Garlic mustard (Alliaria petiolata), an invasive plant of the North American forest understory, is the target of importation biological control programs in both the United States and Canada. After host range testing was completed in the US, a garlic mustard rosette crown-mining weevil from Europe, Ceutorhynchus scrobicollis, was released in Canada in 2018 and has recently been approved for release in the United States. We defined temperature-dependent thresholds for C. scrobicollis oviposition behaviour and cumulative thermal units required for F1 development within garlic mustard rosettes. These thermal requirements of C. scrobicollis were then used to inform a degree-day model to predict the potential geographic distribution of the imported biological control agent in the Great Lakes region of North America. We found that Cumulative Degree Days accumulated during the rosette seasons in 2020 through 2024 were sufficient for C. scrobicollis parasitism of garlic mustard rosettes to take place in Southern Ontario, as well as the central and upper Midwest and Northeast regions of the United States.

Herbaceous perennials must annually rebuild the aboveground photosynthetic architecture from carbohydrates stored in crowns, rhizomes, and roots. Knowledge of carbohydrate utilization and storage can inform management decisions and improve control outcomes for invasive perennials. We monitored the nonstructural carbohydrates in a population of the hybrid Bohemian knotweed [ Polygonum × bohemicum (J. Chrtek & Chrtková) Zika & Jacobson [ cuspidatum × sachalinense ]; syn.: Fallopia × bohemica (Chrtek and Chrtková) J.P. Bailey] and in Japanese knotweed [ Polygonum cuspidatum Siebold & Zucc.; syn.: Fallopia japonica (Houtt.) Ronse Decr.]. Carbohydrate storage in crowns followed seasonal patterns typical of perennial herbaceous dicots corresponding to key phenological events. Starch was consistently the highest nonstructural carbohydrate present. Sucrose levels did not show a consistent inverse relationship with starch levels. Lateral distribution of starch in rhizomes and, more broadly, total nonstructural carbohydrates sampled before dormancy break showed higher levels in rhizomes compared with crowns. Total nonstructural carbohydrate levels in crowns reached seasonal lows at an estimated 22.6% of crown dry weight after accumulating 1,453.8 growing degree days (GDD) by the end of June, mainly due to depleted levels of stored starch, with the estimated minimum of 12.3% reached by 1,220.3 GDD accumulated by mid-June. Depletion corresponded to rapid development of vegetative canopy before entering the reproductive phase in August. Maximum starch accumulation in crowns followed complete senescence of aboveground tissues by mid- to late October. Removal of aboveground shoot biomass in late June to early July with removal of vegetation regrowth in early September before senescence would optimize the use of time and labor to deplete carbohydrate reserves. Additionally, foliar-applied systemic herbicide translocation to belowground tissue should be maximized with applications in late August through early fall to optimize downward translocation with assimilate movement to rebuild underground storage reserves. Fall applications should be made before loss of healthy leaf tissue, with the window for control typically ending by late September in Minnesota.

Palmer amaranth (Amaranthus palmeri S. Watson) is classified as a noxious weed in Minnesota, necessitating its eradication within the state. Manure from livestock fed contaminated feed was identified as a major pathway for the introduction of Palmer amaranth in Minnesota. Black soldier fly larvae (Hermetia illucens L.) (BSFL) are known to feed on organic materials and have been extensively studied for improving manure management. However, little is known about their effect on the fate of weed seeds. Laboratory experiments assessed the effect of BSFL on the fate of Palmer amaranth seeds in dairy manure over a 10-day incubation period. The addition of BSFL during incubation reduced manure weight by 42% compared to a nontreated control. The presence of Palmer amaranth seeds in dairy manure did not impact the biomass accumulation of BSFL during incubation. Palmer amaranth seed recovery from dairy manure was 67%, with no additional reduction observed in the presence of BSFL. Germination of recovered Palmer amaranth seeds dropped to 22% in manure treatments, compared to 64% in a non-manure control seed lot, with no further reduction noted with the addition of BSFL. Overall, mixing Palmer amaranth seeds in manure reduced their emergence to 14%, which could be attributed to the reduction in both seed recovery and germination. However, the addition of BSFL to manure did not affect the number of seeds recovered or their germination.

