Christian J. Willenborg

The Role of Ground Beetles (Coleoptera: Carabidae) in Weed Seed Consumption: A Review

Abstract Weed management is a challenge in all agroecosystems. Given the negative consequences associated with herbicide-based weed management, it is important to consider integrated weed management options with emphasis on strategies such as biological control. Postdispersal weed seed predation by granivorous and omnivorous carabid beetles results in substantial natural suppression of weed populations. Although the role of ground beetles as “generalist predators” in various agroecosystems is known, their contribution to weed management is not well recognized. In this context, this review presents an account of carabids and their granivorous nature, the importance of a seed diet in the life histories of different carabid groups, factors affecting granivory, and their potential role in weed seed management. Below, we discuss the interrelationships among various factors influencing weed seed consumption by carabids, its consequences for weed management, and the need for future research.

Influence of wild oat (Avena fatua) relative time of emergence and density on cultivated oat yield, wild oat seed production, and wild oat contamination

Abstract Wild oat is a serious weed in cultivated oat because there are no herbicides to selectively control it. Considering the effect of time of emergence on weed–crop interference is critical for the development of accurate crop yield loss models and weed density thresholds. Therefore, field experiments were conducted at two locations in Saskatchewan, Canada, in 2002 and 2003 to determine the effect of wild oat density and time of emergence on cultivated oat yield and quality. Wild oat was planted at 50 growing degree day (GDD) intervals ranging from 100 GDD before to 100 GDD after crop planting. Wild oat density ranged from 0 to 320 plants m−2. High densities of early emerging wild oat greatly reduced cultivated oat yield and increased wild oat contamination, with observed oat yield losses as great as 70% and wild oat contamination levels of 15%. Wild oat that emerged before cultivated oat caused considerably more yield and quality loss and had higher reproductive output than wild oat that emerged after cultivated oat. The yield loss caused by individual wild oat plants at low densities (parameter I) ranged from 0.40 to 0.49%. The effect of relative time of wild oat emergence (parameter C) always varied significantly between site-years. However, little variation in absolute values within site-years was observed for cultivated oat yield loss, wild oat seed production, and wild oat contamination, suggesting that relative time of wild oat emergence influences all similarly. The results of this study emphasize both the need to control early emerging wild oat, as well as the importance of time of emergence in the prediction of crop yield loss. Furthermore, our approach of conducting an emergence study based on thermal time is novel and demonstrates a robust, mechanistic method of estimating crop yield losses due to relative time of emergence. Nomenclature: Wild oat, Avena fatua L. AVEFA; oat, Avena sativa L. ‘AC Assiniboia’.

Diverse Rotations and Optimal Cultural Practices Control Wild Oat (Avena fatua)

Abstract In western Canada, more money is spent on wild oat herbicides than on any other weed species, and wild oat resistance to herbicides is the most widespread resistance issue. A direct-seeded field experiment was conducted from 2010 to 2014 at eight Canadian sites to determine crop life cycle, crop species, crop seeding rate, crop usage, and herbicide rate combination effects on wild oat management and canola yield. Combining 2× seeding rates of early-cut barley silage with 2× seeding rates of winter cereals and excluding wild oat herbicides for 3 of 5 yr (2011 to 2013) often led to similar wild oat density, aboveground wild oat biomass, wild oat seed density in the soil, and canola yield as a repeated canola–wheat rotation under a full wild oat herbicide rate regime. Wild oat was similarly well managed after 3 yr of perennial alfalfa without wild oat herbicides. Forgoing wild oat herbicides in only 2 of 5 yr from exclusively summer annual crop rotations resulted in higher wild oat density, biomass, and seed banks. Management systems that effectively combine diverse and optimal cultural practices against weeds, and limit herbicide use, reduce selection pressure for weed resistance to herbicides and prolong the utility of threatened herbicide tools. Nomenclature: Wild oat, Avena fatua L.; alfalfa, Medicago sativa L.; barley, Hordeum vulgare L.; canola, Brassica napus L.; wheat, Triticum aestivum L.

