Why close relatives make bad neighbours: phylogenetic conservatism in niche preferences and dispersal disproves Darwin's naturalization hypothesis in the thistle tribe

The number of exotic plant species that have been introduced into the United States far exceeds that of other groups of organisms, and many of these have become invasive. As in many regions of the globe, invasive members of the thistle tribe, Cardueae, are highly problematic in the California Floristic Province, an established biodiversity hotspot. While Darwin's naturalization hypothesis posits that plant invaders closely related to native species would be at a disadvantage, evidence has been found that introduced thistles more closely related to native species are more likely to become invasive. To elucidate the mechanisms behind this pattern, we modelled the ecological niches of thistle species present in the province and compared niche similarity between taxa and their evolutionary relatedness, using fossil‐calibrated molecular phylogenies of the tribe. The predicted niches of invasive species were found to have higher degrees of overlap with native species than noninvasive introduced species do, and pairwise niche distance was significantly correlated with phylogenetic distance, suggesting phylogenetic niche conservatism. Invasive thistles also displayed superior dispersal capabilities compared to noninvasive introduced species, and these capabilities exhibited a phylogenetic signal. By analysing the modelled ecological niches and dispersal capabilities of over a hundred thistle species, we demonstrate that exapted preferences to the invaded environment may explain why close exotic relatives may make bad neighbours in the thistle tribe.     

Assessment of risk of attack to safflower by Ceratapion basicorne (Coleoptera: Apionidae), a prospective biological control agent of Centaurea solstitialis (Asteraceae)

Ceratapion basicorne (Coleoptera: Apionidae) is a prospective biological control agent of yellow starthistle (Centaurea solstitialis, Asteraceae: Cardueae), which is an important invasive alien weed in the western United States. Previous studies have shown that it is possible for this insect to oviposit on and complete development on safflower (Carthamus tinctorius) under no-choice laboratory conditions; however, it has never been reported as a pest of safflower. Field experiments were conducted at three sites in eastern Turkey during 3 years to evaluate the risk of attack on safflower by this insect in its native range. At two sites where C. basicorne was the only apionid observed, no safflower plants were attacked despite high attack rates on yellow starthistle test plants (48–98% of plants infested). At a third site, where C. basicorne and three other species in the same genus; C. scalptum, C. orientale, and C. onopordi were present, 8–26% of safflower plants were infested, but none of the insects reared from safflower during 3 years were C. basicorne. Other authors have reported rearing C. basicorne from field-collected plants of only Ce. solstitialis, Ce. cyanus, Ce. depressa, and Cnicus benedictus. Our results indicate that C. basicorne does not attack safflower under field conditions and that its introduction would not pose a risk to this crop.     

Field assessment in land of origin of host specificity,infestation rate and impact of <Emphasis Type="Italic">Ceratapion basicorne</Emphasis> a prospective biological control agent of yellow starthistle

Yellow starthistle, Centaurea solstitialis (Asteraceae), is an important invasive alien weed in the western United States. Currently established biological control agents attack only the capitula (flowerheads), and are not effectively controlling the plant in much of its range. The geographic center of diversity for the plant appears to be in Turkey, but no agents have been introduced from this country. Ceratapion basicorne (Coleoptera: Apionidae) is common in Central Turkey, attacking 25–100% of yellow starthistle plants. In a field experiment, Ceratapion spp. attacked 90% of yellow starthistle plants and 88% of milk thistle plants (Silybum marianum) but not seven other plant species, including artichoke and safflower. We suspect that a different species of insect attacked milk thistle, but they emerged before the plants were sampled. Laboratory tests showed that C. basicorne does not oviposit in milk thistle. Ceratapion basicorne appears to be more host specific than was suggested by previous studies of a population in Italy (Clement etal. 1989. Ann. Entomol. Soc. Am. 82: 741–747). The insect is gregarious, and the number of larvae per plant was positively correlated to root diameter. The level of damage to individual plants was positively correlated to the proportion of plants attacked, indicating aggregation both among plants and within plants. Field data did not show any impact of the insect on plant size or number of capitula, but germination rate of seeds produced by infested plants was 15% lower than for uninfested plants at two of three sites studied.     

