Increased activity and reduced sensitivity of glutamine synthetase in glufosinate‐resistant vetiver (Vetiveria zizanioides Nash) cells

Vetiver (Vetiveria zizanioides Nash) cells derived from an inflorescence were cultured in a modified N6 liquid medium supplemented with 10 µm 2,4‐D and 10 mm proline. Exponentially growing cell suspensions were subcultured with a selection medium containing glufosinate (ammonium dl ‐homoalanin‐4‐yl(methyl)phosphinate). The glufosinate‐resistant cells which can grow in a medium containing 5 × 10^?5 M glufosinate was selected by a stepwise selection, and its I₅₀ value was determined to be 4.2 × 10^?5 M. The growth of susceptible cells was inhibited by lower concentrations of glufosinate and its I₅₀ value was 2.5 × 10^?7 M. This indicated that the selected cells were 170‐fold resistant compared with the susceptible cells. Glutamine synthetase (GS) activity of the resistant cells was twice as high as that of the susceptible cells. The I₅₀ values of glufosinate were 3.2 × 10^?5 M and 9.0 × 10^?7 M for GS from the resistant and susceptible cells, respectively. The accumulation of ammonia caused by GS inhibition was higher in the susceptible cells. Absorption of [3,4–¹⁴C]glufosinate was not significantly different between the resistant and susceptible cells. Both cell types did not metabolize glufosinate. These results suggest that the resistance of the selected vetiver cell suspension to glufosinate is mainly due to increased GS activity and its decreased sensitivity to the herbicide.     

Littleseed canarygrass (Phalaris minor) resistance to clodinafop‐propargyl in wheat fields in north‐western India: Appraisal and management

Littleseed canarygrass (～canarygrass) evolved populations that are resistant to isoproturon during the early 1990s in north‐western India. Clodinafop‐propargyl (～clodinafop) was recommended for controlling these populations. It has been used extensively in wheat for the last several years. Recently, poor or no control of canarygrass by clodinafop has been observed in large areas, which could be related to cross‐resistance or multiple resistance. This study was designed to test whether resistance has evolved in canarygrass populations against clodinafop and to explore control of the resistant populations with sulfosulfuron and pinoxaden. Among the 311 canarygrass populations that were tested, 86, 55 and 34 showed variable phytotoxicity (0–99%) due to 0.030, 0.060 and 0.120 kg ha^?1 clodinafop, respectively. Based on the resistance index, 11 populations were “highly resistant”, 60 were “resistant” and the rest (240) were “susceptible” to clodinafop. Five and six clodinafop‐resistant populations showed slight resistance to 0.0125 kg ha^?1 sulfosulfuron and 0.025 kg ha^?1 pinoxaden, respectively. But, sulfosulfuron at 0.025 and 0.050 kg ha^?1 and pinoxaden at 0.050 and 0.100 kg ha^?1 controlled all the canarygrass populations. Clodinafop used for 4 years increased the chance of resistance evolving, whereas its rotation with sulfosulfuron reduced the chance of resistance evolving. This study showed that considerable canarygrass populations have evolved a low‐to‐high degree of resistance against clodinafop. The further use of clodinafop would lead to the spread of resistance in larger areas through the dispersal of resistant seeds. Clodinafop should be replaced with 0.025 kg ha^?1 sulfosulfuron or 0.050 kg ha^?1 pinoxaden. Besides, where canarygrass has not evolved resistance, the yearly rotation of sulfosulfuron with clodanafop or pinoxaden might delay the evolution of resistance.     

Variation in MCPA-resistance in Ranunculus acris L. subsp. acris and its correlation with historical exposure to MCPA

In seven field experiments in which the effect of MCPA on Ranunculus acris subsp. acris in dairy pastures in New Zealand was studied, control varied between sites from 93% to 11% reduction in ground cover. To determine if the populations varied in resistance to MCPA, seedling progeny were sprayed with a range of doses of MCPA in a glasshouse. The LD₅₀ values ranged from 0.36 kg MCPA ha^?1 to 1.5 kg MCPA ha^?1 confirming the existence of genetic variability in resistance between the populations. Correlations between the LD₅₀ values and the control of the populations in the field, suggested that the differences in resistance contributed to the differences in control. A correlation between the LD₅₀ values and historical exposures of the populations to MCPA suggested that selection for resistance had occurred. No evidence of a difference in competitive ability between the least and the most susceptible biotypes was detected, suggesting that resistance would not decline if spraying ceased.     

