Glyphosate resistance in four different populations of Lolium rigidum is associated with reduced translocation of glyphosate to meristematic zones

Weed populations with resistance to glyphosate have evolved over the last 7 years, since the discovery of the first glyphosate‐resistant populations of Lolium rigidum in Australia. Four populations of L. rigidum from cropping, horticultural and viticultural areas in New South Wales and South Australia were tested for resistance to glyphosate by dose–response experiments. All populations required considerably more glyphosate to achieve 50% control compared with a known susceptible population, indicating they were resistant to glyphosate. Translocation of glyphosate within these resistant populations was examined by following the movement of radiolabelled glyphosate applied to a mature leaf. All resistant plants translocated significantly more herbicide to the tip of the treated leaf than did susceptible plants. Susceptible plants translocated twice as much herbicide to the stem meristematic portion of the plant compared with resistant plants. These different translocation patterns suggest an association between glyphosate resistance in L. rigidum and the ability of glyphosate to accumulate in the shoot meristem.     

A target-site mutation is present in a glyphosate-resistant Lolium rigidum population

Annual ryegrass (Lolium rigidum) is the only weed species to have evolved resistance to the broad‐spectrum herbicide glyphosate in Australia. A population that had failed to be controlled by glyphosate was collected from a vineyard in the Adelaide Hills region of South Australia. Dose–response experiments on this population (SLR 77) showed that it was glyphosate resistant, with an LD₅₀ that was 1.9–3.4 times higher than that of a susceptible population (VLR 1). The movement of radiolabelled glyphosate within SLR 77 plants showed that this population did not have the differential glyphosate translocation mechanism of resistance common to several other Australian glyphosate‐resistant populations. Subsequent analysis of shikimic acid accumulation within the plant after glyphosate treatment showed that this population accumulated significantly less shikimic acid than a susceptible population, but more than a glyphosate‐resistant population with the translocation mechanism, indicating the possible involvement of another mechanism of resistance. Sequencing of a portion of the SLR 77 5‐enolpyruvylshikimate‐3‐phosphate synthase gene was carried out and a mutation causing an amino acid change at position 106 from proline to threonine was identified. This mutation is likely to be responsible for glyphosate resistance in this population, as mutations in this position have been found to be responsible for glyphosate resistance in goosegrass (Eleusine indica) from Malaysia. This paper represents the first report of target‐site glyphosate resistance in L. rigidum and provides evidence that this species has at least two mechanisms of glyphosate resistance present in Australia.     

Multiple herbicide‐resistant Lolium rigidum (annual ryegrass) now dominates across the Western Australian grain belt

Lolium rigidum (annual or rigid ryegrass) is a widespread annual weed in cropping systems of southern Australia, and herbicide resistance in L. rigidum is a common problem in this region. In 2010, a random survey was conducted across the grain belt of Western Australia to determine the frequency of herbicide‐resistant L. rigidum populations and to compare this with the results of previous surveys in 1998 and 2003. During the survey, 466 cropping fields were visited, with a total of 362 L. rigidum populations collected. Screening of these populations with the herbicides commonly used for control of L. rigidum revealed that resistance to the ACCase‐ and ALS‐inhibiting herbicides was common, with 96% of populations having plants resistant to the ACCase herbicide diclofop‐methyl and 98% having plants resistant to the ALS herbicide sulfometuron. Resistance to another ACCase herbicide, clethodim, is increasing, with 65% of populations now containing resistant plants. Resistance to other herbicide modes of action was significantly lower, with 27% of populations containing plants with resistance to the pre‐emergent herbicide trifluralin, and glyphosate, atrazine and paraquat providing good control of most of the populations screened in this survey. Ninety five per cent of L. rigidum populations contained plants with resistance to at least two herbicide modes of action. These results demonstrate that resistance levels have increased dramatically for the ACCase‐ and ALS‐inhibiting herbicides since the last survey in 2003 (>95% vs. 70–90%); therefore, the use of a wide range of integrated weed management options are required to sustain these cropping systems in the future.     

