The objective was to determine the magnitude of resistance of A. quitensis H.B.K. biotypes to imazethapyr and chlorimuron-ethyl. Weed biotypes were collected at Zavalla, General Baldissera, Marcos Juárez y Las Rosas. The herbicides were applied at 1/8x, 1/4x, 1/2x, 1x, 5x, 10x and 20x the suggested rate. Weed mortality and reduction of biomass related with untreated plants were evaluated. Herbicides concentrations required to reduce growth by 50% (GR50) were determined. Resistance ratio (GR50 resistant/GR50 susceptible ) was calculated to indicate the degree of resistance. Zavalla showed high susceptibility, with 1/2 of the suggested rate of both herbicides mortality reached 95%. General Baldissera was cross-resistant; with rates 20 times superior to the recommended field use rate mortality was 57.5% and 20% for imazethapyr and clhorimuron-ethyl, respectively. Resistance ratios of this biotype compared with Zavalla were 165 and 246 for imazethapyr and chlorimuron-ethyl, respectively. Las Rosas y Marcos Juárez were resistant to imazethapyr but showed high susceptibility to chlorimuron-ethyl. The resistance patterns showed by the biotypes could be associated with differences in the selection pressure at each location.

El objetivo de este trabajo fue establecer la existencia de biotipos de A. quitensis H.B.K. resistentes a imazetapir y clorimurón-etil. Se utilizaron semillas recolectadas en las localidades de Zavalla, General Baldissera, Marcos Juárez y Las Rosas. Las dosis de herbicidas utilizadas fueron 1/8x, 1/4x, 1/2x, 1x, 5x, 10x y 20x de la dosis de uso recomendada. Se determinó la mortalidad y biomasa de las plantas tratadas. Se calcularon las dosis de herbicidas requeridas para reducir en un 50% la biomasa de las plantas de la maleza (GR50) y se estimó la relación entre GR50 del biotipo resistente y GR50 del susceptible (factor de resistencia). El biotipo Zavalla resultó muy susceptible; con la mitad de la dosis de uso de ambos herbicidas la mortalidad fue 95%. El biotipo General Baldissera presentó resistencia cruzada; con dosis 20 veces superiores a las recomendadas, la mortalidad fue 57,5% y 20% para imazetapir y clorimurón-etil, respectivamente. El factor de resistencia de este biotipo respecto a Zavalla fue 165 y 246 para imazetapir y clorimurón-etil, respectivamente. Los biotipos Las Rosas y Marcos Juárez fueron resistentes a imazetapir y sin embargo resultaron muy susceptibles a clorimurón-etil. Las diferencias en los patrones de resistencia estarían asociadas con distintos niveles de presión de selección en las poblaciones analizadas.

The food preference of Notiobia cupripennis among seeds of eight weedy spring-summer species (Sp-Su) and six autumn-winter (Av-Wi) species more common in the agroecosystems of the South of Santa Fe Province, Argentina, was estimated in this study. The seeds of the species of each group were offered simultaneously and in pairs to adults individually placed. The relation (damaged seeds/available seeds) x 100 of each species was considered as a food preference index. The data were analysed by analysis of variance and Duncan's Multiple Range Test and by a paired difference test, previous arcsine-square-root transformation. The seeds of the Av-Wi species were more damaged (27.65%) than the Sp-Su ones (10.40%). Highly significative differences in the percentage of damaged seeds among the different species were found; dicotyledones were preferred over gramines. The food preference ranking was maintained when the insects were forced to choose between the seeds of two weed species. The food preference would be attributed to differences in seed size, tegument toughness and chemical composition of the seeds in relation to the ability of handling the seeds and the nutritional requirements of the insect.

En este trabajo se estimó la preferencia alimentaria de Notiobia cupripennis por semillas de ocho especies de malezas primavero-estivales (Pr-Es) y de seis especies otoño-invernales (Ot-In) más comunes en los agroecosistemas del Sur de la provincia de Santa Fe, Argentina. Las semillas de las especies de cada grupo fueron ofrecidas en conjunto y de a pares a adultos confinados individualmente. La relación (semillas dañadas/semillas disponibles) x 100 de cada especie fue considerada como índice de preferencia alimentaria. Los resultados fueron analizados mediante análisis de la variancia y test de rangos múltiples de Duncan y mediante un test de diferencia de medias, previa transformación arcoseno de los datos. Las semillas de las especies Ot-In fueron más dañadas (27,65%) que las Pr-Es (10,40%). Se encontraron diferencias altamente significativas en el porcentaje de semillas dañadas entre las diferentes especies; las dicotiledóneas fueron más preferidas que las gramíneas. El orden de preferencia alimentaria se mantuvo cuando los insectos fueron obligados a elegir entre semillas de dos especies de malezas. La preferencia alimentaria se atribuiría a diferencias en el tamaño, la consistencia del tegumento y la composición química de las semillas en relación con la habilidad de manipulación de las mismas y los requerimientos nutricionales del insecto.

