Global spread of wheat curl mite by its most polyphagous and pestiferous lineages

The wheat curl mite (WCM), Aceria tosichella, is an important pest of wheat and other cereal crops that transmits wheat streak mosaic virus and several other plant viruses. Wheat curl mite has long been considered a single polyphagous species, but recent studies in Poland revealed a complex of genetically distinct lineages with divergent host‐acceptance traits, ranging from highly polyphagous to host‐specific. This diversity of WCM genotypes and host‐acceptance phenotypes in Europe, the presumed native range of WCM, raises questions about the lineage identities of invasive WCM populations on other continents and their relationships to European lineages. The goals of this study were to examine the global presence of WCM and determine the relatedness of lineages established in different continents, on the basis of phylogenetic analyses of mitochondrial and nuclear DNA sequence data. Host‐range bioassays of a highly polyphagous WCM lineage were performed to supplement existing data on this lineage's ability to colonise graminaceous and non‐graminaceous hosts. Invasive WCM populations in North and South America and Australia assorted with the only three known polyphagous and pestiferous WCM lineages (‘MT‐1’, ‘MT‐7’ and ‘MT‐8’) from a total of eight currently described lineages. These results show that the most polyphagous lineages were more successful colonisers and reflect a need for extensive surveys for WCM on both crops and wild grass species in invaded continents. The most invasive lineage (‘MT‐1’) was shown to successfully colonise all 10 plant species tested in three families and has spread to North and South America and Australia from its presumed origins in Eurasia.     

Wheat curl mite and dry bulb mite: untangling a taxonomic conundrum through a multidisciplinary approach

Two economically important eriophyoid mites, Aceria tosichella (wheat curl mite; WCM) and Aceria tulipae (dry bulb mite; DBM), were frequently confounded in the world literature until the late 20th Century. Their morphological similarity and ambiguous data from plant‐transfer and virus‐transmission trials contributed to this confusion. Until recently, there was a general lack of knowledge about the existence of species complexes and it was not possible to accurately genotype tested mites. In the present study, two WCM genotypes of divergent host specificity (MT‐1 and MT‐2) and one DBM genotype were tested for the acceptance of Poaceae, Amarylidaceae, and Liliaceae species that were reported or suspected as hosts of WCM or DBM. The MT‐1 lineage colonized all tested plants. Onion‐ and garlic‐associated DBM populations did not colonize tulip and wild garlic, suggesting that host‐acceptance variability exists within A. tulipae s.l. Morphometric analysis did not discriminate closely‐related MT‐1 and MT‐2 genotypes but completely separated both WCM genotypes from DBM based on the larger overall body size of the latter. Three morphological traits combined to discriminate between the DBM and MT‐1 genotypes, both of which can infest Amarylidaceae bulbs. In total, these combined DNA sequence, host‐acceptance, morphometrical results unambiguously separated two WCM and one DBM genotypes. Similar studies on additional lineages of both WCM and DBM should ultimately dispel previous taxonomic confusion between these two species. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111 , 421–436.     

The distribution of wheat curl mite (Aceria tosichella) lineages in Australia and their potential to transmit wheat streak mosaic virus

The wheat curl mite (WCM), Aceria tosichella , is an eriophyid pest of cereals, and the vector responsible for the transmission of wheat streak mosaic virus (WSMV). In a previous study, the taxonomic status of A. tosichella in Australia was assessed using molecular markers. A. tosichella was shown to consist of two genetically distinct lineages likely to represent different species. Here we show that both lineages occupy similar distributions, occurring throughout the entire Australian wheat belt, and that the lineages are often found in sympatry. CLIMEX analysis suggests that tolerance to heat and desiccation limit the distribution of A. tosichella . In the laboratory, only one WCM lineage transmitted WSMV virus under controlled conditions. These results have implications for the management of WCM and WSMV within Australia.     