Simulating evolution of glyphosate resistance in Lolium rigidum II: past, present and future glyphosate use in Australian cropping

Glyphosate is a key component of weed control strategies in Australia and worldwide. Despite widespread and frequent use, evolved resistance to glyphosate is rare. A herbicide resistance model, parameterized for Lolium rigidum has been used to perform a number of simulations to compare predicted rates of evolution of glyphosate resistance under past, present and projected future use strategies. In a 30‐year wheat, lupin, wheat, oilseed rape crop rotation with minimum tillage (100% shallow depth soil disturbance at sowing) and annual use of glyphosate pre‐sowing, L. rigidum control was sustainable with no predicted glyphosate resistance. When the crop establishment system was changed to annual no‐tillage (15% soil disturbance at sowing), glyphosate resistance was predicted in 90% of populations, with resistance becoming apparent after between 10 and 18 years when sowing was delayed. Resistance was predicted in 20% of populations after 25–30 years with early sowing. Risks of glyphosate resistance could be reduced by rotating between no‐tillage and minimum‐tillage establishment systems, or by rotating between glyphosate and paraquat for pre‐sowing weed control. The double knockdown strategy (sequential full rate applications of glyphosate and paraquat) reduced risks of glyphosate and paraquat resistance to <2%. Introduction of glyphosate‐resistant oilseed rape significantly increased predicted risks of glyphosate resistance in no‐tillage systems even when the double knockdown was practised. These increased risks could be offset by high crop sowing rates and weed seed collection at harvest. When no selective herbicides were available in wheat crops, the introduction of glyphosate‐resistant oilseed rape necessitated a return to a minimum‐tillage crop establishment system.     

Interaction between glyphosate and various herbicides for broadleaved weed control

The effects of several herbicides for broadleaved weed control on glyphosate (n-phosphonomethyl glycine) phytotoxicity to wheat (Triticum aestivum L.), barley (Hordeum vulgare L.) and wild oats (Avena fatua L.) were studied in the greenhouse. In tank mixtures, dicamba (3,6-dichloro-o-anisic acid), 2,4-D (2,4-dichlorophenoxy-acetic acid) and bromoxynil (3,5-dibromo-4-hydroxy benzonitrile) reduced the phytotoxicity of threshold rates of glyphosate on all three species. With increasing rates of the herbicides for broadleaved weed control relative to a fixed rate of glyphosate, there was a general trend towards increased antagonism. Increasing glyphosate rates above the threshold level in mixtures containing a fixed rate of herbicides for broadleaved weed control overcame the antagonism. Both the inert and active ingredients of 2,4-D amine and ester appeared to be involved in the antagonism. There was no reduction in glyphosate phytotoxicity on the annual grasses when 2,4-D or bromoxynil were sprayed sequentially at various times in relation to glyphosate. When droplets of bromoxynil and glyphosate were placed side by side on a leaf, glyphosate phytotoxicity was not reduced, whereas when the two herbicides were mixed and applied as one droplet, considerable reduction in glyphosate phytotoxicity occurred. These results suggest that the reduction in glyphosate phytotoxicity caused by tank mixing with herbicides such as 2,4-D, bromoxynil and dicamba may be due to a physical or chemical incompatability within the tank mixture rather than to a biological interaction in the plant.     

Resistance to glyphosate in Lolium rigidum

Annual ryegrass (Lolium rigidum) is a widespread and important weed of Australia and populations of this weed have developed resistance to most major herbicides, including glyphosate. The possible mechanisms of resistance have been examined in one glyphosate-resistant Lolium population. No major differences were observed between resistant and susceptible biotypes in respect of (i) the target enzyme (EPSP synthase), (ii) DAHP synthase, the first enzyme of the target (shikimate) pathway, (iii) absorption of glyphosate, or (iv) translocation. Following treatment with glyphosate, there was greater accumulation of shikimate (derived from shikimate-3-Pi) in susceptible than in resistant plants. In addition, the resistant population exhibited cross-resistance to 2-hydroxy-3-(1,2,4-triazol-1-yl)propyl phosphonate, a herbicide which, although structurally similar to glyphosate, acts at an unrelated target site. On the basis of these observations we speculate that movement of glyphosate to its site of action in the plastid is involved in the resistance mechanism. © 1999 Society of Chemical Industry     

