Available literature indicates that relatively few agricultural leaders and farmers became interested in weeds as a problem before 1200 A.D. or even 1500 A.D. For many centuries, weed control was mostly incidental to tillage for seedbed preparation and growing of crops and to growing and cutting or pasturing of thickly planted crops. Occasional references in literature previous to 1900 mentioned use of mechanical devices and a few inorganic herbicides specifically for weed control.

 State weed laws directed at control of plant diseases were enacted during 1721 to 1766, but weed and seed laws involving weeds directly were not enacted until 100 to 200 years later. Only a few extension type publications on weeds were issued in the United States and Canada between 1860 and 1900. There was a rapid increase in such publications after 1900. Research with inorganic chemicals as herbicides was begun in the 1890's in Europe and in a few states and provinces, and was increased at a rapid pace until the early 1940's. New developments in mechanical and biological control of weeds increased steadily during the same period. However, weed control remained a relatively minor phase of agronomy, botany, horticulture, agricultural engineering, and plant physiology until the early 1950's.

 About 10 years after the discovery of (2,4-dichlorophenoxy)acetic acid (2,4-D) in 1942–1944, the much increased interest of scientists, federal and state governments, industrial companies, and the general public had begun to bear fruit. The word “weed” or “weeds” began to appear in the titles of college courses and extension specialists. Weed conferences had been organized in six regions of the United States and Canada and in 10 states.

  The first meeting of the Weed Science Society of America was held in 1956 and Weed Science was adopted as its official journal. The number of herbicides in general use in the United States and Canada increased from 15 in 1940 to 25 in 1950, and to 100 in 1969. The total support for weed research in 1962 in the United States was six times that in 1950. The number of full-time research and extension workers or their equivalents in part-time workers had increased 20-fold and 13-fold, respectively, over the number in 1940.

 The rate of advancement in the art and science of weed control has increased so rapidly that the progress in each of the recent brief periods 1941 to 1968, 1901 to 1940, and 1800 to 1900 is considered greater than that in all previous periods, beginning about 6000 B.C.

The weed science discipline has changed significantly since 1970. New herbicides have been introduced, many herbicide-resistant weeds have been documented, herbicide-resistant crops have been introduced, federal laws affecting pesticides and weeds have been modified, the number of companies discovering and developing herbicides has been drastically reduced, basic studies on weed biology have received more emphasis, and integrated methods of controlling weeds with nonchemical as well as chemical methods have received increasing attention.

An in vivo shikimate accumulation assay with excised leaf tissue was developed to provide a fast and reliable method for identifying glyphosate-resistant plants. The assay is based on glyphosate-induced accumulation of shikimate. There was a linear accumulation of shikimate in excised leaf discs of soybean and canola treated with 250 μM glyphosate for 48 h. The IC₅₀ for the accumulation of shikimate in soybean and corn leaf discs was 34 and 87 μM, respectively. Leaf discs excised from glyphosate-resistant corn or soybean did not accumulate shikimate when treated with 500 μM glyphosate. Leaf discs taken from a number of field-grown plants accumulated shikimate in a glyphosate dose–dependent manner. The accumulation of shikimate was dependent on light and the age of the leaf from which the disc was taken. The assay worked either in 96-well microtiter plates or in vials, and it clearly differentiated between glyphosate-resistant and -susceptible crops in which the resistance is due to an alteration of the target site for glyphosate. The assay was simple and robust and has the potential to be used as a high throughput assay to detect glyphosate resistance in weeds.

Nomenclature: Glyphosate; canola, Brassica napus L. ‘Hayola 420’; corn, Zea mays L. ‘Pioneer 37M34’, ‘Dekalb DK493RR/BTY’; soybean, Glycine max (L.) Merr., ‘Asgrow A2869’, ‘Asgrow AG3003’.

