High level of metaflumizone resistance and multiple insecticide resistance in field populations of Spodoptera exigua (Lepidoptera: Noctuidae) in Guangdong Province,China

Spodoptera exigua is a phytophagous pest that causes critical economic losses in vegetable crops, and insecticides are commonly used against it in vegetable growing areas. However, excessive and frequent applications of insecticides cause resistance in S. exigua. The current resistance in field populations of S. exigua collected from Huizhou, Guangdong Province, China to 12 insecticides was investigated. S. exiguahad developed very high resistance to lambda cyhalothrin (2925- to 3449-fold), chlorpyrifos (>1786-fold), emamectin benzoate (174- to 867-fold), and metaflumizone (60.3- to 942-fold). High resistance to tebufenozide (51.5- to 75.4-fold) and chlorfluazuron (60.4- to 63.0-fold) was also found. Synergism assays revealed that the resistance to metaflumizone and lambda cyhalothrin was associated with esterase and microsomal oxidases, respectively. The resistance to emamectin benzoate was not affected by detoxification enzymes inhibitors and might be conferred by other mechanisms. The selection of the field population by metaflumizone for 10generations in the laboratory resulted in a 6.1-fold increase in metaflumizone resistance but did not lead to increases in resistance to other insecticides. After metaflumizone selection, susceptibilities to spinosad and endosulfan did not change, and the susceptibilities to indoxacarb, methomyl, pyridalyl, tebufenozide, chlorfluazuron, emamectin benzoate and lambda cyhalothrin decreased slightly. However, no statistically significant differences in the resistance levels were observed among the selected population, its starting strain and the unselected strain. The resistance to chlorantraniliprole noticeably decreased in unselected strain and the strain subjected to selection for 10 generations compared with their starting strains. Lack of cross-resistance to tested insecticides suggested the involvement of multiple mechanisms of resistance and the need for wise application of these insecticides for the management of S. exigua.     

灰飞虱对几类杀虫剂的抗性和敏感性

采用点滴法和稻茎浸渍法分别测定了2006年4月采自江苏省无锡市和浙江省湖州市灰飞虱种群对9种杀虫剂的抗性及17种杀虫剂的敏感性。点滴法测定结果表明,无锡种群和湖州种群对吡虫啉均产生了高水平抗性,抗性倍数分别为79.6倍和44.6倍;对残杀威的抗性倍数分别为76.6倍和40.1倍,属高水平抗性;对甲萘威的抗性倍数分别为29.8倍和45.3倍,属中等-高水平抗性;对二嗪磷、杀螟硫磷、仲丁威、丁硫克百威、醚菊酯、氰戊菊酯的抗性倍数为1.4~8.1倍,属敏感-低水平抗性。毒力测定结果表明氟虫腈、丁烯氟虫腈对无锡和湖州灰飞虱3龄若虫的毒力最高,LC50值为0.21~041 mg/L;噻虫嗪、烯啶虫胺、毒死蜱、吡蚜酮、异丙威、敌敌畏为田间药效试验的推荐用药。还讨论了灰飞虱抗药性的治理。     

Field evolution of insecticide resistance in the brown planthopper (Nilaparvata lugens Stål) in China

The brown planthopper (Nilaparvata lugens Stål) is one of the most destructive pests of rice crops in Asian countries including China, Vietnam, Thailand, etc. Evolution of resistance in this pest insect to isoprocarb, buprofezin, pymetrozine, imidacloprid and other neonicotinoid insecticides has been reported. In order to investigate the current status of resistance to commonly used insecticides, nine field populations of N. lugens were collected from Central China, East China and South China, and resistance to insecticides was monitored from 2009 to 2012. All the 9 field populations collected in 2012 had developed extremely high resistance to imidacloprid, with resistance ratios (RR) ranging from 209.3 to 616.6. Resistance to imidacloprid was much higher in 2012 than in 2009. The RR of thiamethoxam varied from 17.4 to 47.1, and the RR of nitenpyram varied from 1.4 to 3.7 in 2012. Of the 9 field populations, six populations showed higher resistance to nitenpyram in 2012 than in 2011. RR for buprofezin varied from 110.1 to 221.6 in 2012 whereas resistance was at a medium level (RR 20.4 ∼ 39.4-fold) in 2009. RR for pymetrozine ranged from 34.9 to 46.8 in 2012. As for isoprocarb, RR ranged from 21.7 to 38.1 in 2012. The obvious increase in resistance to widely applied insecticides indicates that insecticide resistance management strategies are urgently needed to prevent or delay further increase of insecticide resistance in N. lugens.     