A survey of Lolium rigidum populations in citrus orchards: Factors explaining infestation levels

The presence of herbicide‐resistant Lolium rigidum in Mediterranean (Spanish) citrus orchards was reported in 2005 and it poses a serious threat to crop management. The main objective of this research was to investigate which components could be responsible for the persistence of annual ryegrass populations in Mediterranean mandarin and orange orchards. This is the first study regarding L. rigidum populations in Mediterranean citrus orchards. Surveys were conducted in 55 commercial citrus orchards in eastern Spain in 2013 by interviewing technicians who were working in cooperatives about crop management. The level of infestation by L. rigidum and the presence of harvester ants (Messor barbarus) then were estimated in the same orchards. The variables were subjected to a two‐dimensional analysis and both univariate and multivariate logistic regression models were fitted for each of the three L. rigidum density levels that had been established. The multivariate models showed the significant factors that were associated with various L. rigidum densities: (i) at a low density, the herbicides that were applied, the number of applications in 2013 and the type of irrigation (flood or drip); (ii) at a medium density, the presence of harvester ants; and (iii) at a high density, the herbicides that were applied in 2013. The results indicated that drip irrigation and one application of glyphosate mixed with other herbicides (or herbicides other than glyphosate) were associated with a lower L. rigidum density. The alternative management options that are presented here should help farmers to reduce weed problems in Mediterranean citrus orchards. Future research is required to better understand the presence of herbicide‐resistant populations, as well as the possible beneficial presence of granivorous ant species.     

Monitoring the temporal changes in herbicide-resistant Amaranthus tuberculatus: a landscape-scale probability-based estimation in Iowa

Background

A landscape-scale probability-based sampling of Iowa soybean [Glycine max (L.) Merr.] fields was conducted in 2013 and 2019; Amaranthus tuberculatus [Moq.] J.D. Sauer seed was collected from 97 random geospatial selected fields. The objectives were to evaluate the prevalence and distribution of herbicide-resistant A. tuberculatus (waterhemp) in soybean fields and evaluate temporal changes over 6 years. Amaranthus tuberculatus seedlings were evaluated for resistance to imazethapyr, atrazine, glyphosate, lactofen and mesotrione at 1× and 4× label rates.

Results

Resistance to imazethapyr, glyphosate, lactofen and mesotrione at the 1× rate increased significantly from 2013 to 2019 and was found in 99%, 97%, 16% and 15% of Iowa A. tuberculatus populations in 2019, respectively. Resistance to atrazine at the 4× rate increased over time; atrazine resistance was found in 68% of populations in 2019. Three-way multiple herbicide-resistant A. tuberculatus was the most frequent and increased significantly to 4× rates from 16% in 2013 to 43% of populations in 2019. All A. tuberculatus populations resistant to HPPD-inhibitor herbicides also were resistant to atrazine.

Conclusion

To the best of our knowledge, this is the first probability-based study that presented evolution of A. tuberculatus herbicide resistance over time. The results demonstrated that imazethapyr, atrazine and glyphosate resistance in Iowa A. tuberculatus populations was frequent whereas resistance to lactofen and mesotrione was less frequent. Most Iowa A. tuberculatus populations evolved resistance to multiple sites of action over time. The results of our study are widely applicable given the similarities in weed management practices throughout the Midwest United States. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.     

Testing the role of allelochemicals in different wheat cultivars to sustainably manage weeds

Background

Selecting wheat varieties with allelopathic potential or high competitiveness against weeds is a sustainable solution for organic farming to eliminate the use of synthetic herbicides. Wheat is one of the most economically important crops. This study focuses on screening the allelopathic or competitive potential of four wheat cultivars, Maurizio, NS 40S, Adesso and Element, on two weeds of interest due to acquired herbicide resistance, Portulaca oleracea and Lolium rigidum, through germination and growth bioassays and the identification and quantification of benzoxazinoids (BXZs) and polyphenols (phenolic acids and flavonoids).

