Roger L. Becker

Seed bank dynamics of Lythrum salicaria L.: implications for control of this species in North America

Abstract The seed bank of Lythrum salicaria L. was studied in wetlands of mid-continent North America where this Eurasian species is considered a nuisance. Recruitment under conditions chosen to promote high rates of germination did not exhaust the seed bank in a 1-cm deep layer of wetland soil in experimental flats. Emergence of seedlings from experimentally buried seed decreased linearly ( P = 0.0001, R 2 = 0.89) from 90% at the soil surface to 0% at 2 cm. In the wetlands studied, there were 410 000 L. salicaria seeds m −2 in the top 5 cm of soil; 37% of these were found below a depth of 2 cm. The implications of these results for the control of this species are discussed.

Efficacy and Tolerance to HPPD-Inhibiting Herbicides in Sweet Corn

Abstract The use of POST herbicides has been limited in sweet corn because of the narrow spectrum of weed control or potential crop injury. Field experiments were conducted to evaluate the 4-hydroxyphenyl pyruvate dioxygenase (HPPD)-inhibiting herbicides mesotrione, tembotrione, and topramezone applied POST in sweet corn at three locations. Efficacy of mesotrione, tembotrione, and topramezone applied alone or mixed with atrazine was compared to other labeled POST herbicides following PRE S-metolachlor. Giant foxtail control was greater with tembotrione or topramezone than mesotrione alone or mixed with atrazine. Common lambsquarters, velvetleaf, and common ragweed were controlled 98% or greater with the HPPD-inhibiting herbicides when mixed with atrazine. Tolerance of six sweet corn hybrids was determined in the field when treated with 1× and 2× rates of these herbicides mixed with atrazine. Tolerance of six sweet corn hybrids to these herbicides was determined in the greenhouse when treated with 0.5, 1, 2, 4, 8, and 16 times the labeled rate. Differential hybrid tolerance to each herbicide was observed in both the field and greenhouse evaluations. Tembotrione killed ‘Merit’ in both evaluations. Excluding Merit, hybrids generally had good tolerance to tembotrione and topramezone in the field, but had differential tolerance to mesotrione. With the exception of Merit, hybrids generally had greater tolerance to tembotrione than topramezone and less tolerance to mesotrione in the greenhouse. These HPPD-inhibiting herbicides provide POST weed control, but the potential for sweet corn injury varies among the herbicides and hybrids and warrants further characterization. Nomenclature: Atrazine; mesotrione; S-metolachlor; tembotrione; topramezone;common lambsquarters, Chenopodium album L. CHEAL; common ragweed, Ambrosia artemisiifolia L. AMBEL; giant foxtail, Setaria faberi Herrm. SETFA; velvetleaf, Abutilon theophrasti Medicus ABUTH; corn, Zea mays L

Knowledge networks: an avenue to ecological management of invasive weeds

Abstract Ecological management of invasive weeds will require substantial increases in the application of ecological knowledge and its integration with other forms of knowledge. To enable these increases, we call for purposeful development of knowledge networks in which new knowledge about a complex situation is created by the interaction of different forms of knowledge. We believe that invasive-plant management must be based on a fine-tuning of managed ecosystems, in which operations (e.g., farm activities) are comprehensively adjusted to confront invasives with a wide array of control measures. Land managers must have the primary role in this tuning process because of their holistic knowledge of the ecosystems they manage. Additionally, such ecological management of invasives will require support from new or improved practices in many relevant sectors, e.g., involving extension workers, farm advisors, and researchers of many sorts. Knowledge networks facilitate the creation, application, and integration of knowledge that will be needed to support ecological invasive-plant management. Worldwide, knowledge networks are under very active development as promising solutions to ecological-management challenges. To develop, networks require proactive organization and facilitation. We have developed an experimental knowledge network to facilitate ecological management of field-crop weeds on the basis of collaborative learning groups that help farmers and other professionals develop necessary knowledge. These groups have been favorably evaluated by most participants, and this article describes the results of our project, including our insights into development of such networks.