Monitoring of insecticide resistance in Diaphorina citri Kuwayama (Hemiptera: Psyllidae) from citrus groves of Punjab,Pakistan

The Asian citrus psyllid, Diaphorina citri Kuwayama being a vector of huanglongbing (HLB), citrus greening disease is the most destructive pest of citrus and the management of D. citri is crucial for successful control of HLB. We evaluated adult populations of D. citri from twelve districts of Punjab, Pakistan for resistance to seven different insecticides. Different levels of resistance ratios were observed for all insecticides (chlorpyrifos, bifenthrin, imidacloprid, acetamiprid, thiamethoxam, nitenpyram and chlorfenapyr). Field collected populations of D. citri were highly resistant to imidacloprid as compared to the susceptible population. The resistance ratios were in range of 236.6–759.5, 55.5–212.8, 13.1–46.4, 31.4–216.7, 8.6–89.4-fold for imidacloprid, acetamiprid, chlorfenapyr, nitenpyram, and thiamethoxam, respectively and 39.8–107.1 and 32.7–124.5-fold in case of conventional insecticides i.e., bifenthrin and chlorpyrifos, respectively. Nitenpyram and thiamethoxam, with no or very low resistance should be used in combination or in rotation with other pest management tactics for managing resistance in D. citri. The correlation analysis of the LC₅₀'s of insecticides showing positive and negative correlations among different insecticides in all tested populations, suggests mechanism of cross-resistance. Imidacloprid showed a positive correlation with acetamaprid, but a negative correlation with thiamethoxam from the neonicotenoid group, while the resistance to chlorfenpyr positively correlated with chlorpyrifos and bifenthrin in the pyrethroid group. Multiple resistance mechanisms could be the reason behind the development of such a high resistance in the D. citri.     

Resistance in Maconellicoccus hirsutus (Green) in India to selected insecticides and quantification of detoxifying enzymes imparting resistance

Pink mealybug Maconellicoccus hirsutus (Green) (Hemiptera: Pseudococcidae) is a destructive pest of agricultural and horticultural crops. Insecticides are the major tool used to control M. hirsutus. The present study was conducted to evaluate resistance to the commonly used insecticides acephate, dichlorvos, imidacloprid and buprofezin in M. hirsutus collected from seven different geographical locations of mulberry and vineyards in India. Detoxifying enzymes, namely esterase, glutathione S transferase (GST) and cytochrome P-450 (cyt-P450), were quantified in populations. One population from Erode showed a low level of resistance to acephate (resistance ratio [RR] 10.3-fold), one from Salem showed a low level of resistance to dichlorvos (resistance ratio [RR] 13.7-fold), one from Sangli showed a very low level of resistance to imidacloprid (RR 10.2-fold), and one from Chikkaballapur showed a low level of resistance to buprofezin (RR 14.8-fold). Activity ratios for detoxifying enzymes ranged from 1.8- to 4.9-fold for GST, 1.8- to 3.7-fold for esterase and 1.9- to 2.4-fold for cyt-P450. Furthermore, organophosphate resistance and activity of enzymes (esterase, GST and cyt-P450) were positively correlated. To contain the evolution of resistance to M. hirsutus infestation, buprofezin and imidacloprid could be used, supplemented with biointensive integrated management strategies and regular resistance monitoring programs.     