Glyphosate resistance in common ragweed ( A mbrosia artemisiifolia L.) from Mississippi,USA

Glyphosate is one of the most commonly used broad‐spectrum herbicides over the last 40 years. Due to the widespread adoption of glyphosate‐resistant (GR) crop technology, especially corn, cotton and soybean, several weed species have evolved resistance to this herbicide. Research was conducted to confirm and characterize the magnitude and mechanism of glyphosate resistance in two GR common ragweed ( A mbrosia artemisiifolia L.) biotypes from Mississippi, USA. A glyphosate‐susceptible (GS) biotype was included for comparison. The effective glyphosate dose to reduce the growth of the treated plants by 50% for the GR1, GR2 and GS biotypes was 0.58, 0.46 and 0.11 kg ae ha^?1, respectively, indicating that the level of resistance was five and fourfold that of the GS biotype for GR1 and GR2, respectively. Studies using ¹⁴ C‐glyphosate have not indicated any difference in its absorption between the biotypes, but the GR1 and GR2 biotypes translocated more ¹⁴ C‐glyphosate, compared to the GS biotype. This difference in translocation within resistant biotypes is unique. There was no amino acid substitution at codon 106 that was detected by the 5‐enolpyruvylshikimate‐3‐phosphate synthase gene sequence analysis of the resistant and susceptible biotypes. Therefore, the mechanism of resistance to glyphosate in common ragweed biotypes from Mississippi is not related to a target site mutation or reduced absorption and/or translocation of glyphosate.     

Inheritance of resistance to paraquat in barley grass Hordeum glaucum Steud.

Two biotypes of the grass weed barley grass (Hordeum glaucum), one resistant and the other susceptible to the herbicide paraquat, were studied along with their F₁, F₂ and F₃ progeny to determine the inheritance of paraquat resistance. The plants were sprayed with 50–200 g a.i. paraquat ha^?1. These concentrations killed the susceptible type. The data obtained from segregating populations indicated that paraquat resistance in H. glaucum is controlled by a single nuclear gene with incomplete dominance.     

Monitoring of resistance to spirodiclofen and five other acaricides in Panonychus citri collected from Chinese citrus orchards

BACKGROUND: Citrus red mite, Panonychus citri (McGregor), is one of the most important pesticide‐resistant pests in China. In order better to understand its resistance status, six populations of the mite were collected from Chinese citrus orchards for monitoring of resistance to spirodiclofen and another five acaricides. RESULTS: All the samples collected in the field in 2006 were susceptible to spirodiclofen. However, the LC₅₀ values in populations sampled in 2009 ranged from 3.29 to 418.24 mg L^?1 spirodiclofen, a 127‐fold difference between the least and most sensitive populations. Compared with a susceptible strain, 50‐fold and 90.8‐fold resistance to spirodiclofen was detected in populations sampled from Pinghe and Fuzhou in 2009, as well as cross‐resistance to spirotetramat. The LC₅₀ values for abamectin, fenpropathrin, hexythiazox and pyridaben in the collected samples ranged from 0.041 to 3.52 mg L^?1, from 23.91 to 696.16 mg L^?1, from 13.94 to 334.19 mg L^?1 and from 48.90 to 609.91 mg L^?1 respectively. CONCLUSION: Great variations in resistance to the tested acaricides were observed among the sampled populations. The Pinghe population developed resistance to all the acaricides tested. The Jianning population was susceptible to most acaricides tested, except pyridaben. Resistance management strategies were conducted on the basis of these observations. Copyright © 2010 Society of Chemical Industry     

Preliminary findings of potentially resistant goosegrass (Eleusine indica) to glufosinate‐ammonium in Malaysia

Goosegrass (Eleusine indica), regarded as one of the world's worst weeds, is highly pernicious to cash crop‐growers in Malaysia. Following reports in 2009 that glufosinate‐ammonium failed to adequately control goosegrass populations in Kesang, Malacca, and Jerantut, Pahang, Malaysia, on‐site field trials were conducted to assess the efficacy of glufosinate‐ammonium towards goosegrass in both places. Preliminary screenings with glufosinate‐ammonium at the recommended rate of 495 g ai ha^?1 provided 82% control of the weed at a vegetable farm in Kesang, while the same rate failed to control goosegrass at an oil palm nursery in Jerantut. The ensuing greenhouse evaluations indicated that the “Kesang” biotype exhibited twofold resistance, while the “Jerantut” biotype exhibited eightfold resistance towards glufosinate‐ammonium, compared to susceptible goosegrass populations. The occurrence of glufosinate resistance in goosegrass calls for more integrated management of the weed to prevent escalating resistance and further proliferation of the weed in Malaysia.     