A suspected glyphosate-resistant Italian ryegrass biotype was collected from a filbert orchard near Portland, OR, where glyphosate was applied multiple times per year for about 15 yr. Greenhouse studies were conducted to determine if this biotype was glyphosate resistant. The plants were sprayed with glyphosate (0.01 to 3.37 kg ae ha⁻¹) 14 d after planting and shoot biomass was determined 3 wk after herbicide treatment. Based on the dose–response experiments conducted in the greenhouse, the suspected Italian ryegrass biotype was approximately fivefold more resistant to glyphosate than the susceptible biotype. Plants from both susceptible and resistant biotypes were treated with glyphosate (0.42 and 0.84 kg ha⁻¹) and shikimic acid was extracted 12, 24, 48, and 96 h after treatment. The susceptible biotype accumulated between three and five times more shikimic acid than did the resistant biotype. Leaf segments from both susceptible and resistant biotypes were incubated with different glyphosate concentrations (0.5 to 3000 μM) for 14 h under continuous light. Shikimic acid was extracted from each leaf segment and quantified. At a concentration up to 100 μM, leaf segments from the susceptible biotype accumulated more shikimic acid than leaf segments from the resistant biotype. The epsps gene was amplified and sequenced in both susceptible and resistant biotypes; however, no amino acid change was found in the resistant biotype. The level of resistance in this biotype is similar to that reported for a glyphosate-resistant Italian ryegrass biotype from Chile.

Nomenclature: Glyphosate; Italian ryegrass, Lolium multiflorum Lam. LOLMU; filbert, Corylus avellana L.

Enhanced herbicide metabolism is less common than target site–based herbicide resistance in weeds and often confers resistance to chemically dissimilar herbicides. In a previous study, the mechanism of acetolactate synthase (ALS)-inhibitor resistance in a downy brome biotype was determined to be metabolism. Our research was aimed at determining the multiple resistance pattern in the downy brome biotype, establishing its physiological basis, and investigating its fitness. Dose–response experiments showed that the resistant biotype was also moderately resistant to ethofumesate, clethodim, fluazifop, diuron, and terbacil and highly resistant to the triazine herbicides, atrazine and metribuzin. DNA sequence analysis of the psbA gene, which is the target site of PSII inhibitors, demonstrated a single amino acid substitution from serine to glycine in the resistant biotype at residue 264 in the D1 protein. Thus, the resistant biotype contains two different resistance mechanisms, herbicide metabolism and an altered target site. The resistant biotype produced less shoot dry weight, leaf area, and seed and was shorter than the susceptible biotype. The resistant biotype was also less competitive than the susceptible biotype. Thus, in the absence of herbicides, the frequency of this resistant biotype is unlikely to increase in a population of mixed downy brome biotypes.

Nomenclature: Atrazine, clethodim, diuron, ethofumesate, fluazifop, metribuzin, terbacil; downy brome, Bromus tectorum L. BROTE.

Recent reports indicate that manganese (Mn), applied as a foliar fertilizer in tank mixtures with glyphosate, has the potential to antagonize glyphosate efficacy and reduce weed control. It was hypothesized that Mn² complexed with glyphosate in a similar manner to Ca² , forming salts that were not readily absorbed and, thereby, reducing glyphosate efficacy. This study was conducted to confirm the interaction of Mn² and glyphosate and to measure the effect of Mn on glyphosate absorption and translocation in velvetleaf. In aqueous solutions, Mn² binds with solvent molecules and with chelating agents to form hexacoordinate complexes. The distribution of paramagnetic species, both the free manganous ion ([Mn{H₂O}₆]² ) and the Mn² –glyphosate complex, in Mn–glyphosate solutions at various pH values were analyzed using electron paramagnetic resonance (EPR) spectroscopy. Glyphosate interaction with Mn appeared to increase as the pH was increased from spray solution levels (2.8 to 4.5) to levels common in the plant symplast (7.5). Growth chamber bioassays were conducted to measure absorption and translocation of ¹⁴C-labeled glyphosate in solution with four Mn fertilizers: Mn-ethylaminoacetate (Mn-EAA), Mn-ethylenediaminetetraacetate (Mn-EDTA), Mn-lignin sulfonate (Mn-LS), and Mn-sulfate (MnSO₄). Mn-EDTA did not interfere with glyphosate efficacy, absorption, or translocation. However, both MnSO₄ and Mn-LS reduced glyphosate efficacy, absorption, and translocation. Mn-EAA severely antagonized glyphosate efficacy, and although glyphosate in tank mixtures with Mn-EAA was absorbed rapidly, little was translocated from the treated leaf. The Mn-EAA fertilizer contained approximately 0.5% iron (Fe) not reported on the fertilizer label. Iron is presumed to be partially responsible for the very limited translocation of glyphosate from the treated leaf in Mn-EAA tank mixtures. Adding ammonium sulfate increased the efficacy, absorption, and translocation of glyphosate for each Mn fertilizer tank mixture.

