Genetic Structuring of Western Corn Rootworm (Coleoptera: Chrysomelidae) Populations in the United States Based on Microsatellite Loci Analysis

The western corn rootworm (Diabrotica virgifera virgifera LeConte) (Coleoptera: Chrysomelidae) is a major corn insect pest in North America and is spreading in Europe. Seven polymorphic microsatellite loci were surveyed to characterize genetic structuring of D. v. virgifera populations, based on 595 individuals sampled from 10 locations across nine U.S. states (western Texas and Kansas to New York and Delaware). All populations showed high levels of genetic diversity, with mean allelic diversity ranging from 7.3 to 8.6, and mean expected heterozygosity ranging from 0.600 to 0.670. D. v. virgifera populations exhibited little genetic differentiation as a whole across the geographic range sampled, with a global FSTof only 0.006. Pairwise FST estimates also revealed little genetic differentiation among populations. Most pairwise FST values were nonsignificant, except for those estimated between the Texas population and all others. There was a positive correlation between genetic distance and geographic distance as a whole, but no significant correlation for populations from Kansas to the east coast. There was no evidence for a genetic bottleneck in any D. v. virgifera population sampled. Phylogenetic and principal component analyses support the picture of high genetic similarity over much of the United States. Although high migration rates could produce the same pattern and cannot be ruled out, it seems more likely that the D. v. virgifera populations sampled have had insufficient time for substantial genetic structuring to develop since its recent eastward range expansion from the Great Plains that began ≈50 yr ago

Isolation and characterization of polymorphic microsatellite loci in the boll weevil, Anthonomus grandis Boheman (Coleoptera: Curculionidae)

The boll weevil (Anthonomus grandis Boheman) is a major insect pest of cotton in North America. Dispersal activity poses a threat to ongoing eradication efforts in the US, but little is known about the frequency of long-distance migration. Nuclear molecular markers are needed to assess gene flow in relation to geographical distance. A biotin-enrichment strategy was employed to develop microsatellite markers for the boll weevil. Of 23 loci isolated, 14 were polymorphic with three to 10 alleles per locus. Twelve of the polymorphic loci showed Mendelian inheritance and are likely to be useful in population genetics studies

Genetic structuring of boll weevil populations in the US based on RAPD markers

Randomly amplified polymorphic DNA (RAPD) analysis was performed to infer the magnitude and pattern of genetic differentiation among boll weevil populations from eighteen locations across eight US states and north-east Mexico. Sixty-seven reproducible bands from six random primers were analysed for genetic variation within and between weevil populations. Genetic and geographical distances among all populations were positively correlated, reflecting a pattern of isolation by distance within a larger metapopulation. Gene flow between south-central, western and eastern regions is limited, but migration between locations within regions appears to be relatively frequent up to distances of ∼300–400 km. However, estimates of effective migration were much lower than those estimated from mtDNA-RFLP data reported previously

Population genetics strategies to characterize long-distance dispersal of insects

Population genetics strategies offer an alternative and powerful approach for obtaining information about long-distance movement, and have been widely used for examining patterns and magnitude of insect dispersal over geographic and temporal scales. Such strategies are based on the principle that genetic divergence between local populations reflects the interplay between genetic drift and gene flow, and thus can function as an indicator of dispersal capacity. Relatively new approaches for inferring population history are widely applicable for documenting introduction routes of invasive or quarantine species. These approaches are based on genetic variability calculated from changes in gene frequency of subpopulations, measured using molecular genetic markers. Inferences from population genetics can supplement and corroborate conventional observational approaches for characterizing insect dispersal and have provided important clues to many questions raised in the field of behavior and ecology of insects. Here, we summarize our work on the boll weevil as a case study to illustrate the kinds of information on dispersal capacity and dispersal patterns that can be obtained from population genetics techniques that would be difficult or impossible to acquire in other ways. Then we provide examples of how the molecular markers and population genetics tools have been applied to answer immediate questions of relevance to eradication program managers. Though the latter are idiosyncratic to this particular pest, they demonstrate the kinds and range of problems that can be addressed in other systems through application of population genetics strategies

Coordinated Diabrotica Genetics Research: Accelerating Progress on an Urgent Insect Pest Problem

This article is from American Entomologist 52 (2006): 90.</p

Evaluation of Tolerance to Bacillus thuringiensis Toxins Among Laboratory-Reared Western Bean Cutworm (Lepidoptera: Noctuidae)