Larval densities and field hosts of Ceratapion basicorne (Coleoptera: Apionidae) and an illustrated key to the adults of Ceratapion spp. that feed on thistles in the eastern Mediterranean and Black Sea regions

Many members of the tribe Cardueae are invasive weeds, including yellow starthistle (Centaurea solstitialis L.), one of the most important weeds in the Western United States. We examined the root crowns and stems of yellow starthistle and related plants growing in five countries (Armenia, Republic of Georgia, Greece, Russia, and Turkey) where yellow starthistle is native. In its native range, the root crowns and lower stems of yellow starthistle are frequently attacked by the internal feeding larvae of apionid weevils. We present illustrations and a key to the adults of the six apionid species that we reared from yellow starthistle and its relatives: Ceratapion basicorne (Illiger), C. carduorum (Kirby), C. gibbirostre (Gyllenhal), C. onopodri (Kirby), C. orientale (Gerstaecker), and C. penetrans (Germar). The only apionid we reared from yellow starthistle was C. basicorne. In Turkey, where we collected most intensively, 58% of the yellow starthistle at 20 sites had larvae of this weevil, and at sites where C. basicorne was present, there were an average of 1.8 immatures per yellow starthistle plant. This apionid is currently being further researched for its potential as a biological control agent for yellow starthistle.     

Further investigations of the host range of the carrot fly, Psila rosae (F.)

Techniques were developed for testing umbelliferous species against naturally-occurring populations of carrot fly in the field. The most efficient technique involved exposure of plants to either first or second generations of the insect in the field, followed by caging of infested plots and trapping of emerged flies in water in yellow water dishes. In a series of field experiments between 1981 and 1989 inclusive, a total of 132 umbelliferous species and sub-species were tested against carrot fly. Seventy-eight of these proved to be new hosts, 27 were confirmed as hosts and 27 failed to support any carrot flies. Six non-umbelliferous plant species failed to be colonised by carrot fly. Sources of variability in investigations of the host range of insects are discussed.     

Host Specificity of Altica carduorum Guer. Chrysomelidae: Coleoptera , a Defoliator of Cirsium arvense L. Scop. Asteraceae from North-western China

Traditionally, insects are not approved for release in North America as agents for the biological control of weeds if the larvae complete development on desirable plants in no-choice tests. This criterion excludes the use of the leaf-feeding beetle Altica carduroum Guer. since it can complete development on all Cirsium spp., including rare North American species, although in nature it is only known from the target weed Cirsium arvense. Host suitability is the result of a sequence of selection factors, and this study shows that most discrimination between thistle species is the result of host finding and egg production, and not larval developmental ability. In no-choice tests, the larvae completed development on 18 Cirsium species in five sections of the genus that were tested and on Silybum marianum, but not on other genera in the tribe Cardueae or more distantly related Asteraceae. The adult food consumption on C. arvense was two to eight times greater than on other Cirsium species, many of which, such as C. pitcheri and C. drummondii, received no feeding by over 50 of the beetles in the first 24 h. The best North American thistle for egg production was C. scariosum, but this was only 28 of that on C. arvense. The preference for C. arvense in multiple-choice tests increased with the size 2 of the arena. In the largest, an area of 6 m , 80 of the beetles and eggs were found on C. arvense and none were found on plants outside the subtribe Carduineae. Thus, as theory predicts, host discrimination between thistle species takes place at earlier stages than suitability for larval development, and this should be considered when determining the safety of candidate agents.     

Are bottlenecks associated with colonization? Genetic diversity and diapause variation of native and introduced Rhagoletis completa populations

The success of invasive species appears to be a paradox: despite experiencing strong population bottlenecks, invasive species are able to successfully establish in new environments. We studied how the walnut husk fly, Rhagoletis completa, was able to successfully colonize California from the Midwestern United States, by examining genetic diversity and diapause variation of native and introduced fly populations. Climate plays an important role in the successful establishment of introduced insects, because insect diapause is highly dependent upon external climatic conditions. We examined if: (1) fly populations show signs of a population bottleneck, (2) native and introduced flies differ in diapause length when exposed to California and Midwestern climatic conditions, and (3) population genetic diversity is related to variation in diapause length. We assessed if fly diapause conformed more to a model of establishment by local adaptation or to a model of a highly plastic “general-purpose genotype”. Our results indicate that only two populations close to the original introduced location showed signs of a population bottleneck, and native and introduced populations did not differ in genetic diversity. Genetic diversity increased in the northern introduced populations, suggesting that multiple introductions have occurred. Flies emerged about 2 weeks earlier under the Midwestern treatment than the California treatment, and introduced flies emerged about a week earlier than native flies. All flies emerged when walnuts are typically available in California. Although variance in diapause length differed between populations, it did not vary between populations or regions. Furthermore, genetic diversity was not associated with diapause variation. Therefore, multiple introductions and a “general-purpose genotype” appear to have facilitated the fly’s invasion into California.     