Nomenclature: Glyphosate; velvetleaf, Abutilon theophrasti Medicus. ABUTH.

Bud dormancy is the primary mechanism by which the many perennial weeds escape herbicidal and mechanical control. We developed a 2,654-element Euphorbiaceae cDNA microarray using 1,886 sequenced cDNAs from the model perennial weed leafy spurge, 384 cDNAs from cassava, and 384 control genes from other plant, animal, and bacterial species. This array was used to follow changes in gene expression in root buds of leafy spurge following loss of paradormancy. The differential expression of several genes previously identified as being induced following loss of paradormancy was confirmed by microarray analysis. In addition, genes encoding an asparagine synthase, a phosphate-inducible protein, and a curculin-like (mannose-binding) lectin family protein were found to be rapidly up-regulated upon loss of paradormancy. Several genes involved in flavonoid biosynthesis were found to be rapidly down-regulated upon loss of paradormancy. The potential impact of flavonoid biosynthesis on auxin transport in response to bud growth is discussed.

Nomenclature: Leafy spurge, Euphorbia esula L. EPHES; cassava,

Row orientation in vineyards can affect the quantity of light intercepted by the crop's canopy. Consequently, the light available to weeds growing under the canopy might also be affected, with potential implications for their physiology, growth, and productivity. This hypothesis was tested in 2003 and 2004 in a central California vineyard having rows oriented east–west (EW) and north–south (NS) in a randomized complete block design. In April of both years, potted black nightshade seedlings were placed under grapevines of both row orientations and grown for about 10 wk. Photosynthetically active radiation (PAR) at the weed canopy zone (WCZ) of NS rows was bimodal, with peaks occurring at about 09:30 a.m. and 4:30 p.m. At those times, PAR approached 500 μmol m⁻² s⁻¹ (between 30 and 40% of full sun). In contrast, maximum PAR in the WCZ of EW rows was generally less than 75 μmol m⁻² s⁻¹ throughout the day. The ratio of red to far-red light was also greater in NS than EW rows in the morning and afternoon. In both row orientations, PAR was suboptimal for nightshade because maximum net photosynthesis occurred at light levels ≥ 500 μmol m⁻² s⁻¹, but nightshade in the NS rows had higher net photosynthetic rates than those in EW rows when subjected to higher ambient PAR. Stem extension and phenology of nightshade was not affected by vine row orientation, but plants in EW rows had greater leaf areas, leaf area ratios, leaf weight ratios, and lower specific leaf weights than plants in NS rows. Berry mass, seeds per berry, and estimated seed production was 40, 7, and 20% lower, respectively, for plants in the EW than in the NS rows. Dry mass and total nonstructural carbohydrates (TNC) of nightshade roots were also 25 and 45% lower, respectively, in EW than in NS plants. Thus, grapevine row orientation may affect nightshade fecundity by reducing light in the WCZ.

Nomenclature: Black nightshade, Solanum nigrum L.; grape, Vitis vinifera L.

Intercrops have been associated with greater yields and pest and weed control compared with sole crops. In this field experiment, we investigated agronomic performance and weed suppression by three crops—spring wheat (Triticum aestivum), canola (Brassica napus), and field pea (Pisum sativum)—alone and in all possible combinations at two sites in Manitoba, Canada, from 2001 to 2003. Crop treatments were planted at the same total density (144 seeds m⁻²). The effects of the different crop combinations on weed recruitment and biomass and crop production were studied in both the presence and absence of in-crop herbicides. The agronomic performance of intercrop and sole crop treatments varied greatly across site-years. Some intercrop treatments (e.g., wheat–canola and wheat–canola–pea) tended to produce greater weed suppression compared with sole component crops, indicating synergism among crops within intercrops with regard to weed suppression. Intercrop treatments resulted in land-equivalent ratios (LER) > 1 (i.e., overyielding) in both the presence and absence of in-crop herbicides. In the presence of herbicides, canola–pea was the most consistent intercrop treatment in terms of overyielding for grain (mean LER = 1.22), whereas in the absence of herbicides, wheat–canola–pea produced the most consistent overyielding frequency for dry matter production (mean LER = 1.28). In the presence of herbicides, overall grain yield stability was greatest for the wheat–canola–pea intercrop treatment. Results indicate that annual intercrops can enhance both weed suppression and crop production compared with sole crops.