Resistance monitoring and enzyme activity in three field populations of cowpea aphid (Aphis craccivora) from Egypt

The cowpea aphid (Aphis craccivora) is one of the most important sucking insect pests attacking certain legumes in Egypt particularly faba bean, cowpea and pea. In this study we monitored the resistance level of three field populations of A. craccivora to seven insecticides belonging to three different chemical classes (organophosphates, carbamates and neonicotinoids). The three populations were collected from three governorates in Egypt namely Dakahlia, Qalyobia and Beni Suef. Diagnostic concentrations (LC₉₀ values for susceptible strain) for each insecticide were established using a leaf dipping technique. Resistance monitoring showed that the field population from Dakahlia was highly resistant to all the tested insecticides. In a similar manner, the population from Qalyobia was also resistant to all insecticides except for fenitrothion to which only moderate resistance was observed. The field population from Beni Suef exhibited a lower level of resistance to all the seven tested insecticides.Biochemical assay showed that esterase activity in these three field populations was generally higher as the enzyme activity ratio ranged from 4.3 to 7.8 fold more than that for the susceptible strain. The activity of the other measured detoxifying enzymes (glutathione -S- transferase and mixed function oxidases) was moderate in the populations from Qalyobia and Dakahlia. Nevertheless, the enzyme activity in A. craccivora collected from Beni Suef was variable and differed slightly from the activity measured in the susceptible strain. Monitoring insecticide resistance among the three aphid populations was a proactive approach to detect any shift in insecticide efficiency. The possible occurrence of resistance in the cowpea aphid to the tested insecticides may be due to the higher activity of carboxylesterases. Further studies on the resistance mechanism to these insecticides are needed to provide insights in how to manage and delay the onset of the resistance and thus prolong the performance of insecticides against A. craccivora.     

Stability,cross-resistance and fitness costs of resistance to emamectin benzoate in a re-selected field population of the beet armyworm,Spodoptera exigua (Lepidoptera: Noctuidae)

Spodoptera exigua (Hübner) has a worldwide distribution with a high capacity for damaging a wide range of food, forage and fiber crops. It has been reported extensively from all over the world that populations of this pest species have developed field resistance against many insecticides. The objectives of this study were to determine whether an emamectin benzoate resistant field population of S. exigua re-selected with emamectin benzoate in the laboratory (Ema-SEL) showed cross-resistance to other insecticides, whether resistance was stable under laboratory conditions, and whether there were fitness costs associated with emamectin benzoate resistance. Bioassays at G₁ for the field population, gave resistance ratios (RRs) of 220, 149 and 38-fold for emamectin benzoate, spinosad and lufenuron, respectively, compared with a susceptible laboratory population (Lab-PK). Resistance ratios were increased by 526-fold and 6-fold compared with Lab-PK and the unselected field population (Ema-UNSEL, G₆), respectively after selection with emamectin benzoate (Ema-SEL) for five generations (G₆). Selection with emamectin benzoate had no apparent effect on susceptibility of Ema-SEL to spinosad and lufenuron, instead toxicity to the latter insecticides reduced, suggesting there was no cross-resistance between these compounds. Analysis of various life history traits suggested that the Ema-SEL population had a lower overall fitness (0.38) compared with the Lab-PK (1.0). Lack of cross-resistance and the apparent instability of resistance to emamectin benzoate suggest that spinosad and lufenuron are suitable alternatives for use with emamectin benzoate in resistance management. In addition, the high relative fitness costs observed suggests that emamectin benzoate-resistant insects are at a considerable disadvantage to susceptible populations in the absence of selection pressure although this remains to be tested under field conditions.     