A novel preference for an invasive plant as a mechanism for animal hybrid speciation

Homoploid hybrid speciation--speciation via hybridization without a change in chromosome number--is rarely documented and poorly understood in animals. In particular, the mechanisms by which animal homoploid hybrid species become ecologically and reproductively isolated from their parents are hypothetical and remain largely untested by experiments. For the many host-specific parasites that mate on their host, choosing the right host is the most important ecological and reproductive barrier between these species. One example of a host-specific parasite is the Lonicera fly, a population of tephritid fruit flies that evolved within the last 250 years likely by hybridization between two native Rhagoletis species following a host shift to invasive honeysuckle. We studied the host preference of the Lonicera fly and its putative parent species in laboratory experiments. The Lonicera fly prefers its new host, introduced honeysuckle, over the hosts of both parental species, demonstrating the rapid acquisition of preference for a new host as a means of behavioral isolation from the parent species. The parent taxa discriminate against each other's native hosts, but both accept honeysuckle fruit, leaving the potential for asymmetric gene flow from the parent species. Importantly, this pattern allows us to formulate hypotheses about the initial formation of the Lonicera fly. As mating partners from the two parent taxa are more likely to meet on invasive honeysuckle than on their respective native hosts, independent acceptance of honeysuckle by both parents likely preceded hybridization. We propose that invasive honeysuckle served as a catalyst for the local breakdown of reproductive isolation between the native parent species, a novel consequence of the introduction of an exotic weed. We describe behavioral mechanisms that explain the initial hybridization and subsequent reproductive isolation of the hybrid Lonicera fly. These results provide experimental support for a combination of host shift and hybridization as a model for hybrid speciation in parasitic animals.     

Structure of the phytophagous insect fauna of Onopordum thistles in the northern Mediterranean basin

Thistles of the genus Onopordum (Asteraceae: Cardueae) have become serious weed problems in parts of Australia and North America following introduction from their native Eurasian ranges, and are the target of a biological control programme in Australia. This paper analyses the results of three separate surveys of insects feeding and breeding on thistles of the genus Onopordum in the Mediterranean region. Of the 129 species found feeding, 74 species also bred on these thistles. Endophages comprised 54% of the breeding insects, with species that fed in the capitula and within the rosette crown/root area predominating. The Coleoptera and Hemiptera were the dominant endophage and ectophage taxa, respectively. Differences in the Onopordum fauna were noted between host species and between geographic regions, but this was largely due to the generalist component of the fauna. Onopordum specialists showed little variability across regions or between different species within the host genus. On comparing the fauna of Onopordum with that of other thistle genera, it was suggested that plant-herbivore interactions between Onopordum and its fauna are less highly evolved, in that there is less host specialization and a lesser degree of niche partitioning. This appears to be related to the low rate of speciation and habitat specialization within the host genus itself. The potential of certain genus-specific insects as biological control agents for Onopordum spp. is discussed. It is argued that the absence of a high degree of niche specialization could favour successful biological control, as the agent would be capable of inflicting damage over a wider range of resource gradients.     

Weed biological control in California,USA: review of the past and prospects for the future

Weed biological control in California, USA began in 1940 with the release of a native scale insect on native Opuntia spp. on Santa Cruz Island, just offshore from mainland California. Since then, a total 39 weed species have been targets of biological control releases in California. Releases on 11 weed targets were transfer experiments where agents from related weed hosts were released on a new host. Most of the transfer experiment introductions failed but one weed was successfully controlled. Of the other 28 weeds, release sites for three species were destroyed and for six species releases are too recent to score, but for 19 weeds, their level of control was rated as: complete control (three species), substantial control (five species), and partial (six species), and negligible control (five species). Overall, 42% of the projects provide successful control, a result lower than observed in other countries worldwide. Since 1940, 77 species of agents have been released: 54 species established, 12 species failed to establish, six species had their release sites destroyed, and five species are too early to determine. Establishment rate was 82% but the rate differed among taxonomic orders. Individual agents were scored according to level of impact on their host and Coleoptera obtained the highest average impact score and Diptera the lowest. Mean impact scores over time showed a substantial drop in the 1980s but later increased. Future research efforts that emphasize introduction of high impact agents will further support development of this critical weed control method for California.     