The western bean cutworm, Striacosta albicosta (Smith) (Lepidoptera: Noctuidae), is a destructive insect pest of dry beans and corn within its native range of western Nebraska and eastern Colorado. However, since the initiation of an eastward range expansion of S. albicosta in the late 1990s, economic damage has been observed in the Midwest, and the species has now reached the Atlantic Coast and Quebec. Economic damage to corn occurs by larval feeding on ears, which is not controlled by commercial transgenic hybrids that express Bacillus thuringiensis(Bt) Cry1Ab, but partial control is observed by corn varieties that express Cry1 F toxins. Inadequate protocols for laboratory rearing of S. albicosta have hindered controlled efficacy experimentation in the laboratory and field.Wereport an S. albicosta rearing methodology used to maintain a laboratory colony for 12 continuous generations. Rearing procedures were adapted for Bt toxin diet-overlay assays, revealing that S. albicosta larvae exposed to Bt toxin for 14 d were insensitive to Cry1Ab concentrations up to 25,000 ng/cm2. In contrast, neonates exposed to Cry1 F toxin at ≥250 ng/cm2, showed reduced developmental rates, with estimated effective concentrations of EC50=1,187.5 ng/cm2 and EC95=10,000.5 ng/cm2. The ability to mass produce this pest insect will enhance fundamental research, including evaluation of control tactics and toxin susceptibility

Managing resistance to Bt crops in a genetically variable insect herbivore, Ostrinia nubilalis

To slow the resistance evolution of the European corn borer (ECB) to Cry proteins expressed in transgenic Bacillus thuringensis (Bt) corn, the United States Environmental Protection Agency (EPA) has adopted an insect resistance management (IRM) plan that relies on a “high dose/refuge” strategy. However, this IRM plan does not consider possible ecological differences between the two ECB pheromone races (E and Z). Using carbon isotope analysis, we found that unstructured (non-corn) refuges contribute more to E race (18%) than to Z race (4%) populations of ECB in upstate New York (USA). Furthermore, feeding on non-corn hosts is associated with decreased body mass and reduced fecundity. We also show that the geographic range of E-race ECB is restricted within the range of the Z race and that E-race ECB are increasingly dominant in regions with increasing non-corn habitat. While the proportion of E-race ECB developing in unstructured refuges is higher than previously assumed, low rates of unstructured refuge use by the Z race, evidence for reduced fecundity when reared on non-corn hosts, and complete sympatry within the E race range all argue against a relaxation of current IRM refuge standards in corn based on alternative-host use. We also discuss implications of this research for integrated pest management in vegetables and IRM in Bt cotton

Polymorphic microsatellite loci from the western corn rootworm (Insecta: Coleoptera: Chrysomelidae) and cross-amplification with other Diabrotica spp

Corn rootworms (Diabrotica spp.) make up the major insect pest complex of corn in the US and Europe, and there is a need for molecular markers for genetics studies. We used an enrichment strategy to develop microsatellite markers from the western corn rootworm (Diabrotica virgifera virgifera). Of 54 loci isolated, 25 were polymorphic, and of these, 17 were surveyed for variability in 59 wild individuals. In addition, the potential for cross-amplification of these microsatellites was surveyed for Mexican, northern, and southern corn rootworms. Nine microsatellite loci showed Mendelian inheritance and are likely to be useful in population genetics studies

Molecular genetic variation of boll weevil populations in North America estimated with microsatellites: Implications for patterns of dispersal

The boll weevil (Anthonomus grandis Boheman) is an insect pest of cotton that underwent a well-documented range expansion across the southeastern U.S. from Mexico beginning about 110 years ago. Eleven microsatellite loci were surveyed to infer the magnitude and pattern of genetic differentiation among boll weevil populations from 18 locations across eight U.S. states and northeast Mexico. Estimates of genetic diversity (allelic diversity and heterozygosity) were greater in Southern than Northern populations, and were greater in the west than the east among Northern populations. Boll weevil populations were genetically structured as a whole across the geographic range sampled, with a global F ST of 0.241. South-central populations exhibit classic isolation by distance, but evidence suggests that populations within the Eastern and Western regions have not yet reached genetic equilibrium. Gene flow appears to be relatively high among populations within the Eastern region. Population assignment data and estimates of gene flow indicate that migration between locations separated by \u3c 300 km is frequent. The database of microsatellite genotypes generated in this study now makes it possible, through population assignment techniques, to identify the most likely geographic source of a boll weevil reintroduced to an eradication zone, which will help action agencies decide the most appropriate mitigation response

Inheritance of an extended diapause trait in the Northern corn rootworm, Diabrotica barberi (Coleoptera: Chrysomelidae)

Diapause is an adaptive trait that delays development or reproduction under unfavourable circumstances. The northern corn rootworm,Diabrotica barberi Smith & Lawrence, an important maize, Zea mays L., pest in the Diabroticite species complex, overwinters in diapause during the egg stage. Some NCR populations are adapted to crop rotation by expressing an extended diapause (ED) trait that delays embryonic development for 2 years. This ED trait has increased in frequency and geographic distribution since first reported in Illinois in 1932. Reciprocal single pair crosses among beetles from a laboratory colony with the ancestral 1-year diapause trait and field collected beetles with the 2-year ED trait indicated that ED females laid significantly more ED eggs than did females with the 1-year diapause trait regardless of male genotype. The ED trait was highly heritable [realized heritability (h2) = 0.698 ± 0.314], with genetic dominance (D) of the trait strongly influenced by female genotype. Selection of the ED trait and maintenance of polymorphic diapause phenotypes within maize-soybean cropping systems is discussed in relation to response to a fluctuating environment and as a potentially advantageous life history adaptation