Abstract Native Cirsium species play an important role in landscapes across North America. Hadroplontus litura (F.) (formerly Ceutorhynchus litura), the stem-mining weevil and biological control agent of Canada thistle [Cirsium arvense (L.) Scop.] can complete its life cycle on five Cirsium species native to the upper Midwest. Although these five Cirsium species are within the fundamental host range of H. litura, as determined by host-range tests, we wanted to explore whether phenological differences among Cirsium species help define the field ecological host range of H. litura. The objective of this study was to determine the phenology of Cirsium species native to the upper Midwest in relation to C. arvense and H. litura. Our goal was to explore whether shoots of native Cirsium species could escape H. litura shoot oviposition in spring due to delayed shoot emergence relative to C. arvense. Soil cumulative growing degree days (GDD) were a superior predictor of shoot emergence for perennial Cirsium species or initiation of leaves in biennial Cirsium species, with a 2.4 times larger effect on time to emergence relative to air GDD. All native Cirsium species initiated new leaves or shoots before C. arvense shoot emergence, even when native Cirsium species growth was delayed in the spring. In turn, C. arvense shoots emerged approximately 1 to 3 wk before female H. litura began to lay eggs. As such, all native Cirsium plants had shoots available for H. litura oviposition. There was no phenological separation between native Cirsium and C. arvense shoot emergence or initiation that would render native Cirsium species safe from H. litura attack. Based on the phenology of shoot emergence or initiation in the spring, all tested Cirsium species native to the upper Midwest would be within the ecological host range of H. litura.

Abstract The use of residual herbicides as a part of preemergence (PRE) and in‐season layered treatments has proven to be an effective strategy for controlling late‐emerging broadleaf weed species in annual crops. However, the use of residual PRE herbicides has the potential to negatively affect other crops within an annual two‐crop rotation, such as fall‐planted cover crops. The intent of this study was to determine if PRE herbicides commonly used in the upper Midwest United States would affect stand density, height, and biomass production of fall‐planted cover crops. Field studies were conducted at three locations that differed in soil type and climate. Four PRE herbicides were applied in silage corn (Zea mays L.) in the spring: dimethenamid‐P (single application), dimethenamid‐P + saflufenacil, acetochlor + clopyralid + mesotrione, and a layered treatment of dimethenamid‐P at planting and 30 days after first application. Three cover crops were planted in the fall following silage corn harvest with a no‐till drill: winter cereal rye (Secale cereale L.), winter camelina (Camelina sativa L.), and red clover (Trifolium pratense L.). Although some of the PRE HBs tested affected cover crop spring plant density and height in sandy soils, there was no difference in cover crop biomass production at termination between herbicide treatments, regardless of soil type. These results indicate that the application of these residual PRE herbicides for control of late‐emerging weed species did not interfere with cover crop biomass production as long as soil moisture was not limiting.

Invasive plants cause significant environmental and economic damage, but land managers have few control options. Common tansy (Tanacetum vulgare) is prevalent in many US states and is one of the most reported invasive plants in Minnesota. Controlling common tansy poses a challenge due to its extensive distribution and association with diverse plant communities. A gene drive is being explored as a genetic biocontrol method for the management of several non-native invasives, including common tansy in North America. Gene drives have emerged as a novel biotechnology application with potential to improve public health, promote conservation, and increase agricultural productivity. In common tansy, gene drives could be developed to target genes that would reduce or eliminate female fertility and consequently inhibit common tansy seed production. Using common tansy as an example, we outline risks associated with the use of gene drive technology for invasive plant control and explain how risks may be mitigated. Understanding potential benefits and risks associated with gene drives in the early stages of development is crucial. Mitigating risks, receiving stakeholder input, and navigating the regulatory environment will play an important role in gene drive development and deployment.

In the Upper Midwest, corn (Zea mays L.) and soybean (Glycine max [L.] Merr.) dominate the landscape, but only for six to seven months of the year. Thus, opportunities exist to establish crops that can utilize the remainder of the growing season and contribute to overall farm profitability. One species of interest is pennycress (Thlaspi arvense L.), but a lack of established agronomic best management practices is a barrier to successful crop production. The objectives of this study were to identify a range of cumulative growing degree days (CGDD) corresponding to pennycress physiological maturity, determine the optimal harvest window that maximizes pennycress seed yield and oil content, and characterize changes in pennycress seed attributes over seed maturation. This study was conducted over the 2016-2017 and 2017-2018 growing seasons with 'MN106' pennycress at two locations in Minnesota, USA. Seed dry weight stabilized within the window of maximum seed yield, but oil content did not maximize until after this period. However, there was minimal loss of oil content when pennycress was harvested within the seed yield maximization window. Based on these parameters, as well as seed moisture, it was estimated that pennycress reached physiological maturity between 2230 and 2250 degrees C d CGDD, or about a week prior to harvest maturity in terms of crop phenology. Delaying harvest to harvest maturity resulted in a 26% loss in harvestable seed due to seed shatter compared with the average maximum seed yield of 928 kg ha-1. Ensuring maximum pennycress seed yield and oil content at harvest is imperative to successful production and contribution to farm economic viability.