Population genetics of the wheat curl mite (Aceria tosichella Keifer) in Australia: implications for the management of wheat pathogens

The wheat curl mite (WCM), Aceria tosichella Keifer, is a polyphagous eriophyoid mite and the primary vector of wheat streak mosaic virus (WSMV) and five other viral pathogens in cereals. Previous research using molecular markers and a series of laboratory experiments found A. tosichella in Australia to consist of two genetically distinct lineages, which have broad overlapping distributions and differ in their ability to transmit WSMV under controlled conditions. This pattern of transmission also appears to be apparent in the field, whereby a strong association between WSMV detection and a single WCM lineage has been detected. In this study, we conduct a population genetic analysis and provide information on the genetic structure of the Australian viruliferous WCM lineage. We assessed genetic differentiation of 16 WCM populations using nine microsatellite markers. Strong evidence for extensive gene flow and low genetic structuring throughout the Australian wheatbelt was evident, with an exception for Western Australian and far north Queensland populations that appear to be genetically isolated. The data also indicate genetic patterns consistent with an arrhenotokous parthenogenetic mode of reproduction. Implications of these findings are discussed with reference to the management of WCM and associated cereal pathogens in Australia and overseas.     

Phylogenetic analyses reveal extensive cryptic speciation and host specialization in an economically important mite taxon

The wheat curl mite (WCM) is a major pest in cereal crops around the world and the vector of at least four known pathogens capable of reducing yields in crops such as wheat, corn, barley, oats, millet and rye. Current taxonomy recognizes WCM as a single species, Aceria tosichella; however, recent genetic, physiological and ecological studies have shown that WCM is likely to be a species complex. In this study we assessed genetic variation and phylogenetic relationships among WCM from four continents and a wide range of host plants using DNA sequence data from one mitochondrial gene, one nuclear gene and a single nuclear intergenic spacer region. Phylogenetic analyses revealed 11 unique mite lineages associated with specific plant hosts including wheat and barley. Host associations were consistent across continents, often with a single haplotype dominating a host plant regardless of geographic origin. The genetic and ecological differences identified in this study support the notion that WCM is a species complex in need of major taxonomic revision. These findings have implications for control of WCM globally, particularly within the context of identifying plants that form ‘green bridge’ refuges, assessing disease transmission risk, and identifying resistance in cereal genotypes to WCM and associated pathogens.     

Analysis of microsatellite variation in the spider mite pest Tetranychus turkestani (Acari: Tetranychidae) reveals population genetic structure and raises questions about related ecological factors

The habitat fragmentation that characterizes agricultural systems made up of an array of sympatric crop and weed biotopes might be a major determinant of the population structure of plant‐feeding arthropods. Spider mites are interesting in this regard as several species are highly polyphagous, possibly generating homogenization by plant shifting, but also have the potential of forming host plant races which promotes specialization. This study analyses genetic diversity and structure in natural populations of a spider mite pest, Tetranychus turkestani, collected on both crops and weeds. Five microsatellite markers were used to genotype 283 individuals collected from 15 samples in four locations in southern France. The markers revealed considerable genetic variation, with an average heterozygosity of 0.68. Pairwise Φ_ST estimates calculated between localities showed differentiation in all comparisons. Although geographical distance appears to be a factor that influences T. turkestani population genetic structure at a regional scale, there was no clear evidence for differentiation between mites living on different host plants. A hierarchical analysis of the distribution of the genetic diversity within and between habitats showed that more than 97% of the observed genetic variation accounted for the differentiation between mites collected on the same host plant in a given locality. Significant heterozygote deficiency was found in 11 out of the 15 samples studied. Considering the biology of the mite, a Wahlund effect and inbreeding might explain such an excess of homozygosity. The data support the view that the plants in a given locality are colonized by mites that originate from diverse sources. They also support previous data suggesting that the demographic structure is made up of small demes of inbred individuals. The agricultural‐level implications of the data are discussed, notably the fact that mites may well be capable of moving between host plants, making weeds surrounding the crop fields potential reservoirs for the pest. © 2004 The Linnean Society of London, Biological Journal of the Linnean Society, 2004, 82 , 69–78.     