Multiple herbicide resistance in littleseed canarygrass (Phalaris minor): A threat to wheat production in India

Littleseed canarygrass (Phalaris minor Retz.), a troublesome weed of wheat in India, has evolved multiple herbicide resistance across three modes of action: photosynthesis at the photosystem II site A, acetyl‐coA carboxylase (ACCase), and acetolactate synthase inhibition. The multiple herbicide‐resistant (MHR) populations had a low level of sulfosulfuron resistance but a high level of resistance to clodinafop and fenoxaprop (ACCase inhibitors). Some of the populations had GR₅₀ (50% growth reduction) values for clodinafop that were 11.7‐fold greater than that of the most susceptible population. The clodinafop‐resistant populations also showed a higher level of cross‐resistance to fenoxaprop (fop group) but a low level of cross‐resistance to pinoxaden (den group). Although clodinafop and pinoxaden are from two different chemical families (fop and den groups), their same site of action is responsible for cross‐resistance behavior. The populations that were resistant to four groups of herbicides (phenylureas, sulfonylurea, aryloxyphenoxypropionate, and phenylpyrazolin) were susceptible to the triazine (metribuzin and terbutryn) and dinitroaniline (pendimethalin) herbicides. The P. minor populations that were resistant to the aryloxyphenoxypropionate and phenylurea herbicides were effectively controlled by the sulfonylurea herbicide, sulfosulfuron. In the fields infested with P. minor that was resistant to clodinafop, a sulfosulfuron application (25 g ha^?1) increased the wheat yield by 99.2% over that achieved using the recommended rate of clodinafop (60 g ha^?1). However, the evolution of multiple resistance against the four groups is a threat to wheat production. To prevent the spread of MHR P. minor populations, as well as the extension of multiple resistance to new chemicals, concerted efforts in developing and implementing a sound, integrated weed management program are needed. The integrated approach, consisting of crop and herbicide rotation with cultural and mechanical weed control tactics, should be considered as a long‐term resistance management strategy that will help to sustain wheat productivity and farmers' income.     

Multiple resistance across glufosinate,glyphosate, paraquat and ACCase‐inhibiting herbicides in an Eleusine indica population

An Eleusine indica population was previously reported as the first global case of field‐evolved glufosinate resistance. This study re‐examines glufosinate resistance and investigates multiple resistance to other herbicides in the population. Dose–response experiments with glufosinate showed that the resistant population is 5‐fold and 14‐fold resistant relative to the susceptible population, based on GR₅₀ and LD₅₀ R/S ratio respectively. The selected glufosinate‐resistant subpopulation also displayed a high‐level resistance to glyphosate, with the respective GR₅₀ and LD₅₀ R/S ratios being 12‐ and 144‐fold. In addition, the subpopulation also displayed a level of resistance to paraquat and ACCase‐inhibiting herbicides fluazifop‐P‐butyl, haloxyfop‐P‐methyl and butroxydim. ACCase gene sequencing revealed that the Trp‐2027‐Cys mutation is likely responsible for resistance to the ACCase inhibitors examined. Here, we confirm glufosinate resistance and importantly, we find very high‐level glyphosate resistance, as well as resistance to paraquat and ACCase‐inhibiting herbicides. This is the first confirmed report of a weed species that evolved multiple resistance across all the three non‐selective global herbicides, glufosinate, glyphosate and paraquat.     