Evaluating Seed Shatter of Economically Important Weed Species

The increasing occurrence of herbicide resistance, along with no new herbicide modes of action developed in over 30 yr, have increased the need for nonherbicidal weed management strategies and tactics. Harvest weed seed control (HWSC) practices have been successfully adopted in Australia to manage problematic weeds. For HWSC to be effective, a high proportion of weed seeds must be retained on the plant at crop maturity. This 2-yr (2014, 2015) study evaluated seed shatter of wild oat, green foxtail, wild mustard, and cleavers in both an early (field pea) and late (spring wheat) maturity crop in field experiments at Scott, Saskatchewan. Seed shatter was assessed using shatter trays collected once a week during crop ripening stage, as well as at two crop maturation or harvest stages (swathing, direct-combining). Seed shatter differed among weed species, but was similar between crops at maturity: ca. 30% for wild oat, 5% for cleavers, < 2% for wild mustard, and < 1% for green foxtail. Overall, seed shatter of wild oat occurred sooner and at greater levels during the growing season compared with the other weed species. Viability of both shattered and plant-retained seeds was relatively high for all species. The small amount of seed shatter of cleavers, wild mustard, and green foxtail suggests that these species may be suitable candidates for HWSC. Due to the amount and timing of wild oat seed shatter, HWSC may not reduce population abundance of this grassy weed. Nomenclature: Cleavers, Galium spp. (false cleavers, G. spurium L. GALSP; and catchweed bedstraw, G. aparine L. GALAP); green foxtail, Setaria viridis (L.) Beauv. SETVI; wild mustard, Sinapis arvensis L. SINAR; wild oat, Avena fatua L. AVEFA; field pea, Pisum sativum L.; spring wheat, Triticum aestivum L.

Effects of relative time of emergence and density of wild oat (Avena fatua L.) on oat quality

Selective control of wild oat with herbicides is not possible in oat (Avena sativa L.) crops and, consequently, the high quality that is imperative to market success is continually jeopardized. Field experiments were conducted to determine the effects of relative time of emergence and density of wild oat on oat kernel size and weight. Thousand-kernel weight and percentage plump kernels were generally reduced, and percentage thin kernels were generally increased with increasing wild oat densities and earlier wild oat emergence. However, reductions were generally small and occurred only at the highest wild oat densities, which are not commonly observed in commercial fields. These findings suggest that oat growers should not control wild oat based on perceived reductions in oat physical grain quality. Key words: Competition, emergence, physical grain quality, test weight, 1000-kernel weight

Evaluating the Competitive Ability of Semileafless Field Pea Cultivars

Abstract The inclusion of competitive crop cultivars in crop rotations is an important integrated weed management (IWM) tool. However, competitiveness is often not considered a priority for breeding or cultivar selection by growers. Field pea (Pisum sativum L.) is often considered a poor competitor with weeds, but it is not known whether competitiveness varies among semileafless cultivars. The objectives of this study were to determine if semileafless field pea cultivars vary in their ability to compete and/or withstand competition, as well as to identify aboveground trait(s) that may be associated with increased competitive ability. Field experiments were conducted in 2012 and 2013 at three locations in western Canada. Fourteen semileafless field pea cultivars were included in the study representing four different market classes. Cultivars were grown either in the presence or absence of model weeds (wheat and canola), and competitive ability of the cultivars was determined based on their ability to withstand competition (AWC) and their ability to compete (AC). Crop yield, weed biomass and weed fecundity varied among sites but not years. Cultivars exhibited inconsistent differences in competitive ability, although cv. Reward consistently exhibited the lowest AC and AWC. None of the traits measured in this study correlated highly with competitive ability. However, the highest-yielding cultivars generally were those that had the highest AC, whereas cultivars that ranked highest for AWC were associated with lower weed fecundity. Ranking the competitive ability of field pea cultivars could be an important IWM tool for growers and agronomists. Nomenclature: Field pea, Pisum sativum L.

Depth of Seed Burial and Gender Influence Weed Seed Predation by Three Species of Ground Beetle (Coleoptera: Carabidae)

Abstract Ground beetles are postdispersal weed seed predators, yet their role in consuming buried seeds is not well studied. We conducted greenhouse experiments to investigate how seed burial affects consumption of weed seeds (volunteer canola) by adult ground beetles (Coleoptera: Carabidae). Seed burial depth influenced seed consumption rates as demonstrated by a significant interaction between seed burial depth, carabid species, and gender of the carabid tested. We observed higher seed consumption by females of all species, and greater consumption of seeds scattered on the soil surface compared with seeds buried at any depth. However, there was evidence of seed consumption at all depths. Adults of Pterostichus melanarius (Illiger) and Harpalus affinis (Schrank) consumed more buried seeds than did those of Amara littoralis Mannerheim. Agricultural practices, such as tillage, bury seeds at different depths and based on the results of this study, these practices may reduce seed consumption by carabids. Soil conservation practices that reduce tillage (conservation or zero tillage) will favor greater weed seed predation due, in part, to the high availability of seeds at the soil surface or at shallow soil depths. Nomenclature: Volunteer canola, Brassica napus L.