Nomenclature: Canola, Brassica napus L. ‘Clearfield 46A76’; field pea, Pisum sativum L. ‘DS-Stallworth’; spring wheat, Triticum aestivum L. ‘Clearfield BW755’.

Determining the nature of root and shoot competition can elucidate the competitive ability of an invasive species and direct management strategies. In a set of competition experiments, artichoke thistle (Cynara cardunculus), an exotic invasive perennial forb, was subjected to full or shoot competition with four species: black mustard (Brassica nigra), an exotic annual forb; ripgut grass (Bromus diandrus), an exotic annual grass; purple needle-grass (Nassella pulchra), a native perennial grass; and itself. For shoot competition, a smaller pot nested in a larger experimental pot sequestered the target plant root system. A bare ground invasion experiment, in which all plants were transplanted on the same date, and a community invasion experiment, in which competitor species were planted 1 mo before targets, were conducted. In the bare ground invasion experiment, target plant size was reduced (P ≤ 0.05) when exposed to full competition with the exotic species, but not purple needle-grass. Effects on target plants included reductions in height, number of leaves, rosette diameter, and shoot and leaf dry weight. In the community invasion experiment, full competition with all species reduced target plant growth (P ≤ 0.05). Shoot competition was more important when all species were planted synchronously, whereas root competition was more important when target plant establishment was delayed. In a separate experiment, artichoke thistle was grown under four light levels simulating field conditions under canopies of the same competitors. Midday carbon assimilation decreased linearly with increased shade, indicating the likely effects of shoot competition on artichoke thistle. Results indicated that exotic species are more competitive than native purple needle-grass against artichoke thistle and that restoration directly to native grassland after artichoke thistle removal might be difficult. However, artichoke thistle seedling growth is reduced by root competition from grasses that emerge earlier, indicating that early season management of grasslands to delay artichoke thistle establishment might provide effective control.

Nomenclature: Artichoke thistle, Cynara cardunculus L. CYNCA; black mustard, Brassica nigra (L.) Kock BRANI; purple needle-grass, Nassella pulchra (A. Hitchc.) Barkworth NASPU; ripgut grass, Bromus diandrus Roth BRODI.

Polyethylene mulch is an effective component of weed management in vegetable production. However, nutsedges are persistent and proliferate in these systems. Greenhouse studies evaluated the growth and tuber production of purple nutsedge and yellow nutsedge grown in pots covered with black-opaque polyethylene mulch, clear-colorless polyethylene mulch, or nonmulched. Single, presprouted nutsedge tubers were planted and growth evaluated over 16 wk. Relative to the nonmulched, yellow nutsedge shoot production was reduced 46 and 72% by black and clear mulch, respectively. The number of yellow nutsedge shoots that pierced and emerged through black and clear mulches was reduced 96% relative to emerged shoots in the nonmulched control. Yellow nutsedge in the nonmulched produced 366 tubers per initial tuber, whereas tuber production was reduced 49 to 51% in black and clear mulch. Growth of purple nutsedge shoots and tubers in black polyethylene mulch was similar to the nonmulched. Clear polyethylene reduced purple nutsedge tuber biomass relative to nonmulched, but clear mulch was similar to black mulch in nearly all measured variables. Without mulch, yellow nutsedge produced more shoots, shoot biomass, tubers, tuber biomass, and root biomass than purple nutsedge. However, growth of yellow nutsedge was hindered by polyethylene mulch, whereas differences in purple nutsedge growth among mulches could not be detected. The relative insensitivity of purple nutsedge and sensitivity of yellow nutsedge growth to the physical mulch barrier could lead to a shift in nutsedge species composition in mulched vegetable production.

Nomenclature: Purple nutsedge, Cyperus rotundus L. CYPRO; yellow nutsedge, Cyperus esculentus L. CYPES.