Herbicidal activity on acetolactate synthase‐resistant barnyardgrass (Echinochloa crus‐galli) in Arkansas,USA

The intensive use of the acetolactate synthase (ALS)‐inhibiting herbicides, imazethapyr, penoxsulam and bispyribac‐sodium, in imidazolinone‐resistant (Clearfield) rice increases the risk of the evolution of ALS‐resistant barnyardgrass. In 2009, imazethapyr failed to control barnyardgrass that was collected from a field in Arkansas, USA, following the failure of the herbicide in 2008. A greenhouse experiment was conducted to confirm and document the level of resistance of the biotype against three ALS‐inhibiting herbicides that currently are labeled in rice. The level of control of the resistant biotype at the labeled rate of bispyribac‐sodium of 35 g ai ha^?1 was 10%, penoxsulam at 22 g ai ha^?1 was 0% and imazethapyr at 70 g ai ha^?1 was 25%. The level of mortality of the susceptible biotype was 100% with all the herbicides at the labeled rate. The dose needed to kill 50% of the resistant plants was 49 g ha^?1 of bispyribac‐sodium, 254 g ha^?1 of penoxsulam and 170 g ha^?1 of imazethapyr. For the susceptible biotype, bispyribac‐sodium at 6 g ha^?1, penoxsulam at 10 g ha^?1 and imazethapyr at 12 g ha^?1 killed 50% of the treated plants. Based on these findings, it was confirmed that a barnyardgrass population has evolved cross‐resistance to three ALS‐inhibiting herbicides in rice culture in Arkansas. Furthermore, an experiment was conducted to determine if the ALS‐resistant biotype could be controlled using other mechanisms of action. The results indicated that propanil, a photosystem II inhibitor, and quinclorac, a synthetic auxin, failed to control the resistant biotype at the labeled rates, whereas all the other evaluated herbicides provided effective control of both biotypes.     

Biological characterization of sulfoxaflor, a novel insecticide

BACKGROUND: The commercialization of new insecticides is important for ensuring that multiple effective product choices are available. In particular, new insecticides that exhibit high potency and lack insecticidal cross‐resistance are particularly useful in insecticide resistance management (IRM) programs. Sulfoxaflor possesses these characteristics and is the first compound under development from the novel sulfoxamine class of insecticides. RESULTS: In the laboratory, sulfoxaflor demonstrated high levels of insecticidal potency against a broad range of sap‐feeding insect species. The potency of sulfoxaflor was comparable with that of commercial products, including neonicotinoids, for the control of a wide range of aphids, whiteflies (Homoptera) and true bugs (Heteroptera). Sulfoxaflor performed equally well in the laboratory against both insecticide‐susceptible and insecticide‐resistant populations of sweetpotato whitefly, Bemisia tabaci Gennadius, and brown planthopper, Nilaparvata lugens (Stål), including populations resistant to the neonicotinoid insecticide imidacloprid. These laboratory efficacy trends were confirmed in field trials from multiple geographies and crops, and in populations of insects with histories of repeated exposure to insecticides. In particular, a sulfoxaflor use rate of 25 g ha^?1 against cotton aphid (Aphis gossypii Glover) outperformed acetamiprid (25 g ha^?1) and dicrotophos (560 g ha^?1). Sulfoxaflor (50 g ha^?1) provided a control of sweetpotato whitefly equivalent to that of acetamiprid (75 g ha^?1) and imidacloprid (50 g ha^?1) and better than that of thiamethoxam (50 g ha^?1). CONCLUSION: The novel chemistry of sulfoxaflor, its unique biological spectrum of activity and its lack of cross‐resistance highlight the potential of sulfoxaflor as an important new tool for the control of sap‐feeding insect pests. Copyright © 2010 Society of Chemical Industry     

Characterisation of a triazine-resistant biotype of Bromus tectorum found in Spain

A population of Bromus tectorum infesting an olive grove at Córdoba (Spain) survived simazine use rates of 3.0 kg a.i. ha⁻¹ over two consecutive years. Non‐tillage olive monoculture and two annual simazine applications had been carried out for 10 years. The resistant biotype showed a higher ED₅₀ value (7.3 kg a.i. ha⁻¹) than that of the susceptible control (0.1 kg a.i. ha⁻¹), a 73‐fold increase in herbicide tolerance. The use of fluorescence, Hill reaction, absorption, translocation and metabolism assays showed that simazine resistance in this biotype was caused by a modification of the herbicide target site, since chloroplasts from the resistant biotype of B. tectorum were more than 300 times less sensitive to simazine than those from the susceptible biotype. In addition, non‐treated resistant plants of B. tectorum displayed a significant reduction in the Q_A to Q_B electron transfer rate when compared with the susceptible biotype, a characteristic that has been linked to several mutations in the protein D1 conferring resistance to PS II inhibiting herbicides. Resistant plants showed cross‐resistance to other groups of triazine herbicides with the hierarchy of resistance level being methoxy‐s‐triazines ≥chloro‐s‐triazines > methylthio‐s‐triazines > cis‐triazines. The results indicate a naturally occurring target‐site point mutation is responsible for conferring resistance to triazine herbicides. This represents the first documented report of target site triazine resistance in this downy brome biotype.     