Effect of Galerucella spp. feeding on seed production in purple loosestrife

Abstract The effect of shoot feeding by the biocontrol agents, Galerucella calmariensis and Galerucella pusilla (Coleoptera: Chrysomelidae) on purple loosestrife (Lythrum salicaria) seed production and seed germination was quantified in two Minnesota wetlands. In a wet meadow where Galerucella spp. were present on isolated plants, feeding by adults and larvae during shoot elongation resulted in stunting and malformation of shoot tips. There was a subsequent reduction in purple loosestrife inflorescence length and number of flower buds and seed capsules. As Galerucella spp. larvae preferentially fed on shoot meristems, even low levels of feeding on a whole-plant basis (approximately 10% defoliation) reduced seed production. In a sedge meadow wetland with severe feeding damage (a minimum of 70% leaf defoliation), few to no flower buds formed on plants, and subsequently, few to no seed capsules were produced on purple loosestrife plants. Of the few capsules that were produced, number of seeds per capsule and percent germination of seeds did not differ from control plants. In both wetlands, feeding on a main shoot of purple loosestrife did not result in a compensatory increase in the number of axillary inflorescences. Feeding by Galerucella spp. and the subsequent reduction in number of seeds produced on purple loosestrife plants will decrease the number of seeds available for dissemination to new sites. Fewer seeds will enter the seedbank, and over time, feeding by Galerucella spp. will decrease the number of seeds available for seedling recruitment. The benefit of leaf defoliation on purple loosestrife plants caused by Galerucella spp. feeding has been reported. In this study, we have quantified the additional benefits of reduced seed production from Galerucella spp. feeding on purple loosestrife in North America. Nomenclature: Purple loosestrife, Lythrum salicaria LYTSA; Galerucella calmariensis; Galerucella pusilla.

Public scholarship-linking weed science with public work

Abstract Weed scientists face complex and difficult challenges. Within our discipline, we must increase the sustainability of current weed management approaches and help respond to invasive plants as a component of global change. There also are major challenges that we share with other agricultural disciplines, such as mounting comprehensive efforts to address the problems of current agriculture. We believe that any effective response to these challenges will require public work, i.e., projects in which a diverse group of people work together—across lines of difference (professional, cultural, etc.)—to produce broad-based, systemic innovations that meet complex challenges. We propose that weed scientists should join relevant public-work projects by practicing “public scholarship.” We define public scholarship as original, creative, peer-evaluated intellectual work that is fully integrated in a public-work project. By full integration we mean that the scholars work serves to fuel the social (i.e., collective) learning of the public-work group. This condition requires that the scholar be a full participant in the group rather than just being in a consultative or advisory role. We present several case studies of weed scientists practicing public scholarship. These scientists found this mode of scholarship to be a highly effective means by which to address their professional priorities. Barriers to the practice of public scholarship include the lack of relevant guidelines and norms within academic culture, e.g., with regard to quality-assurance standards. But public scholarship offers weed scientists a new way of responding to increasingly urgent demands to show that our work effectively produces public value in return for public investment. We believe that graduate programs in weed science should begin to offer students opportunities to learn skills that are relevant to public scholarship.

Population biology of garlic mustard (Alliaria petiolata) in Minnesota hardwood forests

Abstract Garlic mustard, a biennial forb native to Europe, has invaded native ecosystems in forested regions in the United States. In anticipation of a biological control program being implemented in the United States for this plant, a garlic mustard monitoring program was initiated. The objective of this study was to characterize garlic mustard populations and the associated plant communities and their response to environmental conditions in Minnesota hardwood forest ecosystems. Additionally, we developed a baseline for long-term studies to determine future benefits and impacts of biological control agents on plant communities infested with garlic mustard, should they be released. To monitor garlic mustard populations, we used a nationally standardized protocol in which data were collected on garlic mustard population density and cover, garlic mustard plant heights and silique production, insect damage to garlic mustard, cover of the associated plant community, and litter cover. We also collected data on available photosynthetically active radiation in the understory. The results underscore the variability in garlic mustard population dynamics. At only 6 of 12 sites did garlic mustard densities follow the predicted two-point cycles due to their biennial life cycle, with the first- or second-year life stage dominating in any given year. Available light did not differ strongly among sites, but shading by adult plants is implicated in keeping the populations of first-year plants low. Sites with greater garlic mustard cover had lower native species richness and cover than sites with lower garlic mustard cover. Absent biological control agents, garlic mustard is currently experiencing very little herbivory in Minnesota with an average of 2% of leaf area removed by herbivores. Our work shows the importance of pre-release monitoring at multiple sites over multiple years to adequately characterize populations. Without control, garlic mustard will likely continue to have negative impacts on northern forests. Nomenclature: Garlic mustard, Alliaria petiolata (Bieb.) Cavara & Grande.