Efficacy of ketoenols on insecticide resistant field populations of two-spotted spider mite Tetranychus urticae and sweet potato whitefly Bemisia tabaci from Greece

Tetranychus urticae (two-spotted spider mite) and Bemisia tabaci (sweet potato whitefly), two of the most important agricultural pests worldwide, have developed resistance to almost all chemical classes of insecticides. Here we investigated the efficacy of the ketoenols spirodiclofen and spiromesifen against, respectively, T. urticae and B. tabaci populations from Greece with variable levels of resistance to other pesticides in order to evaluate their potential role in insecticide resistance management strategies for sustainable control of both pests. No resistance of T. urticae populations against spirodiclofen was observed. Nine out of ten T. urticae populations exhibited moderate levels of resistance to pirimiphos-methyl (up to 23-fold), pyridaben (up to 39-fold) and fenazaquin (up to 42-fold). Two of them exhibited high resistance ratios (RR) to bifenthrin (RR = 81, 351) and one to fenbutatin oxide (RR = 146). Moreover these nine populations were not cross resistant to spirodiclofen (RR < 5). A single population from greenhouse roses exhibited high resistance levels to most of the pesticides tested (RR = 8413, 1494, 434, and 74 to bifenthrin, fenbutatin oxide, abamectin and pirimiphos-methyl, respectively) showed decreased susceptibility to spirodiclofen (RR = 12). In B. tabaci the variation in response to spiromesifen treatments was very low among the eleven field populations. Most of them exhibited high levels of resistance to imidacloprid, and alpha-cypermethrin. The LC₅₀ of spiromesifen ranged from 4.5 to 14 mg/l and the corresponding LC₉₅ values were in general well below the recommended field doses. Ketoenol resistance risk potential was also investigated by applying constant selection pressure under laboratory conditions against populations from both species, but no increase in LC₅₀s was detected. Ketoenols can be used for resistance management purposes for both pests in Greece as an effective chemical tool in rotation with other pesticides.     

Cross-resistance, inheritance and stability of resistance to acetamiprid in cotton whitefly, Bemisia tabaci Genn (Hemiptera: Aleyrodidae)

The cotton whitefly Bemisia tabaci, (Genn.) is an important pest of field crops, vegetables and ornamentals worldwide. Neonicotinoids are considered an important group of insecticides being used against B. tabaci for several years. B. tabaci has developed resistance to some of the compounds of the group. This study was designed to investigate if the selection of B. tabaci with acetamiprid would give a broad-spectrum of cross-resistance and to genetically classify the resistance. At G₁ a low level of resistance to acetamiprid, imidacloprid, thiamethoxam, thiacloprid and nitenpyram was observed with resistance ratios of 3-fold, 8-, 9-, 6- and 5-fold, respectively, compared with a laboratory susceptible population. After selection for eight generations with acetamiprid, resistance to acetamiprid increased to 118-fold compared with the laboratory susceptible population. Selection also increased resistance to imidacloprid, thiamethoxam, thiacloprid, nitenpyram, endosulfan and bifenthrin but no change in susceptibility to fipronil was observed. Furthermore resistance in a field population was stable in the absence of acetamiprid selection pressure. Genetic crosses between resistant and susceptible populations indicated autosomal and incompletely recessive resistance. Further genetic analysis suggested that resistance could be controlled by a single factor. The high level of cross-resistance and stability of incomplete resistance in the field population is of some concern. However, lack of cross-resistance between acetamiprid and fipronil or unstable resistance in the resistant population could provide options to use alternative products which could reduce acetamiprid selection pressure.     

Resistance in the mealybug Phenacoccus solenopsis Tinsley (Homoptera: Pseudococcidae) in Pakistan to selected organophosphate and pyrethroid insecticides

The mealybug Phenacoccus solenopsis is a destructive pest of cotton with the potential to develop resistance to most chemical classes of insecticides. Six populations of P. solenopsis from cotton crops at six different locations in Pakistan were evaluated for resistance to selected organophosphate and pyrethroid insecticides. Resistance ratios (RRs) at LC₅₀ were in the range of 2.7–13.3 fold for chlorpyrifos, 11.6–30.2 fold for profenofos and for the three pyrethroids tested were 10.6–46.4 for bifenthrin, 5.8–25.2 for deltamethrin and 4.1–25.0 for lambda-cyhalothrin. This is the first report of resistance to organophosphate and pyrethroid insecticides in Pakistani populations of P. solenopsis. Regular insecticide resistance monitoring programs are needed to prevent field control failures. Moreover, integrated approaches including the judicious use of insecticides and rotation of insecticides with different modes of action are needed to delay the development of insecticide resistance in P. solenopsis.     

