Gerardo Hernández

Bacillus thuringiensis Cry1Ac resistance frequency in tobacco budworm (Lepidoptera: Noctuidae).

ABSTRACT The tobacco budworm, Heliothis virescens (F.) (Lepidoptera Noctuidae), is one of the most important pests of cotton, Gossypium hirsutum L., that has become resistant to a wide range of synthetic insecticides. Cry1Ac-expressing cotton has proven its effectiveness against this insect since its introduction in North America in 1996. However, the constant exposure of tobacco budworm to this protein toxin may result in the development of resistance to it. To estimate the frequency of alleles that confer resistance to a 1.0 µg of Bacillus thuringiensis Cry1Ac diagnostic concentration in field-collected insects, the second generation (F2) of 1,001 single-pair families from seven geographical regions representing 2,202 alleles from natural populations was screened in 2006 and 2007 without finding major resistant alleles. Neonates of 56 single-pair families were able to develop to second instar on the diagnostic concentration in the initial screen, but only seven of these lines did so again in a second confirmatory screen. Minor resistance alleles to Cry1Ac may be quite common in natural populations of H. virescens. Our estimated resistance allele frequencies (0.0036 – 0.0263) were not significantly different from a previously published estimate from 1993. There is no evidence that H. virescens populations have become more resistant to Cry1Ac.

An empirical test of the F2 screen for detection of Bacillus thuringiensis-resistance alleles in tobacco budworm (Lepidoptera: Noctuidae).

Abstract Insects exposed to genetically modified crops expressing Bacillus thuringiensis (Bt) toxins are under intense selection pressure that could result on widespread Bt resistance. Screening for early indications of Bt resistance developing in targeted Lepidoptera is conducted in many of the regions where genetically modified cotton and corn have been commercialized. Heliothis virescens (F.) (Lepidoptera: Noctuidae) has been selected in the laboratory to have a gene for resistance to Cry1Ac. We used this laboratory line to test the assumptions and theoretical predictions related to detection of recessive Bt-resistant alleles in field populations based on a second generation (F2) screen. By creating single-pair families from mating a heterozygous Cry1Ac-resistant moth with a Cry1Ac-susceptible moth, we simulated the most common genotype when Bt-resistance alleles are at low frequency in the field. The second generation (F2) neonates of single-pair families were screened daily with diagnostic concentration bioassays. Cry1Ac-resistant homozygous larvae were detected, but the proportion of resistant larvae was generally below the theoretical expectation of 6.25% and was influenced by the moth F1 sib-mating density and by the day of oviposition of F2 eggs. Logistical considerations such as F1 sib-mating density and F2 neonate screening are important for the successful implementation of a reliable method.

Glucose Sensing With Complex Polymer-Electrolyte Systems

A new concept of glucose sensing is presented, where glucose/glucose oxidase mixtures are embedded in between two swift heavy ion irradiated thin polymer foils. Depending on the structure of the polymer foils, the glucose sensor calibration curves follow different trends. If the foils were etched, thus giving rise to many parallel nanopores through the polymer, the sensors calibration curve follows the trend as given by the conductivity of the glucose/glucose oxidase mixtures. If, however, unetched polymer foils with a high density of latent ion tracks are used, the trend of the calibration curve is reversed. This is tentatively explained by the interaction of the enzymes with the polymer surfaces.

Offspring from Sequential Matings between Bacillus thuringiensis-Resistant and Bacillus thuringiensis-Susceptible Heliothis virescens Moths (Lepidoptera: Noctuidae)

ABSTRACT The tobacco budworm, Heliothis virescens (F.) (Lepidoptera: Noctuidae), is an economically important pest of the Americas. Females of this species copulate multiple times during their lifetimes, and the presence of sperm from multiple males inside them could allow for a diversity of paternal genotypes in the offspring, unless there was complete precedence of sperm from the first mating. if a female copulates with a male that is insecticide-susceptible and another male that is insecticide-resistant, her progeny could vary in their resistance phenotypes. In some cases, this could impact the evolution of insecticide resistance in a population. We designed a series of experiments to determine whether Bacillus thuringiensis susceptibility is maintained when an H. virescens female that is homozygous for a genetically recessive form of B. thuringiensis resistance copulates with a Cry1Ac-susceptible and a Cry1Ac-resistant males. During the lifetime of double-copulated females, a proportion of F1 progeny were Cry1Ac-resistant. This indicates that when a B. thuringiensis-resistant H. virescens female copulates with two males, with one male being resistant to Cry1Ac, some of the progeny will carry resistance to this insecticide. Due to the polyandrous nature of this species, the above-mentioned scenario is not unrealistic; therefore, results from this study may help understand and manage the evolution of B. thuringiensis-resistance in field populations.

Nuclear track-based biosensing: an overview

ABSTRACT An overview on the development of biosensors based on nuclear tracks in solids is presented. A few decades ago it was found that nuclear tracks in solids can be usefully applied for the creation of biosensors. Meanwhile quite a number of different strategies have emerged to fulfill this task which are summarized here. We ourselves have especially concentrated on the ‘Product Enrichment Strategy’, where the enrichment of products of a given analyte by a suitable biochemical reaction within the narrow confinement of etched ion tracks is used to amplify the product’s signal for high-sensitivity biosensing. We also report on the application of nuclear analysis techniques for obtaining a deeper insight into these sensor structures, for the sake of their further optimization. Last not least we also discuss competing nanopore-based biosensing approaches.

Abundance of Heliothis virescens (Lepidoptera: Noctuidae) during spring in northwestern Mississippi.

Abstract. The tobacco budworm, Heliothis virescens (F.), is thought to be a pest that migrates each year from warmer southern latitudes to colder areas where it cannot survive cold temperatures during the winter. Although northwestern Mississippi is south of 40° N latitude, which is recognized as the upper boundary for overwintering survival of the insect, it is commonly believed that increasing abundance during the spring in this area is because of migration of moths from the south. In this study we used a 10-year set of captures of tobacco budworm male moths from pheromone traps and air temperatures from the area, as well as data on development of tobacco budworm in incubators at a range of constant temperatures. Results indicated that moths that produced eggs before the 258th day of the year have time to accumulate the necessary degree days to produce the last generation of that year, while moths that produced eggs between days 258 and 277 give raise to the first spring generation. Moths re productively active after day 277 will not, under the hypothesis of no larval overwintering survival, produce a generation the following year in northwestern Mississippi. Moths reproductively active between days 80 and 124 will produce the generation observed between days 136 and 162; there is a gap between generations, and this gap will separate generations all year. Because we observed generations overlapping in the data from the pheromone traps, there is a possibility that larval overwintering survival might explain the dynamics of the insect in this area.

Paternity allocation in a mutant Heliothis virescens colony

Abstract Tobacco budworm, Heliothis virescens (F.) (Lepidoptera: Noctuidae), females can copulate multiple times creating the possibility for sperm competition. We used a colony lacking wild pigmentation on the wings (albino-type) for an experiment in which females double mated. Females copulated 2 days apart with two, 2-day-old males, one albino-type and one wild-type, or in the opposite sequence. A third of the females produced offspring from the first mate, and this group was significantly biased toward producing albino-type compared to wild-type progeny. A fourth of the females produced offspring from the second male exclusively and within this group was a significant bias toward wild-type compared to albino-type progeny. Almost half of the females produced offspring sired in equal proportions by both males simultaneously or in alternated paternities throughout all the reproductive life. These results suggest that regardless of the order in which moths mated, wild-type sperm had potential superior competitiveness. Therefore, sperm precedence is not the main driving force behind the paternity allocation mechanism in this strain of tobacco budworm.