Glyphosate-resistant kochia (Kochia scoparia L. Schrad.) in Saskatchewan and Manitoba

Beckie, H. J., Gulden, R. H., Shaikh, N., Johnson, E. N., Willenborg, C. J., Brenzil, C. A., Shirriff, S. W., Lozinski, C. and Ford, G. 2015. Glyphosate-resistant kochia (Kochia scoparia L. Schrad.) in Saskatchewan and Manitoba. Can. J. Plant Sci. 95: 345-349. Previous surveys have documented the occurrence of glyphosate-resistant (GR) kochia in Alberta in 2011 and 2012. To determine the incidence of GR kochia in Saskatchewan and Manitoba, a stratified-randomized survey of 342 sites (one population per site) in southern and central regions of Saskatchewan and a similar survey of 283 sites in southern Manitoba was conducted in the fall of 2013. Mature plants were collected, seed threshed, and progeny screened by spraying with a discriminating glyphosate dose of 900 g ae ha-1 under greenhouse conditions. Screening confirmed 17 GR kochia populations in nine municipalities in west-central or central Saskatchewan, but only two GR populations from different municipalities in the Red River Valley of Manitoba. While the majority of GR kochia populations in Saskatchewan originated in chemical-fallow fields, some populations were found in cropped fields (wheat, Triticum aestivum L.; lentil, Lens culinaris Medik.; GR canola, Brassica napus L.) and non-cropped areas (oil well, roadside ditch). In Manitoba, the two populations occurred in fields cropped to GR corn (Zea mays L.) and soybean (Glycine max L. Merr.). Agronomic and economic impact of this GR weed biotype is compounded because of consistent multiple resistance to acetolactate synthase-inhibiting herbicides. However, GR kochia is susceptible to dicamba, an increasingly important auxinic herbicide used for control of this multiple-resistant weed biotype.

Brassicaceous Weed Seed Predation by Ground Beetles (Coleoptera: Carabidae)

Abstract We used laboratory and field feeding trials to investigate adult carabid beetle preferences for three brassicaceous weed species (rapeseed, wild mustard, and field pennycress) that are pests in canola. All carabid species preferred seeds of rapeseed most and those of field pennycress least and showed intermediate preference for wild mustard seeds. Beetles highly preferred imbibed seeds of all three weed species. Activity–density of carabids and mean weed seed removal were highly correlated in field plots of canola, with activity–density accounting for 67% of the observed variation in seed removal. Our study indicates that seed consumption among carabids is influenced by several factors, including weed species, physiological state of seeds, and carabid activity–density. Carabid seed predation is significant in canola agroecosystems; therefore, understanding these influences has implications for ecological weed management. Nomenclature: Field pennycress, Thlaspi arvense L. THLAR; rapeseed, Brassica napus L. BRSNN, wild mustard, Sinapis arvensis L. SINAR; canola, Brassica napus L.

Sensitivity of Mycosphaerella pinodes to Pyraclostrobin Fungicide

Mycosphaerella blight, caused by Mycosphaerella pinodes, is a destructive disease of field pea that is managed using foliar fungicides. Strobilurin fungicides have been used in western Canada for disease management since 2003. To assess the baseline sensitivities of M. pinodes isolates to the strobilurin fungicide pyraclostrobin, the effective concentration to reduce mycelial growth by 50% (EC50) was determined for 70 isolates collected prior to 2003 from Alberta, Saskatchewan, North Dakota, and Washington State. Each of these isolates was sensitive to pyraclostrobin, with EC50 values ranging from 0.03 to 0.29 mg liter-1. The pyraclostrobin concentrations required to reduce conidia germination by 50% was lower, ranging from 0.008 to 0.041 mg liter-1. In all, 324 isolates collected in 2010 and 2011 were tested for high levels of insensitivity by examining mycelial growth using a discriminatory dose of 5 mg liter-1. Nineteen isolates were highly insensitive to pyraclostrobin, with EC50 values of 80 to 216 mg liter-1. Conidia of these isolates germinated when exposed to a discriminatory dose of 0.1 mg liter-1. Insensitive isolates infected field pea plants treated with pyraclostrobin but sensitive isolates did not. The identification of insensitive isolates indicates that insensitivity may be emerging in the pathogen population.