Gene flow in the green mirid,Creontiades dilutus (Hemiptera: Miridae), across arid and agricultural environments with different host plant species

Creontiades dilutus (Stål), the green mirid, is a polyphagous herbivorous insect endemic to Australia. Although common in the arid interior of Australia and found on several native host plants that are spatially and temporally ephemeral, green mirids also reach pest levels on several crops in eastern Australia. These host‐associated dynamics, distributed across a large geographic area, raise questions as to whether (1) seasonal fluctuations in population size result in genetic bottlenecks and drift, (2) arid and agricultural populations are genetically isolated, and (3) the use of different host plants results in genetic differentiation. We sequenced a mitochondrial COI fragment from individuals collected over 24 years and screened microsatellite variation from 32 populations across two seasons. The predominance of a single COI haplotype and negative Tajima D in samples from 2006/2007 fit with a population expansion model. In the older collections (1983 and 1993), a different haplotype is most prevalent, consistent with successive population contractions and expansions. Microsatellite data indicates recent migration between inland sites and coastal crops and admixture in several populations. Altogether, the data suggest that long‐distance dispersal occurs between arid and agricultural regions, and this, together with fluctuations in population size, leads to temporally dynamic patterns of genetic differentiation. Host‐associated differentiation is evident between mirids sampled from plants in the genus Cullen (Fabaceae), the primary host, and alternative host plant species growing nearby in arid regions. Our results highlight the importance of jointly assessing natural and agricultural environments in understanding the ecology of pest insects.     

Potential new sources of wheat curl mite resistance in wheat to prevent the spread of yield-reducing pathogens

The wheat curl mite (WCM), Aceria tosichella Keifer (Trombidiformes: Eriophyidae), is a major pest in cropping regions of the world and is recognised as the primary vector of several yield-reducing pathogens, primarily affecting wheat. Management of WCM is complicated due to several aspects of the mite’s biology and ecology; however, commercially viable mite resistant wheat varieties may offer practical long-term management options. Unfortunately, mite populations have adapted to previously identified sources of resistance, highlighting the need for further sources of resistance and the value of stacking different resistances to give greater degrees and longevity of control. In this study we assessed the susceptibility of 42 wheat-derived genotypes to mite population growth using a new experimental method that overcomes methodological limitations of previous studies. Experimental wheat lines included a variety of wheat genotypes, related Triticeae species, wheat-alien chromosome amphiploids, and chromosome addition or substitution lines. From these we identify new promising sources of WCM resistance associated with Thinopyrum intermedium, Th. ponticum and Hordeum marinum chromosomes. More specifically we identify group 1J and 5J chromosomes of the L3 and L5 wheat-Th. intermedium addition lines as new sources of resistance that could be exploited to transfer resistance onto homoeologous wheat chromosomes. This study offers new methods for reliable in situ estimations of mite abundance on cereal plants, and new sources of WCM resistance that may assist management of WCM and associated viruses in wheat.     

Effects of host plant on life‐history traits in the polyphagous spider mite Tetranychus urticae

Studying antagonistic coevolution between host plants and herbivores is particularly relevant for polyphagous species that can experience a great diversity of host plants with a large range of defenses. Here, we performed experimental evolution with the polyphagous spider mite Tetranychus urticae to detect how mites can exploit host plants. We thus compared on a same host the performance of replicated populations from an ancestral one reared for hundreds of generations on cucumber plants that were shifted to either tomato or cucumber plants. We controlled for maternal effects by rearing females from all replicated populations on either tomato or cucumber leaves, crossing this factor with the host plant in a factorial design. About 24 generations after the host shift and for all individual mites, we measured the following fitness components on tomato leaf fragments: survival at all stages, acceptance of the host plant by juvenile and adult mites, longevity, and female fecundity. The host plant on which mite populations had evolved did not affect the performance of the mites, but only affected their sex ratio. Females that lived on tomato plants for circa 24 generations produced a higher proportion of daughters than did females that lived on cucumber plants. In contrast, maternal effects influenced juvenile survival, acceptance of the host plant by adult mites and female fecundity. Independently of the host plant species on which their population had evolved, females reared on the tomato maternal environment produced offspring that survived better on tomato as juveniles, but accepted less this host plant as adults and had a lower fecundity than did females reared on the cucumber maternal environment. We also found that temporal blocks affected mite dispersal and both female longevity and fecundity. Taken together, our results show that the host plant species can affect critical parameters of population dynamics, and most importantly that maternal and environmental conditions can facilitate colonization and exploitation of a novel host in the polyphagous T. urticae, by affecting dispersal behavior (host acceptance) and female fecundity.     