Results

The different cultivars showed different abilities to manage surrounding weeds and different capacity to exude or accumulate specialized metabolites in the presence of those weeds. Furthermore, each cultivar behaved differently depending on the weed present in the medium. The most efficient cultivar to control the tested monocot and dicot weeds was Maurizio, as it effectively controlled germination and growth of L. rigidum and P. oleracea while exuding large amounts of benzoxazinones through the roots, especially the hydroxamic acids 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one and dihydroxy-2H-1,4-benzoxaxin-3(4H)-one. By contrast, NS 40S, Adesso and Element showed the potential to control the growth of just one of the two weeds through allelopathy or competition.

Conclusion

This study reveals that Maurizio is the most promising wheat cultivar for sustainable weed control, and that the screening of crop varieties with allelopathic potential, which results in the displacement of synthetic herbicides, is an immediate solution in ecological and sustainable agriculture. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.     

Direct measurement of paraquat in leaf protoplasts indicates vacuolar paraquat sequestration as a resistance mechanism in Lolium rigidum

Sequestration of paraquat away from its target site in the chloroplast has been proposed as a mechanism of paraquat resistance. However, no consensus has been reached as to where paraquat is sequestered. This study quantifies paraquat in leaf protoplasts of paraquat resistant (R) and susceptible (S) Lolium rigidum. Intact protoplasts were prepared from plants treated with commercial dose of paraquat for 2 h. Paraquat absorbed by the leaf protoplasts was determined by light absorption of reduced paraquat following concentration and purification using a cation-exchange resin. Leaf protoplasts from treated paraquat resistant plants contained 2- to 3-fold more paraquat than leaf protoplasts isolated from susceptible plants. Since paraquat is not metabolised in L. rigidum and paraquat readily enters chloroplasts of both R and S plants, this greater amount of paraquat in leaf protoplasts of R plant must be kept away from the target site (chloroplast). This result indicates that paraquat resistance in L. rigidum is associated with a cytoplasmic mechanism, most likely a greater rate of vacuolar sequestration.     

Absence of 3-(2,4-dichlorophenoxy)propionic acid in plants treated with 2,4-dichlorophenoxyacetic acid‡

Experiments were conducted to establish if the formation of 2.4-dichlorophenol (DCP) from 2.4-dichlorophenoxyacetic acid (2,4-D) in plants proceeds via 3-(2,4-dichtorophenoxy) propionic acid (2,4-DP) as an intermediate. Soybean (Glycine max (L.) Merr.), kidney bean (Phaseolus vtitgaris L.), pea (Pisum. sativum L.), smooth bromegrass (Bromus inermis Leyss.), wild oat (Avena fatua L.), yellow foxtail (Setaria glauca(L.) Beauv.), barley (Hordeum vulgare L.), timothy (Phleum pratense L.). and orchardgrass (Dactylis glomerata L.) were treated with equal weights of 2,4-D and 2,4-DP. After 3 days, all plants contained DCP; however, amounts were larger in plants treated with 2.4-DP than in those treated with 2,4-D. The 2.4-DP was absorbed from the leaf surface more efficiently than 2,4-D. No evidence was found for formation of 2,4-DP from 2,4-D in any of the plant species studied. L'absence de l'acide3-(2,4 dichlorophénoxy) propionique dans les plantes traitées avec I'acide 2,4-dichlorophénoxyacétique Des experiences ont été effectuées pour rechercher si la formation de 2,4-dichlorophénoI (DCP) dans les plantes, à partir de l'acide 2.4-dichlorophénoxy acétique (2,4-D) se produit par l'intermêdiaire de l'acide 3-(2,4-dichlorophenoxy) propionique. Le soja (Glycine max(L.) Merr.). le haricot (Phaseolus vuigaris L.), Ic pois (Pisum sativum L.), le brome (Bromus inermis Leyss.), la folle avoine (Avena fatua L.), la sétaire (Setaria glauca(L.) Beauv.). I'orge (Hordeum vulgare L.), la fléole (Phleum pratense L.) et le dactyle (Dactylis ghmerata L.) ont été traités par des poids égaux de 2.4-D et de 2,4-DP. Trois jours aprés, les plantes contenaient du DCP; toutefois, les quantités trouviés furent plus importantes dans les plantes traitées avec le 2,4-DP que dans celles traitées avec le 2,4-D. Le 2,4-DP a été absorbéà partir de la surface foliaire plus efficacement que le 2,4-D. La formation du 2,4-DP à partir du 2,4-D n'a pu être prouvée dans aucune des plantes étudiécs. Die Abwesenheit von 3-(2,4′Dichlorphenaxy)-propionsäure irt mil 2,4-Dichlorphenoxytesäure behandelten Pflanzen Es wurden Versuche durchgeführt um herauszufinden. ob die Bildung von 2,4-Dichlorphenol (DCP) aus 2.4-Dichlor-phenoxyessigsäure (2,4-D) in Pflanzen über 3-(2,4-Dichlor-phenoxy)-propionsaure (2,4-DP) als Zwischcnprodukt ver-läufl. Sojabohnen (Glycine max(L.) Merr.), Gartenbohnen (Phaseolus vutgaris L.), Erbsen (Pisum sativum L.), Wehrlose Trcspe (Bromus inermis Leyss.), Flughafer (Avena fama L.), Gelbe Borsienhirse (Setaria glauca(L.) Beauv.), Gerste (Hordetim vidgarc L.), Wiesenlieschgras (Phleum pratense L.) und Knaulgras (Dactylis ghmerata L.) wurden mit gleichen Mengen an 2,4-D und 2,4-DP behandelt. Nach drei Tagen enthielten alle PHanzen DCP; aber die Menge an DCP war in den mit 2,4-DP behandelten Pflanzen grosser als in den mit 2,4-D behandelten. 2,4-DP wurde von der Blatt-oberflache besser absorbiert als 2.4-D. In keiner der untersuchten Pflanzenarten konnte ein Hinweis für die Bildung von 2,4-DP aus 2,4-D gefunden werden.     