Evaluation of the growth response of six invasive species to past,present and future atmospheric carbon dioxide

The response of plant species to future atmospheric carbon dioxide concentrations [CO(2)] has been determined for hundreds of crop and tree species. However, no data are currently available regarding the response of invasive weedy species to past or future atmospheric [CO(2)]. In the current study, the growth of six species which are widely recognized as among the most invasive weeds in the continental United States, Canada thistle (Cirsium arvense (L.) Scop.), field bindweed (Convolvulus arvensis L.), leafy spurge (Euphorbia esula L.), perennial sowthistle (Sonchus arvensis L.), spotted knapweed (Centaurea maculosa Lam.), and yellow star thistle (Centaurea solstitialis L.) were grown from seed at either 284, 380 or 719 micromol mol(-1) [CO(2)] until the onset of sexual reproduction (i.e. the vegetative period). The CO(2) concentrations corresponded roughly to the CO(2) concentrations which existed at the beginning of the 20th century, the current [CO(2)], and the future [CO(2)] projected for the end of the 21st century, respectively. The average stimulation of plant biomass among invasive species from current to future [CO(2)] averaged 46%, with the largest response (+72%) observed for Canada thistle. However, the growth response among these species to the recent [CO(2)] increase during the 20th century was significantly higher, averaging 110%, with Canada thistle again (+180%) showing the largest response. Overall, the CO(2)-induced stimulation of growth for these species during the 20th century (285-382 micromol mol(-1)) was about 3x greater than for any species examined previously. Although additional data are needed, the current study suggests the possibility that recent increases in atmospheric CO(2) during the 20th century may have been a factor in the selection of these species.     

Supplemental risk evaluations and status of Puccinia carduorum for biological control of musk thistle

Tests of seven rare and endangered native North American Cirsium species and four modern artichoke lines were requested in response to a proposal for introduction of Puccinia carduorum into the United States for biological control of musk thistle (Carduus nutans ssp. leiophyllus). These tests were supplemental to an earlier extensive host-range study that established P. carduorum from musk thistle as host specific, useful for biological control, and suitable for limited field tests in Virginia. Test plants in the current study were evaluated in support of a proposal to use the rust in the western United States, and particularly, in California. None of the test plants in this study had been evaluated in previous assessments and all were either rare, endangered or threatened in California. Tests were conducted in both field and greenhouse settings. Field tests were run for two seasons, and test plants were inoculated by natural spread of the pathogen from source plants inside rings of test plants. Greenhouse tests involved direct inoculation under optimal conditions of dew and temperature (18–20 °C, 16 h) for infection. None of the seven Cirsium species or subspecies tested became infected by P. carduorum, either in field or greenhouse tests, compared to infection of 98% of the individual musk thistle plants (n = 102) from all the studies. Modern artichoke cultivars were tested only by direct inoculation under optimal greenhouse conditions. All artichoke plants (n = 115) either were immune (no macroscopic symptoms, n = 69) or at most, resistant (n = 46); pustules on all but two of the resistant plants were very small (0.30 mm diam). Despite infections on artichokes, P. carduorum could not be maintained on artichokes under optimal greenhouse conditions. These results confirm earlier findings from host-range tests and risk assessments of P. carduorum. This information suggests that rare, threatened, or endangered Cirsium spp. and modern artichoke cultivars are not likely to be adversely affected by the use of P. carduorum for biological control of musk thistle. These data have been reviewed by grower groups and regulatory agencies in a proposal for permission to use the rust for musk thistle control throughout the United States.     

Comparative biology of two rosette crown-feeding flies of the genus Botanophila (Dipt., Anthomyiidae) with potential for biological control of their thistle hosts

The comparative biology of two anthomyiid flies with potential for biological control of their host thistles, Botanophila turcica on Carthamus lanatus and Botanophila spinosa on Onopordum acanthium , was studied using field surveys and collections, and by rearing collected eggs and larvae in the laboratory in southern France. The thistle hosts are significant weeds outside their native range, particularly in Australia. Both flies attack the rosette meristems of their hosts prior to flowering. Larval stages are described together with natural attack rates (21–33% of field plants) and the mortality of field-collected larvae reared in the laboratory. A successful rearing protocol for these flies is also described. The results of preliminary host-specificity tests showed that both species are highly specific, being restricted to their host genus in the tests conducted. Furthermore, Botanophila turcica could not complete development on safflower (a congener of its natural host) under natural conditions. Botanophila turcica had an adult activity period that lasted 6 months from late autumn and laid fewer eggs per host plant than B. spinosa , which was active for 2 to 3 months in spring. Only one parasitoid was observed, an endoparasitic eucoilid, Trybliographa sp., which attacked both fly species, causing 18–23% mortality.     