Purple and yellow nutsedge are the most troublesome weeds of vegetable crops in the southeast United States. Elimination of methyl bromide use will require alternative management programs to suppress nutsedge growth and interference in vegetables. Polyethylene mulch is an effective barrier for most weeds; however, nutsedges can proliferate in beds covered with polyethylene mulch. The influence of polyethylene mulch on shoot production and lateral expansion patterns of single tubers of purple nutsedge and yellow nutsedge over time was evaluated in field studies. Purple nutsedge patch size was similar in the black mulch treatment and nonmulched control after 8 and 16 wk after planting (WAP). By the end of the growing season, purple nutsedge patch size in the black mulch treatment was nearly twice that in the nonmulched control. At 32 WAP, there were 1,550 shoots in the 16.1 m² patch in the black mulch treatment and 790 shoots in the 8.1 m² patch in the nonmulched control. In contrast, yellow nutsedge growth was suppressed in the black mulch treatment, relative to the nonmulched control. Compared with the black mulch treatment at 16 and 24 WAP, the nonmulched control produced nearly three times as many yellow nutsedge shoots (140 shoots at 16 WAP and 210 shoots at 24 WAP) and patches that were twice the size (0.10 m² at 16 WAP and 0.18 m² at 24 WAP). These data indicate that there are significant differences in the growth habits of the two nutsedges species in mulched vegetable systems. The differences in response to black mulch will likely lead to purple nutsedge becoming a greater problem, relative to yellow nutsedge, in vegetable systems. The rapid expansion of a single purple nutsedge shoot to form a patch that is 22.1 m² and containing 3,440 shoots at 60 WAP illustrates the importance of managing this species.

Nomenclature: Purple nutsedge, Cyperus rotundus L. CYPRO; yellow nutsedge, Cyperus esculentus L. CYPES.

As a weed, wheat has recently gained greater profile. Determining wheat persistence in cropping systems will facilitate the development of effective volunteer wheat management strategies. In October of 2000, glyphosate-resistant (GR) spring wheat seeds were scattered on plots at eight western Canada sites. From 2001 to 2003, the plots were seeded to a canola–barley–field-pea rotation or a fallow–barley–fallow rotation, with five seeding systems involving seeding dates and soil disturbance levels, and monitored for wheat plant density. Herbicides and tillage (in fallow systems) were used to ensure that no wheat plants produced seed. Seeding systems with greater levels of soil disturbance usually had greater wheat densities. Volunteer wheat densities at 2 (2002) and 3 (2003) yr after seed dispersal were close to zero but still detectable at most locations. At the end of 2003, viable wheat seeds were not detected in the soil seed bank at any location. The majority of wheat seedlings were recruited in the year following seed dispersal (2001) at the in-crop, prespray (PRES) interval. At the PRES interval in 2001, across all locations and treatments, wheat density averaged 2.6 plants m⁻². At the preplanting interval (PREP), overall wheat density averaged only 0.2 plants m⁻². By restricting density data to include only continuous cropping, low-disturbance direct-seeding (LDS) systems, the latter mean dropped below 0.1 plants m⁻². Only at one site were preplanting GR wheat densities sufficient (4.2 plants m⁻²) to justify a preseeding herbicide treatment in addition to glyphosate in LDS systems. Overall volunteer wheat recruitment at all spring and summer intervals in the continuous cropping rotation in 2001 was 1.7% (3.3 plants m⁻²). Despite the fact that volunteer wheat has become more common in the central and northern Great Plains, there is little evidence from this study to suggest that its persistence will be a major agronomic problem.

Nomenclature: Barley, Hordeum vulgare L.; canola, Brassica napus L.; field pea, Pisum sativum L.; spring wheat, Triticum aestivum L.

Weed seedbanks have been studied intensively at local scales, but to date, there have been no regional-scale studies of weed seedbank persistence. Empirical and modeling studies indicate that reducing weed seedbank persistence can play an important role in integrated weed management. Annual seedbank persistence of 13 summer annual weed species was studied from 2001 through 2003 at eight locations in the north central United States and one location in the northwestern United States. Effects of seed depth placement, tillage, and abiotic environmental factors on seedbank persistence were examined through regression and multivariate ordinations. All species examined showed a negative relationship between hydrothermal time and seedbank persistence. Seedbank persistence was very similar between the two years of the study for common lambsquarters, giant foxtail, and velvetleaf when data were pooled over location, depth, and tillage. Seedbank persistence of common lambsquarters, giant foxtail, and velvetleaf from October 2001 through 2002 and October 2002 through 2003 was, respectively, 52.3% and 60.0%, 21.3% and 21.8%, and 57.5% and 57.2%. These results demonstrate that robust estimates of seedbank persistence are possible when many observations are averaged over numerous locations. Future studies are needed to develop methods of reducing seedbank persistence, especially for weed species with particularly long-lived seeds.

