Herbicidal activity of clove oil and its main constituents eugenol, β-caryophyllene, and α-humulene was studied by measuring their effects on cell membrane integrity in broccoli and common lambsquarters plants at the three- and nine-leaf stage, respectively. Roles of essential oil constituents in the overall phytotoxicity of clove oil, dose-response (10 to 160 mM) relationships of their phytotoxicity, and the effect of light intensity on phytotoxicity of clove oil and eugenol were studied. Most of the phytotoxicity of clove oil (2.5% solution) was due to eugenol, its largest constituent. β-caryophyllene and α-humulene played little or no role. Dose-response relationships showed that at equimolar concentration, eugenol was the most phytotoxic essential oil constituent of the clove oil. On a per unit biomass basis, membrane damage in response to clove oil and eugenol sprays decreased with increasing light intensity. This suggests that efficacy of essential oil in causing plant damage could be affected by light intensity experienced by plants prior to the oil spray.

Managing agricultural pests with an incomplete understanding of the impacts that tactics have on crops, pests, and other organisms poses risks for loss of short-term profits and longer-term negative impacts, such as evolved resistance and nontarget effects. This is especially relevant for the management of weeds that are viewed almost exclusively as major impediments to crop production. Seldom considered in weed management are the benefits weeds provide in agroecosystems, which should be considered for optimal decision-making. Integration of weed costs and benefits will become increasingly important as management for pests transitions away from nearly complete reliance on herbicides and transgenic crop traits as the predominant approach for control. Here, we introduce a weed-management decision framework that accounts for weed benefits and exemplify how in-crop weed occurrence can increase crop yields in which a highly damaging insect also occurs. We highlight a case study showing how management decision-making for common milkweed, which is currently controlled primarily with glyphosate in herbicide-tolerant corn, can be improved by integrating management of the European corn borer (ECB), which is currently controlled primarily by the transgenic toxin Cry1 in Bacillus thuringiensis corn. Our data reveal that milkweed plants harboring aphids provide a food source (honeydew) for parasitoid wasps, which attack ECB eggs. Especially at high ECB population densities (> 1 egg mass leaf–1), maintaining low milkweed densities (< 1 stem m–2), effectively helps to minimize yield losses from ECB and to increase the economic injury level of this aggressive perennial weed. In addition, milkweed is the host for the monarch butterfly, so breeding-ground occurrences of the plant, including crop fields, may help sustain populations of this iconic insect. Using a more-holistic approach to integrate the management of multiple crop pests has the capacity to improve decision-making at the field scale, which can improve outcomes at the landscape scale.

Since the beginning of agriculture, crops have been exposed to recurrent invasion by weeds that can impose severe reductions in crop quality and yield. There have been continuing efforts to reduce the impacts of weeds on production. More than 40 yr ago, overreliance on herbicide technology to reduce weed infestations resulted in the selection of adaptive traits that enabled weed survival and reproduction under herbicide treatments (Délye et al. 2007; Powles and Yu 2010; Vila-Aiub et al. 2008). As a result, herbicide resistance in > 200 weed species has evolved worldwide (Heap 2013; Powles 2008).

Managing weed resistance has become a major challenge for many agricultural producers. Resistance is growing in terms of the number of weeds exhibiting resistance and the number of herbicides to which weeds are becoming resistant. The susceptibility of weeds to herbicides in many regions is a diminishing common pool resource affected by local producer weed control actions and natural conditions. Given the growing number of weeds exhibiting resistance, and the recognition that weed resistance is not a private property issue, we argue that managing resistance must be viewed as a wicked problem with no standard template across regions. Finding farm management approaches that help farmers successfully address weed resistance requires a shared perspective that incorporates an improved understanding of the human dimensions of weed management. Through an analysis of wicked problem characteristics, we argue that a people-centered approach to weed management is necessary. We offer principles learned from tackling other wicked agriculture and resource conservation issues to guide such approaches. Education, technical assistance, incentive schemes and regulatory efforts, and other strategies can play roles in constructing management approaches for herbicide resistance, but will have to vary from current efforts to unravel the mysteries of more effective weed management. Building a more inclusive approach, in terms of stakeholders and disciplines, will be key to achieving progress.