Host plant oviposition preference of Ceratapion basicorne (Coleoptera: Apionidae), a potential biological control agent of yellow starthistle

Ceratapion basicorne is a weevil native to Europe and western Asia that is being evaluated as a prospective classical biological control agent of Centaurea solstitialis (yellow starthistle) in the United States. Choice oviposition experiments were conducted under laboratory conditions to help assess host-plant specificity of the insect. Mean oviposition rates were highest on C. solstitialis (66% of eggs, on a per replicate basis) followed by Centaurea cyanus (bachelor's button 22%), Centaurea melitensis (6%), Centaurea americana (1%), Saussurea americana (3%) and Carthamus tinctorius (safflower 2%). Adult feeding damage followed a similar pattern; however, there was less oviposition relative to the amount of adult feeding on each of the non-target species than on the target host plant, C. solstitialis. Thirteen safflower varieties were tested, and oviposition occurred on eight of them, at low rates. Adult feeding occurred on all safflower varieties tested, although at rates much lower than on yellow starthistle. The results were intermediate between those of previously reported no-choice laboratory and open field experiments. Overall, the combined results support the hypothesis that C. basicorne is not likely to attack any of the non-target plant species tested here except possibly C. cyanus and C. melitensis, which are both invasive alien plants.     

Chemistry of Cirsium and Carduus: a role in ecological risk assessment for biological control of weeds?

Prediction of host plant range and ecological impact of exotic phytophagous insects, such as insects for classical biological control of weeds, represents a major challenge. Recently, the flowerhead weevil (Rhinocyllus conicus Fröl.), introduced from Europe into North America to control exotic thistles (Carduus spp.), has become invasive. It feeds heavily on some, but not all species of native North American thistles (Cirsium spp.). We hypothesized that such non-target use among native plants could be better predicted by knowledge of characteristic chemical profiles of secondary compounds to supplement the results of host specificity testing. To evaluate this hypothesis, we reviewed the literature on the chemistry of Cirsium and Carduus thistles. We asked what compounds are known to be present, what is known about their biological activity, and whether such information on chemical profiles would have better predicted realized host range and ecological effects of R. conicus in North America. We found an extensive, but incomplete literature on the chemistry of true thistles. Two main patterns emerged. First, consistent chemical similarities and interesting differences occur among species of thistles. Second, variation occurs in biologically active groups of characteristic compounds, specifically flavonoids, sterols, alkaloids and phenolic acids, that are known to influence host plant acceptance, selection, and feeding by phytophagous insects. Surprisingly, sesquiterpene lactones, which are characterisitic in closely related Asteraceae, have not been extensively reported for Cirsium or Carduus. The minimal evidence on sesquiterpene lactones may reflect extraction methods vs. true absence. In summary, our review suggests further research on thistle chemistry in insect feeding is warranted. Also, since the exotic Canada thistle (Cirsium arvense) is an invasive thistle of current concern in North America, such research on mechanisms underlying host range expansion by exotic insects would be useful.     

Grapes, galls, and geography: the distribution of nuclear and mitochondrial DNA variation across host-plant species and regions in a specialist herbivore

Studies of patterns of molecular variation in natural populations can provide important insights into a number of evolutionary problems. Among these, the question of whether geographic factors are more important than ecological factors in promoting population differentiation and ultimately speciation has been an important and contentious area in evolutionary biology. Systems involving herbivorous insects have played a leading role in this discussion. This study examined the distribution of molecular variation in a highly specialized gall-forming insect, grape phylloxera (Daktulosphaira vitifoliae Fitch), that is found on both sympatric and allopatric host-plant species of the genus Vitis. In addition, the relationship of insects in the introduced range in the United States to ancestral populations in the native range was examined. Evidence for differentiation along host-plant lines from both nuclear (RAPD) and mitochondrial (COI) DNA was confounded with the effect of geography. Differentiation was found where hosts were allopatric or parapatric, but no evidence was found for such differentiation on two hosts, V. vulpina and V. aestivalis, that are broadly sympatric. The question of population differentiation onto these sympatric hosts can be considered to be resolved--it has not occurred in spite of a long history of association. Evidence was equivocal, but suggestive of a period of divergence in allopatry prior to reestablishment of contact, for insects associated with another host plant species, V. cinerea, found in both sympatric and parapatric populations. A low level of diversity and placement of samples collected from the grape species V. riparia at the tip of a phylogenetic tree supports the hypothesis that this host has been recently colonized from populations from the Mississippi Valley. A polyphyletic origin for biotype B grape phylloxera was supported: Although most samples collected from vineyards in the introduced range in California had similar haplotypes, they were closely related to natives on V. vulpina from the Atlantic Coast-Piedmont region. All samples collected from vineyards in Oregon and Washington were closely related to natives on V. riparia in the northern United States.     