Non-target effect of herbicides: I. effect of glyphosate and hexazinone on soil microbial activity. Microbial population,and in-vitro growth of ectomycorrhizal fungi

The effects of two herbicides, glyphosate (as a 359 g litre^?1 SL) and hexazinone (as a 50gkg^?1 granule) on soil microbial population, carbon dioxide evolution, and in-vitro growth of five species of ectomycorrhizal fungi were investigated. Glyphosate at 0–54 and 3.23 kg a.i. ha^?1 and hexazinone at 1. 2 and 8 kg a.i. ha^?1 did not reduce soil microbial population or carbon dioxide evolution in the long term (6 months). However, there was a significant short-term (2 months) effect of glyphosate on both fungal and bacterial counts at the 0.54 kg ha^?1 treatment. In in-vitro tests, Cenococcum graniforme. Hebeloma crustuliniforme and Laccaria laccata were more susceptible to both herbicides than was Suillus tomentosus. which was, in turn, more susceptible than Paxillus involutus. The growth of all five ectomycorrhizal fungi was significantly reduced when subjected to concentrations above 50 μl formulation litre^?1 (glyphosate) or 50 μg formulation litre^?1 (hexazinone).     

广东省稻菜轮作区中牛筋草对十种除草剂的抗性水平及靶基因序列分析

为明确广东省稻菜轮作区中牛筋草对10种常用除草剂的抗性水平及抗性分子机制,采用整株生物测定法测定广东省稻菜轮作区内8个牛筋草种群P1～P8对草甘膦、草铵膦和乙酰辅酶A羧化酶(acetyl-CoA carboxylase,ACCase)抑制剂类等10种除草剂的抗性水平,并进一步分析P1和P8种群相关靶标酶基因5-烯醇丙酮酰莽草酸-3-磷酸合酶(5-enolpyruvyl-shikimate-3-phosphate synthase,EPSPS)、谷氨酰胺合成酶(glutamine synthetase,GS)和ACCase的部分功能区序列特征。结果显示,牛筋草P1～P8种群对草甘膦抗性指数为敏感种群的5.9倍～17.7倍,其中P8种群对草甘膦的抗性水平最高;8个种群对草铵膦也产生了不同程度的抗性,抗性指数为敏感种群的2.3倍～14.2倍,其中P1种群抗性最高。牛筋草P1和P8种群均对ACCase抑制剂类除草剂精喹禾灵、氰氟草酯和噁唑酰草胺产生了交互抗性;P1种群ACCase基因在第2 041位氨基酸处发生突变,该突变在牛筋草种群中首次发现;而P8种群ACCase基因则在第2 027位氨基...     

Physiological studies of mecoprop-resistance in chickweed (Stellaria media L.)

Glasshouse experiments have confirmed that resistance to mecoprop is present in a biotype of Stellaria media. Resistant plants were less vigorous than the susceptible population and ED⁵⁰ values were calculated as 3·92 and 0·18 kg a.i. ha^?1 respectively. The physiological basis of mecoprop resistance was investigated by studying the responses of each biotype in two experimental systems, namely explant stem elongation and ethylene evolution. These studies have discounted differential ethylene evolution as the basis of resistance but have indicated altered elongation growth in resistant plants. This observation suggests the possibility of reduced mecoprop binding in resistant plants, although the role of mecoprop metabolism remains to be elucidated.     

Characteristics and control of dicamba‐resistant common lambsquarters (Chenopodium album)

The expansion of atrazine‐resistant Chenopodium album (common lambsquarters) since the 1980s has forced New Zealand's maize‐growers to use an additional postemergence herbicide application. The frequent use of dicamba for this has selected for a common lambsquarters population with reduced sensitivity to dicamba. Initial greenhouse experiments with seeds that had been collected from the plants that survived field applications of dicamba showed that these plants could tolerate ≤1.2 kg ha^?1, fourfold the recommended rate. These dicamba‐resistant plants were morphologically distinct from the susceptible population. The leaves of the resistant plants were less dentate and a lighter shade of green. The resistant plants were shorter, had a lower biomass and growth rate and flowered ≤19 days earlier than the susceptible plants. When grown together in various density ratios, the average biomass of both the susceptible and the resistant plants increased as the number of susceptible plants decreased in the mixture. The field experiments demonstrated that the resistant population tolerated dicamba at ≤2.4 kg ha^?1, eightfold the recommended rate. Postemergence applications of bromoxynil, pyridate, nicosulfuron and mesotrione effectively controlled both populations. Nicosulfuron and mesotrione provided long‐term residual control, with nicosulfuron being more effective on the grass weeds. High rates of dicamba damaged the maize plants, resulting in an increased weed cover and reduced grain yield. The number of viable common lambsquarters seeds in the soil seed bank at the end of the growing season declined in the treatments in which common lambsquarters was controlled effectively.     