Abstract One pathway by which Palmer amaranth (Amaranthus palmeri S. Watson) invades new areas is through importation of contaminated livestock feed, which then contaminates land‐applied manure. If contaminated feed is suspected, detection tools are needed to test manure, but traditional methods are time consuming and often inconclusive. Although new genetic seed testing is making detection easier, methods to separate seed from contaminated manure are needed. Six methods were compared for their ability to recover 100 Palmer amaranth seeds added to bedded or nonbedded cattle manure: dry sieving, rinse sieving, manure saturation sieving without blending and with blending, and dispersion sieving without blending and with blending. Seed recovery was highest (>90%) with the rinse sieving method regardless of manure type. The dispersion methods are not recommended as they recovered <24.7% of seeds. Following each method, genetic testing successfully identified Palmer amaranth presence, indicating no interference of recovery method with DNA extraction.

In 1998, Hadroplontus (formerly Ceutorhynchus) litura, a stem-mining weevil, was introduced into a limited area in Minnesota for the biological control of Canada thistle, Cirsium arvense. Although showing a preference for C. arvense, initial host range testing in the 1960s indicated H. litura attacked other native Cirsium species. Before promoting or augmenting biocontrol with H. litura in Minnesota, we wanted to further define the host range of H. litura on native Cirsium species. Our objective was to determine whether H. litura could feed, oviposit and complete development on Cirsium spp. native to the Upper Midwest of the USA. In no-choice tests, female H. litura accepted all native Cirsium species for oviposition. In addition, H. litura was able to complete development to the adult stage on swamp thistle, Cirsium muticum, field thistle, Cirsium discolour, and tall thistle, Cirsium altissimum, and we confirmed the published host range test results of completed development on Flodman’s thistle, Cirsium flodmanii. These Cirsium species are within the fundamental host range of H. litura. No adults were found in development tests with Hill’s thistle, Cirsium pumilum var. hillii, a threatened or species of concern in the Upper Midwest, or Pitcher’s thistle, Cirsium pitcheri, a federally listed threatened species. Larval tunnelling was documented in C. pitcheri. We recommend that field tests be conducted, where search and host acceptance behaviour can occur under field conditions to further define the ecological host range of H. litura.

Garlic mustard [Alliaria petiolata (M. Bieb.) Cavara & Grande] is an invasive Brassicaceae species native to Europe. As obligate biennials, A. petiolata rosettes require a sufficient length of cold during the winter to flower the following spring. As such, mild winter temperatures could limit the species' potential western and southern distribution in North America. The goal of this research was to characterize the vernalization period required for A. petiolata rosettes to develop the capacity to flower. The objectives of this research were: (1) confirm whether A. petiolata rosettes required a vernalization period to flower; (2) define base and upper temperatures that satisfy the vernalization requirement of A. petiolata rosettes under field conditions; (3) determine the vernalization requirement for flowering and calculate chilling degree days (CDD) accumulated during the cold period; and (4) in a common garden, determine the vernalization requirement of A. petiolata rosettes collected from the southern and northern ranges of its distribution in North America and Europe. The probability of flowering increases as A. petiolata rosettes accumulate CDD. This relationship is defined by a binary logistic (logit) function, with base and maximum temperatures of -3 and 4 C, respectively. The regression equation model predicted that 68 and 120 CDD would result in 50% and 99% probability of flowering, respectively, across all locations. Rosettes from five different seed sources varied in the CDD required for flowering when grown in a common garden. Rosettes originating from a Croatia seed source flowered after exposure to fewer CDD than those from Scotland. In North America, rosettes originating from seeds from Arkansas were more likely to flower after exposure to fewer CDD than those from Ohio or Minnesota. Our results may be used to predict the potential distribution of A. petiolata in North America.

AbstractKnotweed (Fallopiaspp.) is an herbaceous perennial from East Asia that was brought to Europe and North America and, despite control efforts, subsequently spread aggressively on both continents. Data are available on knotweed’s modes of sexual and asexual spread, historical spread, preferred habitat, and ploidy levels. Incomplete information is available on knotweed’s current global geographic distribution and genetic diversity. The chemical composition of knotweed leaves and rhizomes has been partially discovered as related to its ability to inhibit growth and germination of neighboring plant communities via phytochemicals. There is still critical information missing. There are currently no studies detailing knotweed male and female fertility. Specifically, information on pollen viability would be important for further understanding sexual reproduction as a vector of spread in knotweed. This information would help managers determine the potential magnitude of knotweed sexual reproduction and the continued spread of diverse hybrid swarms. The potential range of knotweed and its ability to spread into diverse habitats makes studies on knotweed seed and rhizome cold tolerance of utmost importance, yet to date no such studies have been conducted. There is also a lack of genetic information available on knotweed in the upper Midwest. Detailed genetic information, such as ploidy levels and levels of genetic diversity, would answer many questions about knotweed in Minnesota, including understanding its means of spread, what species are present in what densities, and current levels of hybridization. This literature review summarizes current literature on knotweed to better understand its invasiveness and to highlight necessary future research that would benefit and inform knotweed management in the upper Midwest.