Giant Ragweed (Ambrosia trifida) Seed Production and Retention in Soybean and Field Margins

As herbicide-resistant weed populations become increasingly problematic in crop production, alternative strategies of weed control are necessary. Giant ragweed, one of the most competitive agricultural weeds in row crops, has evolved resistance to multiple herbicide biochemical sites of action within the plant, necessitating the development of new and integrated methods of weed control. This study assessed the quantity and duration of seed retention of giant ragweed grown in soybean fields and adjacent field margins. Seed retention of giant ragweed was monitored weekly during the 2012 to 2014 harvest seasons using seed collection traps. Giant ragweed plants produced an average of 1,818 seeds per plant, with 66% being potentially viable. Giant ragweed on average began shattering hard (potentially viable) and soft (nonviable) seeds September 12 and continued through October at an average rate of 0.75 and 0.44% of total seeds per day during September and October, respectively. Giant ragweed seeds remained on the plants well into the Minnesota soybean harvest season, with an average of 80% of the total seeds being retained on October 11, when Minnesota soybean harvest was approximately 75% completed in the years of the study. These results suggest that there is a sufficient amount of time to remove escaped giant ragweed from production fields and field margins before the seeds shatter by managing weed seed dispersal before or at crop harvest. Controlling weed seed dispersal has potential to manage herbicide-resistant giant ragweed by limiting replenishment of the weed seed bank. Nomenclature: Giant ragweed, Ambrosia trifida L. AMBTR; soybean, Glycine max (L.) Merr. Conforme las poblaciones de malezas resistentes a herbicidas se hacen incrementalmente más problemáticas en la producción de cultivos, estrategias alternativas de control de malezas se hacen cada vez más necesarias. Ambrosia trifida, una de las malezas agrícolas más competitivas en cultivos en hileras, ha evolucionado resistencia a múltiples sitios bioquímicos de acción de herbicidas dentro de la planta, lo que ha hecho necesario el desarrollo de métodos nuevos e integrados de control de malezas. Este estudio evaluó la cantidad y duración de la retención de semilla de A. trifida creciendo en campos de soja y márgenes de campos adyacentes. La retención de semilla de A. trifida fue monitoreada semanalmente durante las temporadas de cosecha desde 2012 a 2014 usando trampas de colección de semilla. Las plantas de A. trifida produjeron un promedio de 1,818 semillas por planta, con una viabilidad potencial de 66%. En promedio, A. trifida inició la dispersión de semilla dura (potencialmente viable) y suave (no-viable) el 12 de Septiembre y continuó durante Octubre, con una tasa promedio de 0.75 y 0.44% del total de semillas por día, durante Septiembre y Octubre, respectivamente. Las semillas de A. trifida permanecieron en las plantas hasta la temporada de cosecha de soja en Minnesota, con un promedio de 80% del total de las semillas estando retenidas al 11 de Octubre, cuando la cosecha de soja en Minnesota había sido completada al 75%, en los años de este estudio. Estos resultados sugieren que existe una cantidad de tiempo suficiente para remover A. trifida que haya escapado al control en campos de producción y en márgenes de campos antes de que la semilla sea liberada de la planta, mediante el manejo de la dispersión de semilla de malezas antes o durante la cosecha. El controlar la dispersión de semillas de malezas tiene el potencial de manejar A. trifida resistente a herbicidas al limitar el suministro de nuevas semillas al banco de semillas de malezas.