Neglected refugia of biodiversity: mountainous regions in Mozambique and Malawi yield two novel freshwater crab species (Potamonautidae: Potamonautes)

Phylogenetic relationships amongst the southern African freshwater crab fauna are reinvestigated following the recent collection of morphologically distinct Potamonautes specimens from remote mountainous regions in Malawi and Mozambique. Specimens were subjected to DNA sequencing of three mtDNA loci, cytochrome c oxidase subunit I (COI), 12S rRNA, and 16S rRNA and compared to the 14 described species from the region. Phylogenetic analysis using maximum parsimony and Bayesian inference revealed the presence of two novel evolutionary lineages. The phylogeny demonstrates that Potamonautes obesus (A. Milne‐Edwards, 1868) is sister to a morphologically distinct novel species from Mount Namuli in Mozambique. Two sympatric and genetically distinct species from Mount Mulanje, in Malawi (forms A and B) were recognized. Form B is sister to the large‐bodied South African riverine freshwater crabs and represents a novel lineage whereas the remaining species (form A) from Mulanje, in Malawi was sister to samples from Mounts Inago and Mabu, and in Mozambique was identified as Potamonautes choloensis (Chace, 1953). The two novel evolutionary lineages were genetically distinct and morphologically different from the described species in each of the respective regions. Two new freshwater crab species Potamonautes namuliensis sp. nov. and Potamonautes mulanjeensis sp. nov. , are described in the present study. The samples from Mount Mulanje in Malawi, and Mounts Mabu and Inago in Mozambique represent new distribution records for Potamonautes choloensis. © 2012 The Linnean Society of London, Zoological Journal of the Linnean Society, 2012, 164 , 498–509.     

Molecular phylogenetics of heliothine moths (Lepidoptera: Noctuidae: Heliothinae), with comments on the evolution of host range and pest status

Abstract The Heliothinae are a cosmopolitan subfamily of about 365 species that include some of the world’s most injurious crop pests. This study re‐assesses evolutionary relationships within heliothines, providing an improved phylogeny and classification to support ongoing intensive research on heliothine genomics, systematics, and biology. Our phylogeny estimate is based on two nuclear gene regions, namely elongation factor‐1α (EF‐1α; 1240 bp) and dopa decarboxylase (DDC; 687 bp), and on the barcoding region of mitochondrial cytochrome oxidase I (COI; 708 bp), providing a total of 2635 bp. These were sequenced for 71 heliothines, representing all major genera and nearly all recognized subgenera and species groups, and for 16 outgroups representing all major lineages of trifine Noctuidae. Analysis of the combined data by maximum likelihood, unweighted parsimony and Bayesian methods gave nearly identical topologies, and the individual gene trees showed only one case of potentially strong conflict. Relationships among genera and subgenera are resolved with strong bootstrap support. The earliest‐diverging lineages (c. 200 species in total) consist almost entirely of host specialists, reflecting the inferred ancestral heliothine host range under parsimony. The remaining species form a clade – the Heliothis group – that includes most of the polyphages (30% of heliothines) and all of the major pests. Many other species in the Heliothis group, however, are host specialists. Our results extend previous efforts to subdivide this large clade, and show the most notorious pest groups, the corn earworm complex (Helicoverpa) and the tobacco budworm (Heliothis virescens) group, to be closely related, joining with a small oligophagous genus in what we term the major‐pest lineage. Thus, genomic/experimental results from one model pest may extrapolate well to other pest species. The frequency of evolutionary expansion and contraction in host range appears to increase dramatically at the base of the Heliothis group, in contrast to the case for earlier‐diverging lineages. We ascribe this difference provisionally to differential evolutionary constraints arising from contrasting life‐history syndromes. Host‐specific behaviour and crypsis, coupled with low fecundity and vagility, may discourage host‐range expansion in earlier‐diverging lineages. By contrast, in the Heliothis group, the absence of host‐specific traits, coupled with high vagility and fecundity, may more readily permit expansion or contraction of the host range in response to varying ecological pressures such as host species abundance or differential competition and predation.     