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...     

辽宁省稻田野慈姑对苄嘧磺隆的抗药性

为明确辽宁省不同稻田区野慈姑对苄嘧磺隆的抗性水平,整株测定了辽宁省大石桥(种群R1)、海城(种群R2)、苏家屯(种群R3)和开原(种群R4)共4个水稻产区野慈姑对苄嘧磺隆的抗性水平,并离体测定了各种群叶片体内乙酰乳酸合成酶(ALS)对苄嘧磺隆的敏感性。结果显示,种群R1和R2的抗药性相对较高,抗性指数分别为76.99和49.94,种群R3和R4抗性相对较低,抗性指数分别为12.48和16.91;离体测定结果表明较高水平的ALS活性可能与是否产生抗药性无关,种群R1、R2、R3、R4的抗性指数分别为81.86、67.48、10.56、24.86;抗药性程度依次为R1R2R4R3。表明4个水稻产区野慈姑对苄嘧磺隆均产生了抗药性,而其体内ALS活性降低可能是产生抗药性的原因之一。     

Rapid vacuolar sequestration: the horseweed glyphosate resistance mechanism

BACKGROUND: Glyphosate‐resistant (GR) weed species are now found with increasing frequency and threaten the critically important GR weed management system. RESULTS: The reported ³¹P NMR experiments on glyphosate‐sensitive (S) and glyphosate‐resistant (R) horseweed, Conyza canadensis (L.) Cronq., show significantly more accumulation of glyphosate within the R biotype vacuole. CONCLUSIONS: Selective sequestration of glyphosate into the vacuole confers the observed horseweed resistance to glyphosate. This observation represents the first clear evidence for the glyphosate resistance mechanism in C. canadensis. Copyright © 2010 Society of Chemical Industry     

Chemical control of herbicide-resistant Lolium rigidum Gaud. in north-eastern Spain