Monitoring resistance of field populations of diamondback moth Plutella xylostella L. (Lepidoptera: Yponomeutidae) to five insecticides in South China: A ten-year case study

Diamondback moth Plutella xylostella (L.) is a cosmopolitan pest of crucifers and it is particularly notorious for its resistance to commonly used insecticides. To provide a basis for future resistance management strategies, this study evaluated the resistance status of this pest to five insecticides, namely abamectin, β-cypermethrin, fipronil, monosultap and phoxim, in South China from 1999 to 2009 with 4-6 populations tested each year. Laboratory bioassays using a standard leaf-dip method were conducted on 3rd instar larvae which were the progeny of field collected insects. LC₅₀ values were estimated by probit analysis and resistance factors were calculated by comparing the field populations with a fully insecticide-susceptible lab population. The results showed that the diamondback moth populations generally had low resistance to abamectin from 1999 to 2005, but that resistance increased significantly during 2007-2009 with resistance factors as high as 122.4. Resistance factors for β-cypermethrin in 35 populations tested were high to very high in all 10 years with values ranging from 32.2 to 683.6. Resistance to fipronil had a progressive increase and increased markedly from 2007 to 2009 with one resistance factor of 56.1. Resistance to monosultap was low in 2004, but increased progressively from 2007 to 2009 with one resistance factor of 129.1. Resistance to phoxim was low to moderate from 2004 to 2008, but was moderate and high in 2009 with resistance factors ranging from 58.7 to 129.1. These results indicate that resistance of diamondback moth populations to most commonly used insecticides in South China has increased and is now generally high.     

Biochemical characterization of acetylcholinesterase, cytochrome P450 and cross-resistance in an omethoate-resistant strain of Aphis gossypii Glover

The degree of insecticide resistance, synergism effects, acetylcholinesterase (AChE) activity kinetics, specific activities of detoxification enzymes and cross-resistance were investigated in omethoate resistant and relatively susceptible strains of Aphis gossypii Glover. The resistant cotton aphid strain (ORR) exhibited 231.3-fold resistance to omethoate compared to the susceptible strain (OSS). Synergist piperonyl butoxide (PBO) dramatically increased the toxicity of omethoate in the resistant strain, while triphenyl phosphate (TPP) and diethyl maleate (DEM) did not exhibit synergism effects. The calculated AChE activity, V_max and K_m ratios of ORR to OSS were 0.1, 0.2 and 0.4, respectively. Based on analysis of IC₅₀ indices, enzyme inhibition experiments showed that AChE from the ORR strain was 10.6-, 3.2-, 6.2-, 10.5- and 4.4-fold more insensitive to inhibition by eserine, omethoate, paraoxon, paraoxon-methyl and malaoxon, respectively, than that from the OSS strain. The cytochrome P450-mediated O-demethylation activity (3.7-fold) and ethoxycoumarin-O-deethylase activity (2.6-fold) in the ORR strain were significantly higher than in the OSS strain. Specific activity of carboxylesterase (CarE) and glutathione S-transferase (GST) were not significantly different in both the ORR and OSS strains. Bioassay results indicated the ORR strain had developed high levels of cross-resistance to chlorpyrifos (24.2-fold), malathion (21.1-fold), acephate (10.2-fold), esfenvalerate (30.6-fold), methomyl (22.4-fold), carbofuran (33.2-fold), but had negative cross-resistance to bifenthrin (0.4-fold). Overall, these results demonstrate that reduced AChE sensitivity, combined with increased cytochrome P450 monooxygenase detoxification, plays an important role in the high levels of omethoate resistance and can cause cross-resistance to other insecticides in the ORR strain.     