Molecular identification and ecological characteristics of two cryptic lineages within a cosmopolitan aphid pest, Brachycaudus helichrysi (Hemiptera: Aphididae)

The leaf-curl plum aphid, Brachycaudus helichrysi (Kaltenbach 1843), is a polyphagous cosmopolitan aphid. This serious pest of plum and peach orchards affects the growth of its host plants by curling infested leaves, and can transmit viruses such as the plum pox virus. Phylogenetic analyses have recently shown that B. helichrysi actually comprises two lineages that are morphologically similar but genetically different. In this study, we developed a PCR-RFLP test for differentiating between these two lineages, based on a mitochondrial DNA fragment. We used this test to investigate the patterns of host plant association and the geographical distribution of these lineages, with a large sample of individuals collected from numerous host plants and locations worldwide. We found that one of the lineages was never present on plum trees, and that individuals from both lineages coexisted on many secondary herbaceous hosts at almost all of the sites studied. We then carried out principal component analysis to determine whether the distribution of the two lineages could be explained by climatic features. We found that the lineage absent from plum trees was not found in regions with harsh winters. This suggests that one of the lineages within B. helichrysi may be an obligate parthenogenetic aphid.     

Phylogeography of the parasitic fly Batrachomyia in the Wet Tropics of north-east Australia, and susceptibility of host frog lineages in a mosaic contact zone

Parasites could differentially impact intraspecific host lineages due to genetic, phenotypic, ecological, or behavioural differences between the lineages, or the development of reproductive isolation between them. Batrachomyia (Diptera: Chloropidae) are flies that exclusively parasitize Australian frogs, and in the Wet Tropics rainforest of north‐east Australia larvae are largely restricted to the green‐eyed tree frog Litoria genimaculata (Anura: Hylidae). This frog species consists of two highly divergent genetic lineages that overlap in two nearby, but independent, contact zones. At one contact zone there has been extensive phenotypic divergence and speciation between the lineages whereas, at the other contact relatively lower levels of phenotypic divergence and reproductive isolation suggest that speciation has not occurred. In the present study we tested: (1) whether the deep phylogeographic divergence between northern and southern host populations is mirrored by congruent genetic structuring in the parasite populations and (2) whether the host lineages are differentially impacted by parasitism. We found that the two divergent frog lineages are parasitized by a single lineage of Batrachomyia, which exhibits strikingly little phylogeographic structuring. We found a significant difference in Batrachomyia prevalence between the host lineages at mixed lineage sites in both contact zones, with the magnitude and direction of this effect being consistent in both. The pattern did not differ between the two contacts even though recent phenotypic divergence and speciation has occurred between the lineages at one contact but not the other. Taken together, this suggests a fundamental difference in susceptibility between the genetically divergent host lineages. Using weight relative to body length as a measure of body condition, we found no differential impact of parasitism on the body condition of each host lineage, and no evidence that parasitism impacts the body condition of the host in general. © 2007 The Linnean Society of London, Biological Journal of the Linnean Society, 2007, 92 , 593–603.     