BACKGROUND: Lolium rigidum Gaud. is one of the most common weed species in winter cereals in north‐eastern Spain, with populations that have evolved resistance to herbicides becoming more widespread since the mid‐1990s. Nine trials on commercial fields with herbicide‐resistant L. rigidum were conducted during the cropping seasons 2001–2002 to 2003–2004, testing the efficacy of 20 herbicides and mixtures pre‐ and post‐emergence and as sequential applications. Weed populations chosen had different resistance patterns to chlortoluron, chlorsulfuron, diclofop‐methyl and tralkoxydim, representative of the resistance problems faced by farmers. RESULTS: In pre‐emergence, prosulfocarb mixed with trifluralin, chlortoluron or triasulfuron was effective on six populations. In post‐emergence, iodosulfuron alone or mixed with mesosulfuron gave the best results but did not control three resistant populations. At Ferran 1, none of the herbicide combinations reached 90% efficacy. CONCLUSIONS: The diverse efficacy patterns of the different populations demonstrate the need for detailed knowledge of the populations before using herbicides. Moreover, the unexpected insufficient efficacy of the new herbicide iodosulfuron prior to its field use shows the need to combine herbicides with other non‐chemical weed control methods to control resistant L. rigidum in north‐eastern Spain. Copyright © 2010 Society of Chemical Industry     

UNTERSUCHUNGEN ÜBER AUFNAHME UND TRANSLOKATION VON 14C-MARKIERTEN HERBIZIDEN IN AGROSTEMMA GITHAGO L. UND TUSSILAGO FARFARA L.

Studies of the uptake and translocation of ¹⁴C-labelled 2, 4-D, MCPA and aminotriazole in Agrostemma githago L. and Tussilago farfara L. clarified the behaviour of the herbicides in both species. In A. githago, MCPA was more freely mobile than 2,4-D after application to the leaf; it was distributed in the plant more rapidly and in greater quantity. Similarly, following root uptake MCPA was transported in the shoot in greater amounts than was 2,4-D. There is a clear relationship between the susceptibility of A. githago to MCPA and the mobility of the herbicide in the plant. In T. farfara, 2,4-D and aminotriazole applied to the leaves were equally well absorbed and relatively rapidly translocated. During the period up to 72 h the amounts of herbicide in the plant increased to similar levels; after that, ¹⁴C activity in plants treated with 2,4-D fell slightly whereas there was further accumulation of aminotriazole. Following uptake through the roots, translocation and accumulation in the leaves were considerably greater with aminotriazole than with 2,4-D. The lack of accumulation of 2,4-D could be a factor in the resistance of T. farfara to this herbicidie. Recherches sur l'absorption et la migration d'herbicides marqués au ¹⁴ C dans Agrostemma githago L. et Tussilago farfara L.     

Quantifying the detrimental effect of airborne dust on herbicide efficacy

One side effect of human activity and global climatic change is more airborne dust. This not only has undesirable effects on human health and the environment, but may also adversely affect many agricultural processes, including herbicide efficacy. This is particularly relevant in parts of the world where dust storms are common and limited rainfall means dust persists on plants for protracted periods. Accordingly, a series of greenhouse dose–response pot experiments was carried out to quantify the effect of a dust concentration of 1,500 µg/m³ on the performance of paraquat, glyphosate, sulfosulfuron and 2,4-D+MCPA. Dust-treated and untreated seedlings of Hordeum spontaneum and Sinapis arvensis were sprayed with sulfosulfuron and 2,4-D+MCPA, respectively, while paraquat and glyphosate were applied to both weed species. The efficacy of glyphosate and paraquat decreased significantly in the presence of dust on both species. Similar results were observed for 2,4-D+MCPA on S. arvensis. In contrast, the efficacy of sulfosulfuron on H. spontaneum was not affected by dust. In the presence of dust, the doses of glyphosate, paraquat and 2,4-D+MCPA required for a 90% effect were enhanced by a factor of 1.9 to 2.6. The negative impact on glyphosate efficacy was more pronounced in S. arvensis than H. spontaneum, while it was similar for paraquat. Future studies should consider the amount, duration and composition of dust as well as its impact on other weeds and herbicides. Potential approaches to overcome the detrimental influence of dust on the performance of herbicides were also suggested for future studies.     