Nomenclature: Common lambsquarters, Chenopodium album L. CHEAL; giant foxtail, Setaria faberi Herrm. SETFA; velvetleaf, Abutilon theophrasti Medik. ABUTH.

Site-specific fertilizer application is of primary interest among those investigating site-specific crop management. Site-specific soil moisture levels may be associated with the relative success of certain weed species, and weed competition can be affected by fertilizer application. A field study was conducted to investigate how landscape position–based site-specific nitrogen fertilizer application would influence wild oat interference in spring wheat. Wild oat was allowed to grow in spring wheat at foot and knoll landscape positions in the presence or absence of 80 to 90 kg ha⁻¹ of spring preseed broadcast nitrogen fertilizer. In all three site–years of the study, landscape position did not affect wild oat competitiveness in wheat or relative wild oat biomass, and there were no significant landscape position by nitrogen fertilizer interactions for these variables. The lack of landscape position effect may be attributed to the lack of substantive differences in soil characteristics between landscape positions at the sites used in this study and to normal to above-average seasonal precipitation levels in all three site–years. Wild oat competitiveness in wheat was significantly greater in the presence of nitrogen fertilizer for all three site–years. The results of this study suggest that, when there are not great differences in typical soil characteristics between landscape positions and precipitation levels are normal or above normal, landscape-based site-specific nitrogen fertilizer application does not affect wild oat competitiveness in spring wheat, but preseed spring broadcast nitrogen fertilizer does make wild oat more competitive in spring wheat.

Nomenclature: Wild oat, Avena fatua L. AVEFA; wheat, Triticum aestivum L. TRZAS.

Tomato cultivation in the Mediterranean region is susceptible to infestation by the parasitic weed branched broomrape (Orobanche ramosa), and severe yield losses can result. The effectiveness of solarization, a soil disinfection technique that uses passive solar heating, to control the incidence of broomrape under greenhouse conditions was studied over two growing seasons. Solarization was accomplished by the application of clear polyethylene sheets to moist soil for 58 to 61 d during the hot season. The treatment increased maximum soil temperature by around 10 C, and at 5 cm below the soil surface, a temperature of more than 45 C was reached for 34 to 58 d, whereas this temperature was not reached at all in the first season and not for 20 d (second season) in unmulched soil. In solarized soil, no broomrape shoots emerged, and neither haustoria nor underground tubercles of the parasite were found on tomato roots. The treatment killed about 95% of buried viable seed, and induced secondary dormancy in the remaining 5%. In nonsolarized plots, broomrape shoots were present at a high density, decreasing plant growth and fruit production. Fruit yield was 133 to 258% higher in the solarized as compared with the nonsolarized treatment. Based on these results, we suggest that soil solarization, which precludes chemical contamination and is suitable for organic farming, is an appropriate technology where the risk of branched broomrape infestation is high.

Nomenclature: Branched broomrape, Orobanche ramosa L. ORARA; tomato, Lycopersicon esculentum Mill. ‘Ikram’.

A greenhouse experiment was conducted to evaluate the herbicidal activity of five aliphatic (ethyl, propyl, butyl, allyl, and 3-methylthiopropyl) and three aromatic (phenyl, benzyl, and 2-phenylethyl) isothiocyanates (ITCs) on Palmer amaranth, pitted morningglory, and yellow nutsedge. All ITCs were applied to soil at 0, 10, 100, 1,000, and 10,000 nmol g⁻¹ of soil and incorporated. All ITCs had a deleterious effect on Palmer amaranth and pitted morningglory emergence. LC₅₀ values for Palmer amaranth emergence inhibition from aliphatic and aromatic ITCs ranged from a low of 32 nmol g⁻¹ of soil for phenyl ITC to a high of 941 nmol g⁻¹ of soil for propyl ITC. Pitted morningglory was slightly more tolerant than Palmer amaranth to each of the ITCs, with LC₅₀ values for emergence ranging from 347 to 2,855 nmol g⁻¹ of soil for 3-methylthiopropyl and butyl ITC, respectively. Yellow nutsedge was the most tolerant of the three species, with LC₅₀ values for ethyl, butyl, benzyl, and 2-phenylethyl being greater than the highest evaluated concentration of 10,000 nmol g⁻¹ of soil. Phenyl and 3-methylthiopropyl at 10,000 nmol g⁻¹ of soil were the most effective ITCs against yellow nutsedge, reducing emergence by 92%. Effectiveness of the ITCs varied across structure and species, but 3-methylthiopropyl and phenyl ITC were generally the most efficacious for the three species evaluated.

