Binding energy of localized biexcitons in quantum wells

A variational calculation of the ground state energy of a biexciton in a GaAs/AlGaAs quantum well is presented. The well width fluctuations leading to trapping of the biexcitons are modeled by a parabolic potential. The results obtained for different well widths are compared with recent experimental data. Good agreement is obtained both for the biexciton binding energy and for the Haynes factor. We find that the structure of a biexciton is similar to the one of the H_2 molecule.Comment: 9 pages, 4 figure

Monitoring insecticide resistance in the bollworm Helicoverpa armigera (Hübner) from 1998 to 2002 in Cote d'Ivoire, West Africa

Helicoverpa armigera (Hübner) is the major insect pest of the cotton crop in West Africa. Populations recently developed resistance to pyrethroids via the overproduction of oxidases. To control this pest, a resistance management strategy was applied in the major West African cotton growing countries from 1999 onwards. In Côte d'Ivoire insecticide resistance of H. armigera was monitored in field strains from 7998 to 2002 using insecticide coated vial tests and topical applications of insecticides with third-instar larvae. Vial tests with discriminating doses of cypermethrin were used directly on field-collected larvae at the end of the cotton season. The percentage of resistant larvae varied around a mean of 67% with 6 [mu]g cypermethrin/vial and around 13% with 30 [mu]g/ vial in different years and places. Topical applications with various insecticides were used in the laboratory on the first generation of populations collected from cotton (Gossypium hirsutum), tomato (Lycopersicum esculentum) or a strongly infested ornamental flower (Antirrhinum majus). The resistance factors calculated from dose-mortality regressions varied from 5 to 38 with deltomethrin. In the Bouaké area they were always higher for strains collected from cotton at the end of the season. Concerning the pyrethroid alternatives currently used in Côte d'Ivoire, no reduction in susceptibility in the cotton field strains was detected for endosulfan or profenofos showing their potential for use in a resistance management strategy. These results suggest a relative stability of the pyrethroid resistance in H. armigera from 1998 to 2002 and confirm the success of the resistance management strategy. (Résumé d'auteur

Sea state monitoring using coastal GNSS-R

We report on a coastal experiment to study GPS L1 reflections. The campaign was carried out at the Barcelona Port breaker and dedicated to the development of sea-state retrieval algorithms. An experimental system built for this purpose collected and processed GPS data to automatically generate a times series of the interferometric complex field (ICF). The ICF was analyzed off line and compared to a simple developed model that relates ICF coherence time to the ratio of significant wave height (SWH) and mean wave period (MWP). The analysis using this model showed good consistency between the ICF coherence time and nearby oceanographic buoy data. Based on this result, preliminary conclusions are drawn on the potential of coastal GNSS-R for sea state monitoring using semi-empirical modeling to relate GNSS-R ICF coherence time to SWH.Comment: All Starlab authors have contributed significantly; the Starlab author list has been ordered randomly. Submitted to GR

Spinosad : a new chemistry to solve the pyrethroid resistance of Helicoverpa armigera (Hübner) in West Africa

The major insect pest of cotton and vegetable in West Africa, the bollworm Helicoverpa armigera (Hübner), recently developed resistance to pyrethroids via the overproduction of oxidases, leadmg to control failures in the field. One way to overcome this problem is restriction in pyrethroid use. Tested for six years in cotton, spinosad (Laser®) proved to be efficient for the control of H. armigera, Furthermore, no cross-resistance has been detected in a selected pyrethroid-resistant strain. Spinosad can be recommended to manage bollworm resistant populations in West Africa. (Résumé d'auteur

The Eddy Experiment: accurate GNSS-R ocean altimetry from low altitude aircraft

During the Eddy Experiment, two synchronous GPS receivers were flown at 1 km altitude to collect L1 signals and their reflections from the sea surface for assessment of altimetric precision and accuracy. Wind speed (U10) was around 10 m/s, and SWH up to 2 m. A geophysical parametric waveform model was used for retracking and estimation of the lapse between the direct and reflected signals with a 1-second precision of 3 m. The lapse was used to estimate the SSH along the track using a differential model. The RMS error of the 20 km averaged GNSS-R absolute altimetric solution with respect to Jason-1 SSH and a GPS buoy measurement was of 10 cm, with a 2 cm mean difference. Multipath and retracking parameter sensitivity due to the low altitude are suspected to have degraded accuracy. This result provides an important milestone on the road to a GNSS-R mesoscale altimetry space mission.Comment: All Starlab authors have contributed significantly; the Starlab Author list has been ordered randoml

Devenir de deux insecticides dans le sol d'une culture cotonnière en Côte d'Ivoire

Des études de résidus d'insecticides montrent que les teneurs sont faibles dans la fibre et davantage encore dans les graines ou l'huile de coton, mais qu'elles peuvent être importantes dans les déchets d'égrenage qui peuvent représenter une source de contamination s'ils sont épandus sur le sol. Les études de résidus dans les sols et l'eau des régions cotonnières sont rares en Afrique de l'Ouest. Afin d'évaluer les teneurs résiduelles d'insecticides dans le sol, nous avons entrepris l'étude du devenir du triazophos et de la deltamethrine sur la station expérimentale de l'IDESSA, Institut des Savanes de Bouak

Knowledge-Attitudes-Practices About Malaria Among Communities in Southern Benin

Malaria still remains the main public health problem in Benin. We explored the determinants that influenced malaria treatment as well as protective behaviors, to generate a framework of useful ideas as alternative strategies against malaria. A cross-sectional survey of the knowledge, attitudes and practices (KAP) was conducted at Hozin, Vakon and Agblangandan districts in southern region of Benin. Descriptive statistics were computed and mixed logistic regression helped evaluating the relationship between frequency of each category of severity of malaria and sex group, educational level, treatment, means of self-protection against mosquitoes and identification of the cause of malaria. A significant proportion 750 (81.3%) (p<0.001) of participants stated that malaria was caused by mosquitoes. The respondents who mentioned sun as the cause of malaria, have trivialized more malaria in a proportion of about 59.30% (OR=2.67 [95% CI 1.61-4.44]) followed by those who have reported the cause of body weakness (43.68%) (OR=2.97 [95% CI 1.68-5.28]). Poor knowledge justifies the trivialization of the disease and poor management of malaria control means. National Malaria Control Programs should improve access to education, especially for women and could help improving prevention and control behaviours against malaria in communities

Scirtothrips dorsalis hood (thysanoptera, thiripidae), un nouveau ravageur du cotonnier en Côte d'Ivoire

L'objectif de cet article est de présenter le cycle biologique sur cotonnier de #S. dorsalis#, les dégâts occasionnés sur cette culture ainsi que leur incidence sur la production. La dynamique de population de ce ravageur est décrite sur la station de Bouaké en 1997 ainsi que l'effet de la pluviosité sur la régulation des population