Predictable risk to native plants in weed biological control

Data on field host use of 112 insects, 3 fungi, 1 mite, and 1 nematode established for biological control of weeds in Hawaii, the continental United States, and the Caribbean indicate that the risk to native flora can be judged reliably before introduction. Virtually all risk is borne by native plant species that are closely related to target weeds. Fifteen species of insects introduced for biological control use 41 native plant species; 36 of which are congeneric with target weeds, while 4 others belong to two closely allied genera. Only 1 of 117 established biological organisms uses a native plant unrelated to the target weed. Thus the elements of protection for the native flora are the selection of weed targets that have few or no native congeners and the introduction of biological control organisms with suitably narrow diets.     

Climate change and plant invasions: restoration opportunities ahead?

Rather than simply enhancing invasion risk, climate change may also reduce invasive plant competitiveness if conditions become climatically unsuitable. Using bioclimatic envelope modeling, we show that climate change could result in both range expansion and contraction for five widespread and dominant invasive plants in the western United States. Yellow starthistle ( Centaurea solstitialis ) and tamarisk ( Tamarix spp.) are likely to expand with climate change. Cheatgrass ( Bromus tectorum ) and spotted knapweed ( Centaurea biebersteinii ) are likely to shift in range, leading to both expansion and contraction. Leafy spurge ( Euphorbia esula ) is likely to contract. The retreat of once-intractable invasive species could create restoration opportunities across millions of hectares. Identifying and establishing native or novel species in places where invasive species contract will pose a considerable challenge for ecologists and land managers. This challenge must be addressed before other undesirable species invade and eliminate restoration opportunities.     

Biology and host specificity ofChamaesphecia hungarica andCh. Astatiformis (Lep.: Sesiidae) two candidates for the biological control of leafy spurge,Euphorbia esula (Euphorbiaceae) in North America

Leafy spurge (Euphorbia esula (s.1.)) is an herbaceous perennial and serious weed of Eurasian origin that has been accidentally introduced into North America. The two European root-boring mothsChamaesphecia hungarica andCh. astatiformis are univoltine and overwinter as mature larvae. Both species have a lower survival rate on leafy spurge than on their field hosts, and thus are not optimal candidates for the biological control of leafy spurge. However, the rate of larval development and larval growth on the target weed and on the two field hosts is nearly the same. The experimental host range of both species is restricted to a few species in the subgenusEsula within the genusEuphorbia. The two species occupy different habitats in the steppe biome and are targeted for similar leafy spurge habitats in North America.     

Molecular biogeography and host relations of a parasitoid fly

Successful geographic range expansion by parasites and parasitoids may also require host range expansion. Thus, the evolutionary advantages of host specialization may trade off against the ability to exploit new host species encountered in new geographic regions. Here, we use molecular techniques and confirmed host records to examine biogeography, population divergence, and host flexibility of the parasitoid fly, Ormia ochracea (Bigot). Gravid females of this fly find their cricket hosts acoustically by eavesdropping on male cricket calling songs; these songs vary greatly among the known host species of crickets. Using both nuclear and mitochondrial genetic markers, we (a) describe the geographical distribution and subdivision of genetic variation in O. ochracea from across the continental United States, the Mexican states of Sonora and Oaxaca, and populations introduced to Hawaii; (b) demonstrate that the distribution of genetic variation among fly populations is consistent with a single widespread species with regional host specialization, rather than locally differentiated cryptic species; (c) identify the more‐probable source populations for the flies introduced to the Hawaiian islands; (d) examine genetic variation and substructure within Hawaii; (e) show that among‐population geographic, genetic, and host song distances are all correlated; and (f) discuss specialization and lability in host‐finding behavior in light of the diversity of cricket songs serving as host cues in different geographically separate populations.