Doveweed is a summer annual that is difficult to control in turfgrass. Photosystem II inhibitors have the potential to control doveweed, but research is limited on the efficacy of these herbicides. The objectives of this research were to evaluate (1) the differential tolerance levels of doveweed to atrazine and simazine, (2) the influence of application placement and rate on herbicide efficacy, and (3) uptake and metabolism of these herbicides in doveweed. In greenhouse experiments, the time required to injure doveweed 50% was three to five times faster for atrazine than simazine. Simazine soil or foliar + soil application reduced doveweed biomass 77% from the nontreated, but foliar-only treatments reduced biomass 51%. Application placements for atrazine equally reduced shoot biomass 96% from the nontreated. In a dose–response experiment, atrazine and simazine required ≤ 1.8 kg ha−1 and ≥ 5.1 kg ha−1 to injure doveweed 50% from 8 to 16 d after treatment (DAT), respectively. Doveweed required 79% less atrazine to reduce biomass 50% from the nontreated compared with simazine. In laboratory experiments, doveweed had similar root absorption levels of 14C-atrazine and 14C-simazine. Metabolism of both herbicides linearly increased from 1 to 7 DAT, but parent herbicide levels averaged 39 and 25% of the extracted radioactivity from 14C-atrazine and 14C-simazine, respectively. Doveweed metabolized 14C-simazine to three major metabolites, including hydroxysimazine, that each ranged from 24 to 29% of the extracted radioactivity. Hydroxyatrazine was the only major metabolite (> 10% of total 14C extracted) of 14C-atrazine. Overall, doveweed has slower metabolism of atrazine compared with simazine and is the basis for differential tolerance levels to these herbicides.

The genetic basis of weedy and invasive traits and their evolution remain poorly understood, but genomic approaches offer tremendous promise for elucidating these important features of weed biology. However, the genomic tools and resources available for weed research are currently meager compared with those available for many crops. Because genomic methodologies are becoming increasingly accessible and less expensive, the time is ripe for weed scientists to incorporate these methods into their research programs. One example is next-generation sequencing technology, which has the advantage of enhancing the sequencing output from the transcriptome of a weedy plant at a reduced cost. Successful implementation of these approaches will require collaborative efforts that focus resources on common goals and bring together expertise in weed science, molecular biology, plant physiology, and bioinformatics. We outline how these large-scale genomic programs can aid both our understanding of the biology of weedy and invasive plants and our success at managing these species in agriculture. The judicious selection of species for developing weed genomics programs is needed, and we offer up choices, but no Arabidopsis-like model species exists in the world of weeds. We outline the roadmap for creating a powerful synergy of weed science and genomics, given well-placed effort and resources.

Herbicide degradation in soil is a major research issue, as evidenced by the number of refereed articles on the subject (Figure 1). The approximate number of total citations per year has increased from about 70 in the mid-1990s to approximately 170 per year in the last few years (Figure 1a). From a weed science perspective, publications in the journals Weed Science and Weed Technology have tended to decline from an average of eight per year in the 1995 to 2005 interval to about three per year in the last 10 yr (Figure 1b). This discrepancy of total citations vs. Weed Science Society of America journals may be due to funding availability and other more immediate research needs in weed science. Other reasons might be a lack of reader interest (indicating low potential impact in this specific topic area) or the perception that Weed Science and Weed Technology are light venues for such papers and therefore not the first choice for publication. The authors believe this research topic to be important and relevant to the discipline of weed science, even more so as herbicide use patterns become more complicated because of glyphosate-resistant (GR) weeds.

Palmer amaranth, a dioecious summer annual forb, originating in Sonoran desert washes, compromises crop yields in much of the southern United States and its range is expanding northward. Appropriate tactics for managing this weed proactively in the Upper Midwest will depend on characterizing its damage niche, the geographic range in which it can reduce crop yields. We implemented a common garden study in 2011 and 2012, planting eight accessions of Palmer amaranth from the southern and midwestern United States, into soybean crops in southern, central, and northern Illinois, at a population density of 8 plants m−2 with a biocontainment protocol. Once Palmer amaranth plants initiated flowering, they were removed and burned. Weed survival, flowering, and weed biomass were measured, in addition to soybean yield and weather data. Analyses indicated that Palmer amaranth's damage niche in Illinois soybean was independent of weed genotype or maternal environment. Despite competing only briefly, Palmer amaranth reduced soybean yields in all site–years, indicating its damage niche in Illinois, and much of the Midwest, is limited primarily by seed immigration rate. These results highlight the urgent need for weed managers to learn Palmer amaranth identification, prevent seed introduction, and maintain a policy of zero seed return.

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.