Winter survival of late emerging purple loosestrife (Lythrum salicaria) seedlings

Abstract In wetlands, drought or managed late-summer drawdowns create exposed mudflats that provide an excellent substrate for germination of purple loosestrife seeds. If late-emerging purple loosestrife seedlings survive the winter, new or expanding populations of purple loosestrife will result. Spring survival was determined for overwintered purple loosestrife seedlings from seeds planted at weekly intervals in late summer or fall of the previous year. Seedlings of purple loosestrife that emerged from late July to early August had the greatest survival rates and the greatest shoot dry weight, and they were the tallest the following spring. However, 37% of purple loosestrife seedlings that emerged in late August, although stunted, generated a crown that was able to overwinter successfully and regrow the following spring. The number of growing degree days accumulated from planting date to October 6 (the average date of first frost for Minneapolis and St. Paul, MN) was 1,424 for seedlings from seeds planted on July 21 but only 219 for seedlings from seeds planted on September 15. Purple loosestrife seedlings that emerge during late summer through early September in Minnesota may survive the winter to create additional purple loosestrife weed problems in wetland mudflats caused by artificial drawdowns or droughts. Nomenclature: Purple loosestrife, Lythrum salicaria L. LYTSA.

Seedbank Depletion and Emergence Patterns of Giant Ragweed (Ambrosia trifida) in Minnesota Cropping Systems

In the midwestern United States, biotypes of giant ragweed resistant to multiple herbicide biochemical sites of action have been identified. Weeds with resistance to multiple herbicides reduce the utility of existing herbicides and necessitate the development of alternative weed control strategies. In two experiments in southeastern Minnesota, we determined the effect of six 3 yr crop-rotation systems containing corn, soybean, wheat, and alfalfa on giant ragweed seedbank depletion and emergence patterns. The six crop-rotation systems included continuous corn, soybean—corn—corn, corn—soybean—corn, soybean—wheat—corn, soybean—alfalfa—corn, and alfalfa—alfalfa—corn. The crop-rotation system had no effect on the amount of seedbank depletion when a zero-weed threshold was maintained, with an average of 96% of the giant ragweed seedbank being depleted within 2 yr. Seedbank depletion occurred primarily through seedling emergence in all crop-rotation systems. However, seedling emergence tended to account for more of the seedbank depletion in rotations containing only corn or soybean compared with rotations with wheat or alfalfa. Giant ragweed emerged early across all treatments, with on average 90% emergence occurring by June 4. Duration of emergence was slightly longer in established alfalfa compared with other cropping systems. These results indicate that corn and soybean rotations are more conducive to giant ragweed emergence than rotations including wheat and alfalfa, and that adopting a zero-weed threshold is a viable approach to depleting the weed seedbank in all crop-rotation systems. Nomenclature: Giant ragweed, Ambrosia trifida L. AMBTR, alfalfa, Medicago sativa L., corn, Zea mays L., soybean, Glycine max (L.) Merr., wheat, Triticum aestivum L.

Alternatives to Atrazine for Weed Management in Processing Sweet Corn

Atrazine has been the most widely used herbicide in North American processing sweet corn for decades; however, increased restrictions in recent years have reduced or eliminated atrazine use in certain production areas. The objective of this study was to identify the best stakeholder-derived weed management alternatives to atrazine in processing sweet corn. In field trials throughout the major production areas of processing sweet corn, including three states over 4 yr, 12 atrazine-free weed management treatments were compared to three standard atrazine-containing treatments and a weed-free check. Treatments varied with respect to herbicide mode of action, herbicide application timing, and interrow cultivation. All treatments included a PRE application of dimethenamid. No single weed species occurred across all sites; however, weeds observed in two or more sites included common lambsquarters, giant ragweed, morningglory species, velvetleaf, and wild-proso millet. Standard treatments containing both atrazine and mesotrione POST provided the most efficacious weed control among treatments and resulted in crop yields comparable to the weed-free check, thus demonstrating the value of atrazine in sweet corn production systems. Timely interrow cultivation in atrazine-free treatments did not consistently improve weed control. Only two atrazine-free treatments consistently resulted in weed control and crop yield comparable to standard treatments with atrazine POST: treatments with tembotrione POST either with or without interrow cultivation. Additional atrazine-free treatments with topramezone applied POST worked well in Oregon where small-seeded weed species were prevalent. This work demonstrates that certain atrazine-free weed management systems, based on input from the sweet corn growers and processors who would adopt this technology, are comparable in performance to standard atrazine-containing weed management systems. Nomenclature: Atrazine; dimethenamid; mesotrione; tembotrione; common lambsquarters, Chenopodium album L.; giant ragweed, Ambrosia trifida L.; morningglory species, Ipomea spp.; velvetleaf, Abutilon theoprasti Medik.; wild-proso millet, Panicum miliaceum L.; sweet corn, Zea mays L.