Transgenic maize engineered to express insecticidal proteins from the bacterium Bacillus thuringiensis (Bt) has become widely adopted in U.S. agriculture. In 2009, Bt maize was planted on more than ...22.2 million hectares, constituting 63% of the U.S. crop. Using statistical analysis of per capita growth rate estimates, we found that areawide suppression of the primary pest Ostrinia nubilalis (European corn borer) is associated with Bt maize use. Cumulative benefits over 14 years are an estimated $3.2 billion for maize growers in Illinois, Minnesota, and Wisconsin, with more than $2.4 billion of this total accruing to non-Bt maize growers. Comparable estimates for Iowa and Nebraska are $3.6 billion in total, with $1.9 billion for non-Bt maize growers. These results affirm theoretical predictions of pest population suppression and highlight economic incentives for growers to maintain non-Bt maize refugia for sustainable insect resistance management.
In this work, we study experimentally the impact of different gate dielectric stacks on the subthreshold behavior of cryogenic MOSFETs. While in room temperature devices, silicon nitride deteriorates ...the off-state of MOSFETs it turns out that at cryogenic temperatures an appropriately thin, grown silicon nitride layer in combination with a high-k gate dielectric counteracts the saturation of the inverse subthreshold slope and inflection phenomena. As a result, steep slope cryogenic MOSFETs with strongly improved subthreshold behavior are demonstrated.
A collaborative research effort by scientists in several states and in Canada has produced information to develop a formal risk assessment of the impact of Bt corn on monarch butterfly (Danaus ...plexippus) populations. Information was sought on the acute toxic effects of Bt corn pollen and the degree to which monarch larvae would be exposed to toxic amounts of Bt pollen on its host plant, the common milkweed, Asclepias syriaca, found in and around cornfields. Expression of Cry proteins, the active toxicant found in Bt corn tissues, differed among hybrids, and especially so in the concentrations found in pollen of different events. In most commercial hybrids, Bt expression in pollen is low, and laboratory and field studies show no acute toxic effects at any pollen density that would be encountered in the field. Other factors mitigating exposure of larvae include the variable and limited overlap between pollen shed and larval activity periods, the fact that only a portion of the monarch population utilizes milkweed stands in and near cornfields, and the current adoption rate of Bt corn at 19% of North American corn-growing areas. This 2-year study suggests that the impact of Bt corn pollen from current commercial hybrids on monarch butterfly populations is negligible.
Laboratory tests were conducted to establish the relative toxicity of Bacillus thuringiensis (Bt) toxins and pollen from Bt corn to monarch larvae. Toxins tested included Cry1Ab, Cry1Ac, Cry9C, and ...Cry1F. Three methods were used: (i) purified toxins incorporated into artificial diet, (ii) pollen collected from Bt corn hybrids applied directly to milkweed leaf discs, and (iii) Bt pollen contaminated with corn tassel material applied directly to milkweed leaf discs. Bioassays of purified Bt toxins indicate that Cry9C and Cry1F proteins are relatively nontoxic to monarch first instars, whereas first instars are sensitive to Cry1Ab and Cry1Ac proteins. Older instars were 12 to 23 times less susceptible to Cry1Ab toxin compared with first instars. Pollen bioassays suggest that pollen contaminants, an artifact of pollen processing, can dramatically influence larval survival and weight gains and produce spurious results. The only transgenic corn pollen that consistently affected monarch larvae was from Cry1Ab event 176 hybrids, currently <2% corn planted and for which re-registration has not been applied. Results from the other types of Bt corn suggest that pollen from the Cry1Ab (events Bt11 and Mon810) and Cry1F, and experimental Cry9C hybrids, will have no acute effects on monarch butterfly larvae in field settings.
The density of corn pollen on leaves of milkweed plants inside and outside of cornfields was measured in several studies from different localities. The purpose was to obtain a representative picture ...of naturally occurring pollen densities to provide a perspective for laboratory and field studies of monarch larvae feeding on milkweed leaves with Bt corn pollen. Pollen density was highest (average 170.6 grains per cm2) inside the cornfield and was progressively lower from the field edge outward, falling to 14.2 grains per cm2at 2 m. Inside the cornfield, and for each distance from the field edge, a frequency distribution is presented showing the proportion of leaf samples with different pollen densities. Inside cornfields, 95% of leaf samples had pollen densities below 600 grains per cm2and the highest pollen density observed was 1400 grains per cm2, which occurred in a study with a rainless anthesis period. All other studies had rainfall events during the anthesis period. A single rain event can remove 54-86% of the pollen on leaves. Leaves on the upper portion of milkweed plants, where young monarch larvae tend to feed, had only 30-50% of the pollen density levels of middle leaves.