Foram conduzidos experimentos de campo durante quatro anos agrícolas em Zavalla e Oliveros, Argentina, com o objetivo de otimizar o controle químico de capim-massambará (Sorghum halepense (L.) Pers.), usando o modelo do calendário térmico em comparação com outros critérios de decisão (altura da planta daninha ou dias após a semeadura). O efeito do herbicida foi determinado por avaliação visual, densidade de perfilhos originados de rebrota de rizomas e de touceira e caule, e rendimento de grão de soja. As aplicações segundo o critério do calendário térmico mostraram o melhor controle. A altura da planta daninha na primeira época evidenciou pouca variabilidade entre tratamentos, mas foi muito variável pela segunda e terceira época, atingindo, em alguns casos, valores maiores que 120 cm. Os rendimentos da cultura na primeira e segunda época não diferiram entre si e superaram o rendimento da terceira época. A maior densidade de rebrota de rizomas foi observada na primeira época de aplicação, e o maior rebroto de touceira e caule foi determinado na terceira época. Os parâmetros associados com a eficiência mostraram o melhor comportamento na segunda época. Contudo, a altura da planta nesse momento pode interferir com a aplicação do herbicida e a variabilidade mostrada por esse parâmetro salienta o risco de determinar a época de controle usando só um critério de decisão.

Field studies were established in Zavalla and Oliveros, Argentina, during four years in order to optimize Johnsongrass (Sorghum halepense (L.) Pers.) chemical control by means of the thermal calendar model in comparison with other criteria (weed height or days after sowing). The effect of three application dates of postemergence herbicides was determined by visual control, density of tillers originated from rhizome bud regrowth, and from crown and shoot bud regrowth, and soybean yield. Following the thermal calendar model criterion, applications during the second date afforded the best control. Weed height for the first date showed little variability between experiments but was highly variable in the second and third application dates, achieving in some cases values greater than 120 cm. For all years, no significant differences were detected for crop yield between the first and second application dates, and yields were always lower for the third date. The greatest rhizome bud regrowth was observed for the earliest application date and the highest crown and shoot bud regrowth was determined for the last application date. Parameters associated with control efficiency showed the best behaviour for the second date. However, plant height at this moment may interfere with herbicide application and the variability exhibited by this parameter highlights the risk of determining control timing using only one decision criterion.

The objective was to evaluate the losses of the superficial bank of Amaranthus quitensis seeds, due to insect predation, in a soybean crop and in the subsequent fallow, in two tillage systems. Experiments were conducted during 1994/95 and 1995/96. To estimate predation rates, trays covered with wire meshes to prevent rodent predation, and with fine sheer net (tulle) in the control treatment were used; 100 weed seeds were sown in each tray, and the number of remaining seeds was registered every 15 days. Pitfall traps were used to identify insects species occurring in the field and to estimate their abundance. The carabid Notiobia cupripennis was captured in pitfall traps, the higher population found during March (4.5 and 5.8 insect/trap in conventional tillage and 2.7 and 3.3 insect/trap in no-tillage). The highest predation rates was also registered in March (5.6% and 8% in conventional tillage and 2.7% and 3.8% in no-tillage) and both systems were significantly different as to abundance and predation. No significant differences were found from April to June, and the number of insects and predation rates decreased. There was a positive correlation between the predation rates and the values of insect abundance in both years.

El objetivo fue evaluar las pérdidas del banco superficial de semillas de Amaranthus quitensis H.B.K. (yuyo colorado) debidas a la predación por insectos en un cultivo de soja y en el barbecho posterior, en dos sistemas de laboreo. Los experimentos se realizaron durante las campañas 94/95 y 95/96. Para calcular la tasa de predación se emplearon bandejas cubiertas con tejido para evitar el ingreso de roedores y con tela de tul en los tratamientos testigos; en cada una se sembraron 100 semillas de la maleza y cada 15 días se registró el número de semillas remanentes. Para determinar los insectos presentes y su abundancia se emplearon trampas "pitfall". Entre los insectos capturados se encontró el carábido Notiobia cupripennis, su mayor abundancia se registró en marzo (4,5 y 5,8 insectos/trampa en convencional y 2,7 y 3,3 insectos/trampa en siembra directa), coincidiendo con las tasas de predación más altas (5,6% y 8% en convencional y 2,7% y 3,8% en siembra directa); tanto en abundancia como en predación se observaron diferencias significativas entre ambos sistemas. A partir de este mes, las diferencias no fueron significativas, el número de insectos y la tasa de predación disminuyeron. En ambos años existió una correlación positiva entre estas variables.