于对照组,内禀增长率、周限增长率以及种群加倍时间与对照组无显著差异。扩散试验证明毒死蜱对二斑叶螨有较强的刺激扩散作用。【结论】以上结果说明毒死蜱亚致死剂量对不同发育阶段的二斑叶螨发育和繁殖的影响不同,毒死蜱对二斑叶螨的刺激扩散作用可能是引起二斑叶螨再猖獗的原因之一。     

An experimental test of potential host range in the ant parasitoid Apocephalus paraponerae

1. The work reported here tested experimentally whether specialisation in Apocephalus paraponerae was due to physiological interactions that limit the parasitoid to the host ant Paraponera clavata. The suitability of other ant species as hosts was tested, and behavioural traits that may promote a high degree of specificity within this host–parasitoid system are discussed. 2. Data for development time, number of puparia, and adult eclosion success for A. paraponerae ovipositing in the regular host P. clavata are provided. A new method for testing host suitability in parasitoids of ants is described. Eggs of A. paraponerae were transferred directly into potential ant hosts. The development time, number of puparia, and adults eclosed for the eggs transferred into other ant host species are compared with comparable data from P. clavata. 3. Seven ant species within the Ponerinae are suitable for the development of A. paraponerae. The potential host range is greater than the actual host range of A. paraponerae. Flies are not limited solely by host suitability of related ant species for the development of larvae. Host location and acceptance behaviours are proposed as the primary reasons for host specialisation. The large size of the primary host P. clavata, and the ability of multiple females to raise many offspring successfully from those hosts may influence the specialisation of A. paraponerae on P. clavata.