Nomenclature: Palmer amaranth, Amaranthus palmeri S. Wats. AMAPA; pitted morningglory, Ipomoea lacunosa L. IPOLA; yellow nutsedge, Cyperus esculentus L. CYPES.

Weeds are an important plant resource for insects, although feeding by insects on weeds can have both positive and negative effects on crop productivity. Weeds also indirectly affect crops via their influence on beneficial insects, and by harboring plant and insect diseases. Weeds may affect the ability of dispersing insects to locate crop plants. The host relationship between insects and plants is highly variable, ranging from very specialized to generalized feeding behaviors. Despite the myriad interactions of weeds and insects, many aspects of the relationship are predictable. Most insects, including crop pests, are specialists, and preadapted to feed only on some plants, often within a single plant family. Even polyphagous insects often have a distinct preference hierarchy, feeding more widely only when preferred hosts are unavailable. Use of plants by insects is a dynamic interaction, with characteristics of the insect (e.g., mandible structure) and the plant (e.g., allelochemicals) affecting feeding behavior. Thus, weeds that are closely related to crops are particularly important in harboring insects that attack those crops. Crop production practices should seek to sever the taxonomic association between the crop and the weeds found within the crop, and nearby, by eliminating weeds related to the crop. This will make it less likely that insects will move easily from weed to crop plants, that damaging population densities of insects will develop in the field, and that insect vectors that harbor plant diseases will be harbored in the field. Particularly important integrated pest management practices include crop rotation, reduced use of chemical herbicides, and management of weeds in noncultivated areas.

Annual crop fields typically are simple habitats dominated by a few plant species where pesticides play a major role in managing weed and insect infestations. Recently, there has been significant interest in the potential to reduce reliance on pesticides by manipulating plant species and communities to benefit natural enemies of insects and weeds. Such efforts aim to enhance natural enemy impact by providing appropriate food, shelter, and hosts, and efforts typically are accomplished by manipulation of plant species, populations, or communities. Habitat management is generally viewed as an important factor in maintaining stable insect and natural enemy populations in agricultural systems and may have a similar function in increasing weed seed predation. Crop and noncrop habitats provide resources to natural enemies either directly through floral nectar and pollen, indirectly by increased host or prey availability, or through emergent properties of the habitat such as by moderating the microclimate. These critical resources for natural enemies can be provided in agricultural ecosystems at several scales: within fields, at field margins, or as a component of the larger landscape. Because individual natural enemy species may require quite specific resources at different times and spatial scales, not all attempts to manipulate habitat diversity are equally effective. We review the role of plant resources, including weeds, in supporting natural enemy communities and provide case studies of how varying plant diversity at different spatial scales can influence the effectiveness of biological control in agricultural landscapes.

Interactions between weeds and organisms in other pest categories are inevitable. Weeds are plants and therefore ecologically are producers. All other pest organisms are consumers; they are herbivores or pathogens and can thus use weeds directly as a food source. Beneficial organisms are primary carnivores that feed on herbivores; weeds can support beneficials indirectly when they feed on herbivores living on weeds. Weeds can also serve to mask crop plants from herbivore pests; the mechanisms by which this occurs are still debated. Presence of a weed canopy modifies ecosystem microclimate and provides shelter for pests and beneficials that would otherwise not survive. Tactics used to control pests can have impacts on nontarget organisms in other pest categories. Changes in tillage for weed control can impact population development of other pests. Pesticides can affect nontarget organisms resulting in unanticipated changes in crop tolerance and pest control. Development of true integrated pest management programs requires a multidisciplinary approach that incorporates interactions between organisms in different pest categories.