Interpopulation variability and adaptive potential for reduced glyphosate sensitivity in Alopecurus myosuroides

Glyphosate use in the United Kingdom has more than doubled in the last 20 years. Much of this increase is driven by efforts to control herbicide resistant weeds, particularly Alopecurus myosuroides, prior to crop drilling. There is precedent for evolution of glyphosate resistance in similar situations, raising concerns over the sustainability of glyphosate use in the UK. We used dose–response experiments to examine variation in glyphosate sensitivity amongst 40 field‐collected A. myosuroides populations. No populations were resistant to glyphosate, but ED₉₀ values ranged between 354 and 610 g a.i. ha^?1. Five populations had ED₉₀ values significantly higher than the unexposed control population collected from a site at Rothamsted Research with no previous glyphosate exposure. Recurrent selection experiments were performed to determine whether variation in glyphosate sensitivity had a heritable basis. Following two rounds of selection, five of six field populations evolved significantly reduced sensitivity to glyphosate, with R/S ratios, based on estimated ED₅₀ values, ranging from 1.2 to 1.5. These results confirm that there is a heritable basis to variation in glyphosate sensitivity. The response to selection was modest. Evolved populations were not highly resistant to glyphosate, although some twice‐selected individuals survived recommended field rates. These results do not represent definitive proof of the potential of A. myosuroides to evolve glyphosate resistance, although they do indicate caution is needed when considering the sustainability of increased glyphosate use to control this herbicide resistance‐prone species.     

Imidacloprid‐susceptible Nilaparvata lugens individuals exceeded resistant individuals in a mixture population with density pressure

BACKGROUND

Fitness costs associated with insecticide resistance in pest insects have mainly been studied under optimal laboratory conditions. However, resistant insects face more stressors than just insecticides in the field, and how the resistant population reacts to these stressors is of practical importance for the control of pest insects such as the brown planthopper Nilaparvata lugens. The aim of the present study was to explore the impact of population density on the competitiveness of resistant and susceptible individuals.

RESULTS

Two isogenic N. lugens populations, a highly imidacloprid‐resistant population (HZ‐R) with a resistance ratio (RR) of 227.10 and a relatively susceptible population (HZ‐S) with an RR of 2.99, were created from a field‐resistant population (HZ; RR 62.51). The high resistance levels of HZ‐R and HZ were mainly attributable to the overexpression of multiple cytochrome P450 (CYP) genes such as CYP6ER1, CYP6AY1, CYP6CW1 and CYP4CE1 compared with HZ‐S, this being supported by piperonyl butoxide synergism. HZ‐R was observed to be more resistant to thiacloprid and etofenprox compared with HZ and HZ‐S. Most interestingly, in high population density treatments, HZ‐S individuals were much more competitive than HZ‐R individuals.

CONCLUSION

Imidacloprid‐resistant individuals of N. lugens are less competitive than their susceptible counterparts under density pressure. © 2017 Society of Chemical Industry     

Simulating evolution of glyphosate resistance in Lolium rigidum I: population biology of a rare resistance trait

Despite frequent use for the past 25 years, resistance to glyphosate has evolved in few weed biotypes. The propensity for evolution of resistance is not the same for all herbicides, and glyphosate has a relatively low resistance risk. The reasons for these differences are not entirely understood. A previously published two‐herbicide resistance model has been modified to explore biological and management factors that account for observed rates of evolution of glyphosate resistance. Resistance to a post‐emergence herbicide was predicted to evolve more rapidly than it did to glyphosate, even when both were applied every year and had the same control efficacy. Glyphosate is applied earlier in the growing season when fewer weeds have emerged and hence exerts less selection pressure on populations. The evolution of glyphosate resistance was predicted to arise more rapidly when glyphosate applications were later in the growing season. In simulations that assumed resistance to the post‐emergence herbicide did not evolve, the evolution of glyphosate resistance was less rapid, because post‐emergence herbicides were effectively controlling rare glyphosate‐resistant individuals. On their own, these management‐related factors could not entirely account for rates of evolution of resistance to glyphosate observed in the field. In subsequent analyses, population genetic parameter values (initial allele frequency, dominance and fitness) were selected on the basis of empirical data from a glyphosate‐resistant Lolium rigidum population. Predicted rates of evolution of resistance were similar to those observed in the field. Together, the timing of glyphosate applications, the rarity of glyphosate‐resistant mutants, the incomplete dominance of glyphosate‐resistant alleles and pleiotropic fitness costs associated with glyphosate resistance, all contribute to its relatively slow evolution in the field.