Host‐associated differentiation in a highly polyphagous,sexually reproducing insect herbivore

Insect herbivores may undergo genetic divergence on their host plants through host‐associated differentiation (HAD). Much of what we know about HAD involves insect species with narrow host ranges (i.e., specialists) that spend part or all their life cycle inside their hosts, and/or reproduce asexually (e.g., parthenogenetic insects), all of which are thought to facilitate HAD. However, sexually reproducing polyphagous insects can also exhibit HAD. Few sexually reproducing insects have been tested for HAD, and when they have insects from only a handful of potential host‐plant populations have been tested, making it difficult to predict how common HAD is when one considers the entire species' host range. This question is particularly relevant when considering insect pests, as host‐associated populations may differ in traits relevant to their control. Here, we tested for HAD in a cotton (Gossypium hirsutum) pest, the cotton fleahopper (CFH) (Pseudatomoscelis seriatus), a sexually reproducing, highly polyphagous hemipteran insect. A previous study detected one incidence of HAD among three of its host plants. We used Amplified fragment length polymorphism (AFLP) markers to assess HAD in CFH collected from an expanded array of 13 host‐plant species belonging to seven families. Overall, four genetically distinct populations were found. One genetically distinct genotype was exclusively associated with one of the host‐plant species while the other three were observed across more than one host‐plant species. The relatively low degree of HAD in CFH compared to the pea aphid, another hemipteran insect, stresses the likely importance of sexual recombination as a factor increasing the likelihood of HAD.     

From forest to plantation? Obscure articles reveal alternative host plants for the coffee berry borer,Hypothenemus hampei (Coleoptera: Curculionidae)

The coffee berry borer, Hypothenemus hampei (Coleoptera: Curculionidae: Scolytinae), is the most devastating insect pest of coffee throughout the world. The insect is endemic to Africa but can now be found throughout nearly all coffee‐producing countries. One area of basic biology of the insect that remains unresolved is that of its alternative host plants, i.e. which fruits of plants, other than coffee, can the insect survive and reproduce in. An in‐depth survey of the literature revealed an article by Schedl listing 21 genera in 13 families in which the insect was collected, mainly in the Democratic Republic of Congo. This overlooked reference, together with information provided in other early articles, suggests that H. hampei is polyphagous, and could provide, if confirmed in the field, critical information on the evolution of this insect's diet, ecology and host range. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, ??, ??–??.     

Evolutionary radiation of Asteromyia carbonifera (Diptera: Cecidomyiidae) gall morphotypes on the goldenrod Solidago altissima (Asteraceae)

Population divergence of phytophagous insects is often coupled to host‐plant shifts and is frequently attributed to the divergent selective environments associated with alternative host‐plants. In some cases, however, divergence is associated with the use of alternative host‐plant organs of a single host species. The basis of within‐host radiations such as these remains poorly understood. In the present stusy, we analysed the radiation of Asteromyia gall midges occurring both within one host plant species and within a single organ on that host. In this system, four morphologically distinct Asteromyia gall forms (morphs) coexist on the leaves of goldenrod Solidago altissima. Our analyses of amplified fragment length polymorphism and DNA sequence data confirm the genetic differentiation among midges from three gall morphs and reveal evidence of a genetically distinct fourth gall morph. The absence of clear gall morph related clades in the mitochondrial DNA derived phylogenies is indicative of incomplete lineage sorting or recent gene flow, suggesting that population divergence among gall forms is recent. We assess the likely history of this radiation and use the results of phylogenetic analyses along with ecological data on phenology and parasitism rates to evaluate potential hypotheses for the mode of differentiation. These preliminary analyses suggest that diversification of the Asteromyia gall morphs is likely shaped by interactions between the midge, a symbiotic fungus, and parasitoid enemies. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 95 , 840–858.     