Palmer amaranth-a fast-growing, challenging-to-control noxious weed that significantly reduces crop yields-was first found in Minnesota in September 2016 in conservation plantings sown with Palmer amaranth contaminated seed mixes. Minnesota Department of Agriculture (MDA) designated Palmer amaranth as a Prohibited Noxious Weed in 2015 and listed it as a Noxious Weed Seed in 2016 by emergency order. A genetic test to identify Palmer amaranth was simultaneously developed by multiple laboratories, providing a tool to limit its spread as a contaminant in seed. Seed companies adopted genetic testing methods for labeling seed for sale, thus reducing introductions via the seed pathway. Additionally, MDA determined that manure spread on crop fields from contaminated screenings fed to livestock resulted in new infestations. Limiting spread via these and other potential pathways was critical to successfully reducing the impact of Palmer amaranth. MDA, University of Minnesota (UMN) Extension, Conservation Corps Minnesota and Iowa (CCMI), farmers, and other partners are working to eradicate these infestations before they can spread. In 2016, 35 sites were sown with Palmer amaranth-contaminated seed mixes. Palmer amaranth was found at eight (23%) of these sites. Management with intensive scouting, torching, prescribed burning, and herbicide application was implemented in 2016 and 2017. By 2018, no Palmer amaranth was found at any of these sites. Similar success to newer infestations in 2018, 2019, and 2020 was achieved using the same methods. MDA recorded management activities and documented a comprehensive timeline of Palmer amaranth in Minnesota. This timeline provides a story of success and challenges in combating and eradicating Palmer amaranth.

Light is an essential requirement for proper plant growth and development. Growth chamber experiments were conducted to determine whether artificial alteration of light quality (reducing the red to far-red ratio-R:FR) differentially affected the growth and development of giant foxtail and wild proso millet, two troublesome annual grass weeds in the United States. Growth phenotypes of both weeds were examined under two R:FR regimes (0.28-reduced R:FR and 1.12-unaltered R:FR) in the absence of competition (control conditions) and under intraspecific and interspecific competition. The reduced R:FR simulated shaded (below-canopy) R:FR conditions in the field while the unaltered R:FR treatment simulated direct sunlight (above-canopy) conditions. Averaged across weed species, reducing the R:FR increased plant height, but reduced tiller production and above-ground biomass under no plant competition (P<0.05). In the presence of competition, reducing the R:FR increased plant height and internode length but reduced the number of tillers and leaf area across weed species. No phenotypic differences were observed for weeds tested under intraspecific or interspecific competition. Our study has shown that the response of both weeds to artificial R:FR alteration is similar to that observed under shaded field conditions. Therefore, by replacing bordering plants with a crop, controlled experiments can be used to test the effect of crop canopies on weed suppression when selecting cultivars to be planted in areas where certain weed species are prevalent, minimizing weed-related yield losses.

Crop canopy architecture is known to affect weed performance. Field experiments were conducted to examine the effect of altered crop canopy architecture and light interception on growth and development of wild proso millet and giant foxtail, two problematic weed species. Crop canopy architecture was manipulated by planting two sweet corn varieties contrasting in canopy architecture (Bonus-has a dense leaf canopy and Sprint-has an open leaf canopy) at two row spacings (51-cm and 76-cm rows). Results showed that sweet corn variety, rather than row spacing, altered crop canopy architecture, which in turn altered photosynthetically active radiation (PAR) and red:far-red light ratio (R:FR) received by both weeds. The competitive Bonus canopy had a higher (P<0.05) PAR and R:FR than Sprint at anthesis and harvest. Bonus also more effectively suppressed weed growth and development than Sprint, and weeds growing on Bonus plots had reduced tiller numbers, reduced biomass, lower population densities, and reduced seed production (P<0.05). These responses were attributed to the Bonus canopy having a higher canopy area index, which intercepted more light resulting in lower PAR and R:FR received by both weeds. This study suggests that crop variety selection is an important consideration for weed suppression in row-cropping systems. Key words: Crop canopy architecture, light interception, photosynthetically active radiation, red:far-red ratio, row spacing, sweet corn, wild proso millet, giant foxtail.