Susceptibility to Insecticides and Ecological Fitness in Resistant Rice Varieties of Field Nilaparvata lugens Stl Population Free from Insecticides in Laboratory

A population of rice brown planthopper(BPH) Nilaparvata lugens collected from a paddy field in Hangzhou was successively reared on susceptible rice Taichung Native 1(TN1) in a laboratory free from insecticides for more than 14 generations. The changes in susceptibility to insecticides and ecological fitness on different resistant rice varieties were monitored in each generation. The resistance ratio to imidacloprid sharply declined with the succession of rearing generations without insecticides from 359.94-fold at F1 to 6.50-fold at F14 compared with the susceptible strain, and the resistance ratio to chlorpyrifos was from 9.90-fold at F1 to 5.94-fold at F14. Nymphal duration and weights of newly hatched female adults were significantly affected by rice variety, generation and their interactions, but nymphal survival was significantly affected by the generation only. The ratio of brachypterous adults in males was affected by the generation and generation × variety interaction, whereas no difference was found in females. Nymphal duration extended with increasing generations, and the female nymphal duration was shorter in the susceptible variety TN1 than those in the resistant varieties IR26 and IR36. In addition, the female adult weight in TN1 was higher than those in IR26 and IR36. These results indicated that the resistance of field BPH population to insecticides was reversed after several generations of no-exposure to insecticides, and the ecological fitness in TN1 was higher than those in IR26 and IR36. These findings suggested the rational and reduced use of insecticides in combination with the manipulation of resistant rice varieties would be effective for BPH management.     

褐飞虱对吡虫啉敏感性的时空变化及现实遗传力

为科学用药,采用稻茎浸渍法测定了我国7省（区）42个田间褐飞虱种群对吡虫啉的敏感性时空变化。结果表明,在1996－2003年,除了1997年广西桂林种群为低水平抗性（6.3倍）外,苏、皖、桂3省13个种群对吡虫啉为敏感至敏感性下降（<5倍）;然而,2005年苏、浙、皖、赣、湘、桂6省（区）16个大田种群对吡虫啉的抗性迅速上升,达高水平至极高水平抗性（79～811倍）;2006年,除江苏通州大田种群的抗性为627倍外,苏、浙、皖、赣、湘、桂、闽7省（区）11个种群的抗性为150～322倍,比2005年有一定程度下降,这可能与暂停使用吡虫啉有关。室内用吡虫啉对褐飞虱筛选18代,其抗性由筛选前的208.3倍上升到筛选后的1110.8倍,抗性现实遗传力（h2）为0.1414。这暗示褐飞虱对吡虫啉产生高水平抗性后,如继续使用吡虫啉防治,其抗性可能会进一步上升。还讨论了稻褐飞虱抗药性的治理策略。     

Insecticide resistance in Brazilian populations of the cotton leaf worm, Alabama argillacea

The cotton leaf worm, Alabama argillacea, is a key cotton pest in Brazil and is managed with repeated insecticide applications. Reports of insecticide control failures have recently increased, particularly with pyrethroids. The present work assessed the resistance status of A. argillacea to a number of different insecticides currently used in cotton crops. Bioassays were conducted to estimate the response of 2nd-instar A. argillacea populations to deltamethrin, chlorpyrifos, endosulfan, abamectin and spinosad. A leaf dip bioassay with diluted insecticide formulations was performed in the laboratory with five to nine populations depending on the insecticide. LC₅₀ values were estimated by probit analysis after correction for control mortality data and used to calculate the resistance ratios (RR). All assessed populations exhibited varied and significant levels of resistance to all insecticides tested, but only moderate levels of resistance to deltamethrin were observed (RR = 52.3). The LC₅₀ values for deltamethrin were higher than 30 mg/l for most populations, and above the field rate (12.5 mg/l). This suggests that the frequency of resistant individuals in these populations was likely above the critical frequency. There was low to moderate resistance to abamectin, chlorpyrifos, endosulfan and spinosad formulations (the highest RRs observed were 4.2, 8.4, 11.1 and 23.5, respectively). Despite the moderate levels of resistance to pyrethroids in A. argillacea, overall results indicate the presence of low to moderate resistance of A. argillacea to insecticides currently used against cotton pests in Brazil.     