Divergent host plant adaptation drives the evolution of sexual isolation in the grasshopper Hesperotettix viridis (Orthoptera: Acrididae) in the absence of reinforcement

Early stages of lineage divergence in insect herbivores are often related to shifts in host plant use and divergence in mating capabilities, which may lead to sexual isolation of populations of herbivorous insects. We examined host preferences, degree of differentiation in mate choice, and divergence in cuticular morphology using near‐infrared spectroscopy in the grasshopper Hesperotettix viridis aiming to understand lineage divergence. In Kansas (USA), H. viridis is an oligophagous species feeding on Gutierrezia and Solidago host species. To identify incipient mechanisms of lineage divergence and isolation, we compared host choice, mate choice, and phenotypic divergence among natural grasshopper populations in zones of contact with populations encountering only one of the host species. A significant host‐based preference from the two host groups was detected in host‐paired feeding preference studies. No‐choice mate selection experiments revealed a preference for individuals collected from the same host species independent of geographic location, and little mating was observed between individuals collected from different host species. Female mate choice tests between males from the two host species resulted in 100% fidelity with respect to host use. Significant differentiation in colour and cuticular composition of individuals from different host plants was observed, which correlated positively with host choice and mate choice. No evidence for reinforcement in the zone of contact was detected, suggesting that divergent selection for host plant use promotes sexual isolation in this species. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 100 , 866–878.     

The impact of plant associations on Macrosteles quadrilineatus management in carrots

Habitat diversification can influence the interactions of insects with plants and this can be used in agroecosystems for the management of pest populations. Plant diversification can be achieved through planting crops, such as trap crops, or by adjusting weed management. Aster leafhopper, Macrosteles quadrilineatus Forbes (Hemiptera: Cicadellidae), is a polyphagous species that uses cereals, vegetables, and weeds as host plants. The influence of weeds on M. quadrilineatus abundance was investigated experimentally in carrot [Daucus carota L. cv. Canada (Apiaceae)] field plots by adjusting the level of management of two groups of weeds (broadleaf and grass) and by comparing it to weed‐free plots. The preference of M. quadrilineatus for different cereal and weed species relative to carrots was tested in choice test assays. Habitat context influenced the abundance of M. quadrilineatus in the field experiments. The presence of border crops such as oat, rye, barley, wheat, and triticale did not significantly attract or repel this insect to carrot plots compared to the no‐border treatment. However, spelt‐bordered plots had 42% fewer M. quadrilineatus than three treatments, triticale, wheat, and barley, that had the highest insect abundance. The type of weed management affected M. quadrilineatus abundance in carrot plots, but not the frequency of herbicide application. Plots that had carrot growing with broadleaf‐weeds had about 59% fewer M. quadrilineatus compared with those growing with crabgrass or carrot alone. In the greenhouse choice tests, grasses (e.g., cereals) attracted and broadleaf‐weeds repelled M. quadrilineatus relative to carrots. In summary, carrot growers may be able to manage this pest by reducing the interaction of cereal cover crops with carrots and eliminating grassy weeds in commercial production fields.     

The ecological differentiation of asexual lineages of cotton aphids: alate behaviour, sensory physiology, and differential host associations

The cotton aphid ( Aphis gossypii Glover) is a cosmopolitan pest species consisting of various host plant-associated lineages that appear to have different patterns of insecticide resistance, electrophoretic banding, and reproductive output across alternative host species. Debate continues as to whether these differences represent intraspecific variation or indicate a complex of cryptic or sibling species. We studied the host plant-related ecology of two asexual lineages of A. gossypii associated with the families Malvaceae and Cucurbitaceae, respectively, in particular the host plant finding and acceptance processes of the alate forms. The two lineages proved to be differentiated across host plants in terms of: (1) their landing and settling behaviour, nymphal production rate, and subsequent colony establishment (i.e. nymphal survival and development) and (2) their sensory abilities in relation to host plant-derived odours. Furthermore, genetic structuring according to host plant type was detected using the gene for elongation factor 1-α. The magnitude of the genetic differentiation across lineages was low, perhaps indicating a relatively recent divergence. The results demonstrate: (1) the presence of (at least) two cryptic species within the current definition of A. gossypii , as anticipated in previous studies, and (2) the practicalities of using a multidisciplinary approach to understanding the 'limits' of asexual species. If sexuals can be generated from each of these lineages, this conclusion will be open to further testing.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 97 , 503–519.