Commercial sweet corn (Zea mays convar. saccharata var. rugosa) production has a proportionally high potential for nutrient loss to waterways, due to its high nitrogen (N) requirements and low N use efficiency. Cover crops planted after sweet corn can help ameliorate N lost from the field, but farmers are reluctant to utilize cover crops due to a lack of economic incentive. Pennycress (Thlaspi arvense L.) is a winter annual that can provide both economic and environmental benefits. Five N-rates (0, 65, 135, 135 split and 200) were applied pre-plant to sweet corn. After the sweet corn harvest, pennycress was planted into the sweet corn residue with two seeding methods and harvested for seed the following spring. Residual inorganic soil N (Nmin), pennycress biomass, biomass N and yield were measured. The nitrogen rate and seeding method had no effect on pennycress yield, biomass, or biomass N content. The nitrogen rate positively affected Nmin at pennycress seeding, wherein 200N plots had 38–80% higher Nmin than 0N plots, but had no effect on Nmin at pennycress harvest. Control treatments without pennycress had an average of 27–42% greater Nmin. In conclusion, pennycress can act as an effective N catch crop, and produce an adequate seed yield after sweet corn without the need for supplemental fertilization.

In this paper, we describe lessons learned and protocols developed after a decade of rearing Ceutorhynchus scrobicollis Nerenscheimer and Wagner in a Biosafety Level 2 containment facility. We have developed these protocols in anticipation of approval to release C. scrobicollis in North America for the biocontrol of garlic mustard. The rearing protocol tried to minimize the potential of attack by the adult parasitoid, Perilitus conseutor, which may be present in field collected C. scrobicollis from Europe to prevent inadvertent introduction of parasitoids into North America. All C. scrobicollis used for our quarantine rearing were field collected near Berlin, Germany. We have successfully reared C. scrobicollis on caged garlic mustard plants in a growth chamber by alternating temperatures and photoperiods to simulate those in its native range. In Germany, C. scrobicollis produces one generation per year and F1 adults emerge in late May. In containment, a new generation of adults emerged an average of 108 days after adults were placed on plants. We found the optimal time spent to collect F1 adults was four weeks after the appearance of the first F1 adult, with 95% of potential adults collected. Simulating a three-month summer aestivation period, followed by a week of fall, and three weeks of winter conditions resulted in optimum levels of oviposition in F1 females. Larvae first hatched 8- to-10 days after adults were placed on plants at 15/14 C day/night temperatures with a 9.5 hour photoperiod. We therefore recommend that C. scrobicollis adults are removed from garlic mustard rosettes after 8 days. This will maximize the period of female oviposition while minimizing the time when larvae are available for attack from P. conseutor.

European buckthorn is an exotic problematic invasive woody species that has displaced native plant species in Minnesota woodlands. Buckthorn is also an overwintering host for oat crown rust and soybean aphids, which can cause significant crop yield losses. The overall goal of this study was to test multiple buckthorn control methods and examine the establishment of native plant species in colonized areas. Specific objectives were to 1) determine the effectiveness of buckthorn control methods when applied in different seasons, 2) monitor seedling recruitment and resprouting ability of buckthorn saplings following treatment, 3) monitor recruitment and survival of native plant species following treatment, and 4) characterize buckthorn carbohydrate fluctuations and considerations for timely and effective buckthorn management. Field experiments conducted for two years in two locations (Eagle Lake Regional Park and Battle Creek Regional Park, Minnesota, U.S.A), tested four buckthorn control treatments: 1) cutting only; 2) cutting+stump treatment with herbicide (triclopyr); 3) cutting+stump treatment with herbicide+burning, and 4) cutting+burning. Untreated controls were included in each experiment. Across management seasons, the cutting+stump treatment with herbicide resulted in higher seedling densities for buckthorn and other species the next season compared to cutting only without herbicide application. Spring management resulted in the lowest seedling density the next season for both buckthorn and other plant species, and spring control treatments that included herbicide and burning resulted in higher buckthorn and native species seedling densities than treatments without burning. Because seasonal total nonstructural carbohydrate levels in buckthorn crowns were highest in the fall season, we recommend applying systemic herbicides in the fall when carbohydrates are translocated for storage to facilitate herbicide translocation and efficacy. Our study shows that integrating multiple buckthorn control methods reduces buckthorn populations and increases native species diversity. For long-term control of buckthorn seedling establishment, follow-up treatments like applying foliar herbicide sprays can be used in addition to prescribed burning.