二化螟抗药性监测和对三唑磷抗性的遗传分析

采用点滴法测定了2001~2002年浙、苏、皖、赣等4省水稻二化螟4龄幼虫对氟虫腈、阿维菌素、三唑磷及杀虫单等的抗性;用三唑磷高抗种群经室内筛选纯化后与室内敏感种群进行正反交、回交,对三唑磷抗性进行了初步遗传分析。结果表明：(1) 通过对4省7个种群的检测,发现浙江瑞安种群对氟虫腈有低水平抗性（抗性倍数为6.5倍）,其他种群虽存在敏感度差异（1.0~1.6倍）,但均属敏感范围;(2) 浙、苏、皖三省6个不同种群对阿维菌素的敏感度变化小于2.3倍,未检测到明显抗药性;(3) 4省10个种群对三唑磷敏感度变化很大（LD50值为有效成分0.0052~1.0114 μg/头）,其中瑞安种群具有高水平抗性（163.1倍）,温州种群为中等水平抗性（18.2倍）,常熟、锡山和金坛3种群为低水平抗性（6.7~9.7倍）,新洋农场、楚州、连云港、宜丰和太湖5种群为敏感（0.8~2.9倍）;(4) 4省9个种群对杀虫单均有不同程度抗药性,瑞安和宜丰种群达高水平抗性（113.7和57.6倍）,温州、常熟、金坛和太湖4种群为中等水平抗性（11.0~29.7倍）,新洋农场和锡山种群为低水平抗性（6.7和7.5倍）,灌云种群抗性最低,为3.7倍;(5)瑞安种群对三唑磷抗性属单个主基因控制的不完全显性性状,但不排除微效修饰基因的存在。     

Selection,mechanism, cross resistance and stability of spinosad resistance in Spodoptera litura (Fabricius) (Lepidoptera: Noctuidae)

Spodoptera litura is one of the most destructive pests in Pakistan and in many other regions of the world. A field collected population of S. litura was selected with spinosad for eleven generations under controlled laboratory conditions to study the cross resistance, mechanism and stability of spinosad resistance in S. litura. The resistance to spinosad in S. litura increased 3921-fold (after eleven generations of selection with spinosad) as compared to a susceptible population of S. litura. No cross resistance between spinosad and emamectin benzoate, methoxyfenozide, fipronil, indoxacarb, profenofos, lufenuron or deltamethrin was found in the spinosad-selected population of S. litura. To find the possible mechanism of spinosad resistance in S. litura two synergists, Piperonyl butoxide (PBO), S, S, S-tributyl phosphorotrithioate (DEF) were tested on the susceptible and resistant strains and on the un-selected field population. The values of the synergism ratios of PBO and DEF were 2.33 and 1.06 for the spinosad-selected strain, 1.36 and 1.06 for the un-selected field population and 1.14 and 1.00 for the susceptible strain, respectively. As high PBO ratio indicates the role of microsomal O-demethylase in causing spinosad resistance in S. litura. The spinosad-resistant and field populations of S. litura were reared without any selection pressure from the 12th to the 16th generation (G₁₂–G₁₆). The spinosad resistance decreased from 3921 to 678-fold in the spinosad-resistant population and from 31.1 to 15.1-fold in the un-selected population of S. litura as compared to the susceptible strain. Spinosad resistance in S. litura has a high reversion rate (−0.15) which indicates that spinosad resistance in S. litura is unstable and can be easily managed by switching off the selection pressure for a few generations or alternating with insecticides having different modes of action.     

