Jordi Izquierdo

A Thermal Time Model to Predict Corn Poppy (Papaver rhoeas) Emergence in Cereal Fields

Abstract Corn poppy is the most abundant broad-leaved weed in winter cereals of Mediterranean climate areas and causes important yield losses in wheat. Knowledge of the temporal pattern of emergence will contribute to optimize the timing of control measures, thus maximizing efficacy. The objectives of this research were to develop an emergence model on the basis of soil thermal time and validate it in several localities across Spain. To develop the model, monitoring of seedling emergence was performed weekly during the growing season in a cereal field located in northeastern Spain, during 3 yr. Cumulative thermal time from sowing date was used as the independent variable for predicting cumulative emergence. The Gompertz model was fitted to the data set of emergences. A base temperature of 1.0 C was estimated through iteration for maximum fit. The model accounted for 91% of the variation observed. Model validation in several localities and years showed general good performance in predicting corn poppy seedling emergence ( values ranging from 0.64 to 0.99 and root-mean-square error from 4.4 to 24.3). Ninety percent emergence was accurately predicted in most localities. Results showed that the model performs with greater reliability when significant rainfall (10 mm) occurs within 10 d after crop sowing. Complemented with in-field scouting, it may be a useful tool to better timing control measures in areas that are homogeneous enough regarding climate and crop management. Nomenclature: Corn poppy; Papaver rhoeas L.

Spatial distribution of weed diversity within a cereal field

Weeds are key components of agroecosystems because they support biological diversity within crop fields. Ecological indicators of weed diversity are usually assessed on a field scale, but weeds are distributed unevenly within fields. Here, we explore spatial distribution of diversity in weed seed banks, and how a failure of grass weed control may affect biodiversity indicators. We studied spatial distribution and stability of species richness, Shannon’s diversity index, and Pielou’s evenness index of a weed seed bank, site-specifically and for the entire field, over three years in a commercial winter wheat field regularly treated with narrow- and broad-leaved herbicides. 254 soil samples were taken on 10 m × 10 m grids at the beginning of each season. Seeds were identified by germination in a greenhouse and indices were assessed for each point. The spatial structure of the indices was described by spherical isotropic semivariograms. Our findings show that diversity and evenness computed for the entire field both decreased by 63% after lack of grass weed control, and increased 32% and 31%, respectively, the following year. However, richness, diversity and evenness were not homogeneous across the field. Diversity and evenness became increasingly patchy over time, as shown by a spatial dependence increasing by 21% and 40%, respectively, after two years. This finding is related to the reduction in the patch extension of broad-leaved species due to broad-leaved herbicide application each year and the expansion of grass patches due to the lack of grass control in the first year. Spatial location of patches was not stable over time. Nevertheless, weedy areas remained on the field and represent plant diversity caches that may contribute to maintaining global biodiversity. This information is missed if a single biodiversity index is computed for the entire field. Knowledge of spatial distribution of weed diversity within a field will be useful for identifying wildlife conservation areas and understanding changes in biodiversity in cropland ecosystems better.

Spatial distribution and temporal stability of prostrate knotweed (Polygonum aviculare) and corn poppy (Papaver rhoeas) seed bank in a cereal field

Abstract The knowledge of weed distribution in a field is a key factor to manage weeds effectively. The feasibility of using weed distribution maps for site-specific weed control will largely depend on the stability of the spatial distribution of the populations. Seed banks are the most reliable way of telling the areas weediness, but the effect of regular herbicide applications on its stability is largely unknown. A field experiment was conducted during 3 yr in a winter wheat field under herbicide treatments with the aim of studying the seed banks spatial distribution of prostrate knotweed and corn poppy and the spatiotemporal stability of their populations. Soil samples were taken each year on the same locations, and seed abundance was measured by germination in greenhouse. Both species accounted for more than 10% of the broad-leaved weed seed bank and they were selected for further analysis. Prostrate knotweed seed-bank density decreased 76% and corn poppy 88% in 3 yr. Spatial distribution was described by spherical isotropic semivariograms. Distance of spatial dependence (range) of prostrate knotweed and corn poppy decreased 33 and 11% respectively, and the spatial variability (sill) decreased 96 and 99%. Yearly spatial seed distribution was compared for each species and no temporal stability was observed over a 3-yr period. The lack of stability was attributed to the important decrease of seed density over time and the increase in the short-range variability (nugget). However, for prostrate knotweed, the location of minima and maxima were roughly the same between years, allowing farmers to extend the period of use of the weed distribution maps. Although spatial distribution of seed banks can be affected by processes that promote fast changes in the densities of weed populations, this fact does not mean that weed distribution maps could not be used in consecutive seasons. Nomenclature: Corn poppy, Papaver rhoeas L.; prostrate knotweed, Polygonum aviculare L; winter wheat, Triticum aestivum L.

Characterization and Modeling of Itchgrass (Rottboellia cochinchinensis) Biphasic Seedling Emergence Patterns in the Tropics

Abstract Itchgrass is an aggressive weed species in tropical agroecosystems. Because of phytosanitary restrictions to exports, pineapple producers must use a zero tolerance level for this species. An understanding of itchgrass seedling emergence would help producers to better time POST control. The objective of the present study was to characterize itchgrass seedling emergence patterns and develop a predictive model. Multiple field experiments were conducted in four agricultural fields in Costa Rica between 2010 and 2011 for a total of 9 site-years. Itchgrass consistently showed a biphasic emergence pattern, with a first emergence phase that was faster and more consistent across site-years than the second one. Weibull + logistic models based on chronological time (R2adj  =  0.92) and thermal time with Tbase  =  20 C (R2adj  =  0.92) provided the best fit for the combined emergence data for two experimental locations in 2010. Both models predicted itchgrass seedling emergence adequately for most site-years, but the thermal-time model was more accurate (R2adj  =  0.64 to 0.86) than the chronological model (R2adj  =  0.31 to 0.74), especially when temperatures were high. Both models showed high accuracy in the first emergence phase but tended to underestimate emergence rate during the second phase. The models predicted 50% emergence at 14 d or 80 growing degree days and the stabilization of the first emergence phase at approximately 25 d or 200 growing degree days. Thus, these models can be used to properly time itchgrass POST control. More research is needed to understand the regulatory mechanisms responsible for the variability of the second emergence phase. Nomenclature: Itchgrass, Rottboellia cochinchinensis (Lour.) W.D. Clayton, pineapple, Ananas comosus (L.) Merr.

Population Density and Distribution of Wheat Bugs Infesting Durum Wheat in Sardinia, Italy

Abstract Wheat is a very important crop in Italy, and is infested by wheat bugs belonging to the genera Eurygaster (Hemiptera: Scutellaridae) and Aelia (Hemiptera: Pentatomidae). Many wheat bug infestations have been reported in the north, south, and center of Italy, both in the past as well as recently. The present study was carried out in Sardinia, Italy, during two years (2007 and 2008). The objective of this study was to determine the species and distribution of wheat bugs in durum wheat fields in Sardinia, and to estimate their population density in order to know the incidence of the pest on the island. Sampling took place twice a year (May and June) in three zones, representative of durum wheat cropping in the island. Four species of wheat bugs were found; the predominant species was Eurygaster austriaca (Schrank), followed by Aelia germari (Kuster), Eurygaster maura L., and Aelia acuminata L. The average density of wheat bugs was low (1.1 individuals/m2), but in certain areas it was above the damage threshold (4 individuals/m2). For this reason, the conclusion of the study is that this pest should be monitored in order to control outbreaks and prevent their further spread.