Survival and growth of monarch larvae, Danaus plexippus (L), after exposure to either Cry1Ab-expressing pollen from three Bacillus thuringiensis (Bt) corn (Zea mays L.) events differing in toxin ...expression or to the insecticide, λ-cyhalothrin, were examined in field studies. First instars exposed to low doses (≈22 grains per cm2) of event-176 pollen gained 18% less weight than those exposed to Bt11 or Mon810 pollen after a 5-day exposure period. Larvae exposed to 67 pollen grains per cm2on milkweed leaves from within an event-176 field exhibited 60% lower survivorship and 42% less weight gain compared with those exposed to leaves from outside the field. In contrast, Bt11 pollen had no effect on growth to adulthood or survival of first or third instars exposed for 5 days to ≈55 and 97 pollen grains per cm2, respectively. Similarly, no differences in larval survivorship were observed after a 4-day exposure period to leaves with 504-586 (within fields) or 18-22 (outside the field) pollen grains per cm2collected from Bt11 and non-Bt sweet-corn fields. However, survivorship and weight gain were drastically reduced in non-Bt fields treated with λ-cyhalothrin. The effects of Bt11 and Mon810 pollen on the survivorship of larvae feeding 14 to 22 days on milkweeds in fields were negligible. Further studies should examine the lifetime and reproductive impact of Bt11 and Mon810 pollen on monarchs after long-term exposure to naturally deposited pollen.
Electrostatically-defined quantum dots (QDs) in silicon are an attractive platform for quantum computation. Localized single electron spins define qubits and provide excellent manipulation and ...read-out fidelities. We propose a scalable silicon-based qubit device that can be fabricated by industry-compatible processes. The device consists of a dense array of QDs localized along an etched silicon nano-ridge. Due to its lateral confinement, a simple dense array of metallic top-gates forms an array of QDs with controllable tunnel-couplings. To avoid potential fluctuations because of roughness and charged defects at the nano-ridge sidewall, the cross-section of the nano-ridge is trapezoidal and bounded by atomically-flat {111} facets. In addition to side-gates on top of the low-defect oxidized {111} facets, we implement a global back-gate facilitated by the use of silicon-on-insulator. The most relevant process modules are demonstrated experimentally including anisotropic wet-etching and local oxidation of the silicon nano-ridge, side-gate formation with chemical-mechanical polishing, and top-gate fabrication employing the spacer process. According to electrostatic simulations, our device concept allows forming capacitively-coupled QD double-arrays or adjacent charge detectors for spin-readout. Defining a logical qubit or realizing a single electron conveyor for mid-range qubit-coupling will be future applications.
Crops producing toxins derived from the bacterium Bacillus thuringiensis (Bt) are widely planted to manage insect pests including western corn rootworm, Diabrotica virgifera virgifera LeConte ...(Coleoptera: Chrysomelidae), which is a significant pest of maize in the United States and Europe. However, the widespread use of Bt maize places intense selection pressure on pest populations to evolve resistance, and field‐evolved resistance to Bt maize by western corn rootworm has been documented in the United States. In conjunction with non‐Bt refuges, fitness costs of Bt resistance can delay resistance evolution. Fitness costs arise in the absence of Bt toxin when individuals with resistance alleles have lower fitness than Bt‐susceptible genotypes. We quantified the level of resistance and fitness costs of resistance for a strain of western corn rootworm with laboratory‐selected resistance to transgenic maize (Zea mays L.) producing Bt toxin Cry3Bb1. Survival to adulthood on Cry3Bb1 maize was more than twofold higher for resistant insects vs. susceptible insects, which is similar to the magnitude of resistance first observed in the field. Fitness costs were measured in two experiments; the first used maize hybrids and the second used inbred lines. The experiment with maize hybrids compared resistant and susceptible strains while the experiment with maize inbreds compared resistant, susceptible and heterozygous genotypes. The only non‐recessive fitness cost detected (i.e. cost affecting heterozygotes) was for adult size. Recessive fitness costs (i.e. costs affecting the resistant strain) were observed for developmental rate, female survival and egg viability. However, when reared on non‐Bt maize, the resistant strain also displayed higher fecundity, higher survival for males and greater adult longevity compared to the susceptible strain. These results suggest that resistance to Bt maize by western corn rootworm may not impose substantial fitness costs, and consequently, may evolve quickly and persist once present.
The movement of miniature inverted repeat transposable elements (MITEs) modifies genome structure and function. We describe the microsatellite-associated interspersed nuclear element 2 (MINE-2), that ...integrates at consensus WTTTT target sites, creates dinucleotide TT target site duplications (TSDs), and forms predicted MITE-like secondary structures; a 5' subterminal inverted repeat (SIR; AGGGTTCCGTAG) that is partially complementary to a 5' inverted repeat (IR; ACGAAGCCCT) and 3'-SIRs (TTACGGAACCCT). A (GTCY)n microsatellite is hitchhiking downstream of conserved 5'MINE-2 secondary structures, causing flanking sequence similarity amongst mobile microsatellite loci. Transfection of insect cell lines indicates that MITE-like secondary structures are sufficient to mediate genome integration, and provides insight into the transposition mechanism used by MINE-2s.
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