Regional susceptibilities to 12 insecticides of melon and cotton aphid,Aphis gossypii (Hemiptera: Aphididae) and a point mutation associated with imidacloprid resistance

The melon and cotton aphid Aphis gossypii Glover (Hemiptera; Aphididae) is one of the most serious pests worldwide. We surveyed insecticide susceptibility in A. gossypii field populations to 12 insecticides (6 neonicotinoids, 3 pyrethroids and 3 others) to examine resistance ratios. The levels of insecticide resistance were extremely high, especially to neonicotinoids, such as acetamiprid, clothianidin, thiacloprid and imidacloprid. To identify the neonicotinoid resistance mechanisms, we used an imidacloprid-resistant (IMI-R) strain as a model strain. IMI-R showed an extremely high resistance ratio and also cross-resistance to all the test neonicotinoids. However, there was little or no cross-resistance to the other insecticides, including sulfoxaflor. Synergist tests and enzyme activity assays suggested the absence of resistance mechanisms based on enhanced detoxification enzymes, such as cytochrome P450, esterase and glutathione S-transferase. One point mutation was found in the beta1 subunit loop D region of the nicotinic acetylcholine receptor (nAChR) of the IMI-R strain. This R81T point mutation was also found in field populations collected from 5 regions. Therefore, the R81T point mutation was identified as an important mechanism of imidacloprid resistance in A. gossypii.     

Susceptibility levels of Tuta absoluta (Meyrick) (Lepidoptera: Gelechiidae) to minor classes of insecticides in Brazil

The tomato leafminer, Tuta absoluta (Meyrick), is one of the most important pests of tomato worldwide. The use of chemicals is still the major tactic of control against this pest in Brazil, where spray overuse leads to resistance, frequently associated with control failures in the field. In this study, a survey of the susceptibility to indoxacarb, metaflumizone, chlorfenapyr, cartap, and abamectin aimed to determine the resistance status of T. absoluta populations. Also, the major enzyme systems associated with metabolic resistance were assessed to infer variability. The LC₅₀ values varied among the populations for abamectin (0.54–3.38 mg a.i./L), cartap (93.1–589.8 mg a.i./L), chlorfenapyr (0.62–2.83 mg a.i./L), indoxacarb (0.86–2.89 mg a.i./L), and metaflumizone (0.35–7.44 mg a.i./L). Resistance ratios varied among populations being 6.2-, 6.4-, 4.6-, 3.3-, and 21.2-times for abamectin, cartap, chlorfenapyr, indoxacarb, and metaflumizone, respectively. Only the cartap confidence limits of the LC₈₀ bracketed the recommended label concentration for three populations (Anápolis, Guaraciaba do Norte, and Tianguá), suggesting control failures. No cross-resistance was observed between indoxacarb and metaflumizone, and natural variation may explain the variability of response to this latter insecticide. The activity of enzymes frequently associated with metabolism of insecticides significantly differed among populations, and glutathione S-transferases and cytochrome P450-dependent monooxygenases were variable among the populations of T. absoluta, while alpha and beta-esterases were very homogeneous. T. absoluta resistance to abamectin and cartap has not been critical in Brazil despite their long use and together with chlorfenapyr appear to be an interesting option of rotation with indoxacarb, for which there is no cross-resistance to metaflumizone.     

Monitoring resistance of Helicoverpa armigera to different insecticides used in cotton in Spain

Helicoverpa armigera is the key pest of cotton in Spain, resulting in many insecticide treatments against it. The resistance status of H. armigera to different insecticides currently used in cotton was evaluated in Spain in two different seasons, 1999 and 2004. Four populations were tested in total, two in each season. Toxicological bioassays were conducted in the laboratory, and performed on third instar larvae by topical application of the insecticides. LD₅₀'s were estimated by probit analysis and resistance factors (RF) were calculated at the LD₅₀ level. Four insecticides were evaluated, but only endosulfan reached a moderate resistance level (RF = 11.4), and the others (methomyl, chlorpyrifos and lambda-cyhalothrin) showed low resistance (RF between 1.9 and 6.0). Such results indicate the generally low resistance of H. armigera to most of the insecticides used against this pest in cotton in Spain. Possible explanations for this situation are discussed.