Glyphosate‐resistant Italian ryegrass (Lolium multiflorum) on rice paddy levees in Japan

The rapid range expansion of naturalized Italian ryegrass (Lolium multiflorum Lam.) in farmland is a serious problem in Fukuroi city in Shizuoka Prefecture, Japan. Glyphosate has been used to control Italian ryegrass in the levees of rice paddy fields and wheat fields for ～20 years, but this weed in Fukuroi city is poorly controlled by glyphosate. In order to elucidate the level of resistance to glyphosate in Italian ryegrass populations, seed bioassays and a foliar application experiment, using seeds collected from 16 wild populations in and around Fukuroi city and from three susceptible cultivars, were conducted. For the susceptible cultivars and one population from a site where glyphosate had not been applied for >10 years, the shoot length in the seed bioassays was greatly suppressed at a glyphosate concentration of 10 mg ai L^?1 and no seedling survived after the foliar application of glyphosate at a rate of 2.3 kg ai ha^?1. Nine wild populations from levees in the southern part of Fukuroi city showed vigorous shoot growth at a glyphosate concentration of 10 mg ai L^?1 and had at least a 78% survival rate after the application of glyphosate at 2.3 kg ai ha^?1. Four wild populations from levees in the northern part of Fukuroi city showed a slight suppression of the shoot growth as a result of the glyphosate treatment and their survival rates ranged from 20 to 64%. The results suggested that resistance to glyphosate has evolved in the wild populations of Italian ryegrass that are growing on the levees. This is the first report of a glyphosate‐resistant weed in Japan.     

Resistance of barnyardgrass (Echinochloa crus-galli) to atrazine and quinclorac

Two populations of Echinochloa crus-galli (R and I) exhibited resistance to quinclorac. Another population (X) exhibited resistance to quinclorac and atrazine. The R and I populations were collected from monocultures of rice in southern Spain. The X population was collected from maize fields subjected to the application of atrazine over several years. The susceptible (S) population of the same genus was collected from locations which had never been treated with herbicides. The quinclorac ED₅₀ value (dose causing 50% reduction in shoot fresh weight) for the R and I biotypes were 26- and 6-fold greater than for the S biotype. The X biotype was 10 times more tolerant to quinclorac than the S biotype and also showed cross-resistance to atrazine, being 82-fold more resistant to atrazine than the R, I and S biotypes. Chlorophyll fluorescence and Hill reaction analysis supported the view that the mechanism of resistance to atrazine in the X biotype was modification of the target site, the DI protein. Quinclorac at 20 mg litre^-1 did not inhibit photosynthetic electron transport in any of the test biotypes. The quinclorac I₅₀ values (herbicide dose needed for 50% Hill reaction reduction) of the S population was over 50000-fold higher than the atrazine I₅₀ value for the same S population, indicating that quinclorac is not a PS II inhibiting herbicide. Propanil at doses greater than 0·5 kg ha^-1 controlled all the biotypes. © 1997 SCI     

Evolution of resistance to simazine in Senecio vulgaris L.

Fruits were collected from populations of S. vulgaris growing on commercial fruit farms and progenies were tested for susceptibility to simazine. The five least susceptible populations, which originated from sites where simazine had been applied annually for periods ranging from six to ten years, were subjected to two generations of artificial selection for simazine resistance. Two control populations which originated from unsprayed sites were treated similarly. One population originating from Malpas, Cheshire showed a significant response to selection. The simazine resistance of the other six populations was not improved by artificial selection. Selected progenies of the Malpas population were completely unaffected by simazine at 2.8 kg ha^?1. The potential for evolution of resistance to simazine in S. vulgaris is discussed and the effects of ecological and genetic factors on the rate of evolution are assessed.