Weeds and native plants should be considered when endeavoring to manage and control plant pathogens of cultivated plants. Whether as a pest itself, vector of a pathogen, or reservoir of a pathogen or its vector, weeds can significantly influence disease incidence. The relationship between these factors plays a critical role in determining disease incidence and impact. Weeds can interact with pathogen management in several ways, including provision of weed biological control, parasitic weeds can directly serve as vectors of plant pathogens, weeds can serve as reservoir alternative hosts for pathogens and vectors, weeds may be obligate alternate hosts for some pathogens, and herbicides used for weed control can interact with plant pathogens. A recent concern is the advent and deployment of plants genetically engineered for pathogen resistance, raising the question of “super weeds” resulting from genetic drift of genes from crops into surrounding weed populations, the impact of which has yet to be determined.

The literature relating to the impact of other pests on weeds of agroecosystems is minimal. A great deal of literature discusses the effect of organisms used for biological control of weeds; however, pest organisms used as biological control agents are not the subject of this paper. The objective of this review is to present what is known about the impact of insect, pathogen, and nematode pests on weeds; to outline some of the gaps in our knowledge; to present concepts from the ecological literature that might provide insight; and to discuss implications for integrated pest management. The limited data that are available suggest that weeds require fewer resources to survive in the presence of the pest complex than the crop and that weeds would potentially have a greater ability to survive, compete, and reproduce in a competitive environment compared to the planted crop. We suggest that three categories of weed response to polyphagous crop pests may occur in agricultural fields: susceptible weed species or biotypes that host the pest with severe effects on growth and fecundity and therefore are of limited concern in terms of competition for resources; tolerant weed species that host the pest without severe effects on growth and fecundity, resulting in effective competition with the crop and larger pest populations; and resistant weed species that do not host the pest but compete effectively with the crop. We propose the hypothesis that the weed community in many agricultural fields is dominated by plant species that are tolerant or resistant to the endemic pest complex, particularly the soil pest complex, because of constant selection pressure from these pests.

Weeds are alternative hosts for plant-parasitic nematodes and have long been recognized for their ability to maintain nematode populations targeted for suppression by various management strategies. The impact of weeds as alternative hosts depends largely on nematode feeding behavior, which is determined by the level of host specialization required for the parasite to feed successfully. In general, the more specialized feeding adaptations are associated with greater crop damage, more diverse nematode management options, and greater negative impact from weeds. Besides serving as alternative hosts, certain weeds can protect nematodes from pesticides and the environment, provide nematode suppression through antagonism, contribute to changes in future nematode biotic potential, or exert indirect effects through competition with crops or by the effects of weed control strategies on nematode populations. Shrinking nematicide options and increasing environmental concerns are making integrated pest management (IPM) a necessity for nematode management in many crops. A prominent similarity between most major weeds and plant-parasitic nematodes is that both are place-bound organisms that are passively dispersed. Weed–nematode interactions in agricultural production systems may be more intricate and complex than the simple function of weeds as alternative hosts. Their relationship may represent a normal adaptation resulting from the limited mobility of both groups of organisms and the obligate parasitism of phytophagous nematodes. The challenge that faces weed scientists and nematologists is to identify effective, compatible IPM strategies that address weed and nematode management collectively.

Plant model systems have contributed greatly to the dramatic progress in understanding the fundamental aspects of plant biology. Using model weeds will also help facilitate focused funding and research in the weed science community. Criteria for developing model weeds require attention to weedy characteristics that impart economic losses and a wide geographic distribution, attributes that present the potential for political and scientific support. Expressed sequence tag (EST) databases for model weeds are the most practical approach to identifying new genes and obtaining data on the gene expression underlying weedy characteristics. Weeds such as Canada thistle, eastern black nightshade, johnsongrass, jointed goatgrass, leafy spurge, waterhemp, and weedy rice are proposed as model systems.

Nomenclature: Canada thistle, Cirsium arvense (L.) Scop CIRAR; common waterhemp, Amaranthus rudis Sauer AMATA; eastern black nightshade, Solanum ptycanthum Dun. SOLPT; johnsongrass, Sorghum halepense (L.) Pers SORHA; jointed goatgrass, Aegilops cylindrica Host. AEGCY; leafy spurge, Euphorbia esula L. EUPES; red rice (weedy rice), Oryza sativa L. ORYSA; tall waterhemp, Amaranthus tuberculatus (Moq.) J. D. Sauer AMATU.