Biochemical study of resistance to imidacloprid in B biotype Bemisia tabaci from Guatemala

Systemic uptake bioassays using excised cotton leaves confirmed resistance to imidacloprid in a Guatemalan population of the tobacco whitefly Bemisia tabaci Gennadius. Polyacrylamide gel electrophoresis of naphthyl esterases identified the insects as B-types. Upon collection from the field, resistance was determined to be 58-fold relative to a susceptible strain originating in the Imperial Valley of California. Resistance levels increased to 126-fold in this population during its continuous exposure to systemically treated cotton. In biochemical investigations, there was no detectable NADPH-dependent mixed function oxidase metabolism of 14C-imidacloprid at any time during the selection process. In contrast, microsomal preparations from housefly abdomens readily produced significant amounts of the mono-hydroxy and olefin derivatives of the parent compound. Detoxification of imidacloprid by housefly MFOs may account for reports of lower toxicity of the insecticide towards this insect compared with whiteflies, despite similar binding properties between imidacloprid and the nicotinic acetylcholine receptors in both species.     

Susceptibility to insecticides in the Q biotype of Bemisia tabaci is correlated with bacterial symbiont densities

BACKGROUND: The presence of symbiotic microorganisms may influence an insect's ability to tolerate natural and artificial stress agents such as insecticides. The authors have previously shown that Rickettsia in the B biotype of the whitefly Bemisia tabaci (Gennadius) (Homoptera: Aleyrodidae) increases this insect's susceptibility to several insecticidal compounds. This communication reports a comparison of the susceptibilities of three isofemale strains of the Q biotype of B. tabaci harbouring different bacterial complements to major insecticides from different chemical groups: one strain harboured only Arsenophonus, one harboured Rickettsia and Arsenophonus and one harboured Arsenophonus and Wolbachia. RESULTS: The presence of different symbiont combinations in the three strains had a significant influence on their susceptibility to most of the insecticides tested. Thiamethoxam, imidacloprid, pyriproxyfen and spiromesifen had a significant influence on strains that had the double infections Rickettsia–Arsenophonus and Wolbachia–Arsenophonus, which also carried higher amounts of symbionts as assessed by quantitative real‐time PCR. No significant differences in mortality rates were observed when the tested strains were treated with diafenthiuron. CONCLUSION: The results suggest a correlation between the presence of high bacterial densities in B. tabaci and the insect's ability to detoxify toxic compounds such as insecticides. Copyright © 2009 Society of Chemical Industry     

Age-specific expression of resistance to a neonicotinoid insecticide in the whitefly Bemisia tabaci

Neonicotinoid insecticides retain a crucial role within many chemical and integrated control strategies for the tobacco whitefly, Bemisia tabaci Gennadius, in spite of the establishment of potent and widespread resistance in many areas. Metabolic resistance mechanisms mediated by overexpression of P450-dependent monooxygenases have been implicated in neonicotinoid resistance in the two most prevalent B. tabaci biotypes. Further characterisation of resistance to the neonicotinoid imidacloprid in populations of both these B- and Q-types is reported.Expression of resistance to imidacloprid was age specific in B- and Q-type strains of B. tabaci. The highest observed resistance ratio at LC(50) expressed in prepupal nymphs was 13, compared with at least 580 in their adult counterparts. For all strains, resistance expressed in immatures was not sufficiently potent to compromise recommended imidacloprid application rates.Targeting neonicotinoids towards immature life stages of B. tabaci may circumvent the protection conferred by current mechanisms of resistance, simultaneously reducing the selection pressures imposed. However, such tactics may enhance the expression of existing resistance mechanisms in immatures, or promote the establishment of novel ones expressed in all life stages.     

Toxicity of spiromesifen to the developmental stages of Bemisia tabaci biotype B

BACKGROUND: Spiromesifen is a novel insecticidal/acaricidal compound derived from spirocyclic tetronic acids that acts effectively against whiteflies and mites via inhibition of acetyl‐CoA‐carboxylase, a lipid metabolism enzyme. The effects of spiromesifen on the developmental stages of the whitefly Bemisia tabaci (Gennadius) were studied under laboratory conditions to generate baseline action thresholds for field evaluations of the compound. RESULTS: Adult B. tabaci mortality rate after spiromesifen treatment (5 mg L^?1) was 40%. Treatment with 0.5 mg L^?1 reduced fecundity per female by more than 80%, and fertility was almost nil. LC₅₀ for eggs was 2.6 mg L^?1, and for first instar 0.5 mg L^?1. Scanning electron microscopy revealed that eggs laid by treated adult females had an abnormally perforated chorion, and females were unable to complete oviposition. Light and fluorescent microscopy showed significantly smaller eggs following treatment, and smaller, abnormally formed and improperly localized bacteriomes in eggs and nymphs. The number of ovarioles counted in females treated with 5 mg L^?1 was significantly reduced. Spiromesifen showed no cross‐resistance with other commonly used insecticides from different chemical groups, and resistance monitoring in Israel showed no development of field resistance to this insecticide after 1 year of use. CONCLUSION: The strong effect on juvenile stages of B. tabaci with a unique mode of action and the absence of cross‐resistance with major commonly used insecticides from different chemical groups suggest the use of spiromesifen in pest and resistance management programmes. Copyright © 2008 Society of Chemical Industry     

Gene expression in pyriproxyfen-resistant Bemisia tabaci Q biotype

Pyriproxyfen is a biorational insecticide that acts as a juvenile hormone (JH) analogue and disrupts insect development with an unknown molecular mode of action. Pyriproxyfen is one of the major insecticides used to control the whitefly Bemisia tabaci (Gennadius) and comply with integrated pest management (IPM) programmes, resulting in minimal effects on the environment, humans and beneficial organisms. During the last few years, resistance to pyriproxyfen has been observed in several locations in Israel, sometimes reaching a thousandfold or more. No information exists about the molecular basis underlying this resistance that may lead to understanding the mode of action of pyriproxyfen and developing molecular markers for rapid monitoring of resistance outbreaks. In this communication, a cDNA microarray from B. tabaci was used to monitor changes in gene expression in a resistant B. tabaci population. Based on statistical analysis, 111 expressed sequence tags (ESTs) were identified that were differentially upregulated in the resistant strain after pyriproxyfen treatment. Many of the upregulated ESTs observed in the present study belong to families usually associated with resistance and xenobiotic detoxification such as mitochondrial genes, P450s and oxidative stress, genes associated with protein, lipid and carbohydrate metabolism and others related to JH-associated processes in insects such as oocyte and egg development.     

Current status of insecticide resistance in Q biotype Bemisia tabaci populations from Crete

BACKGROUND: A major problem of crop protection in Crete, Greece, is the control of Bemisia tabaci (Gennadius) (Homoptera: Aleyrodidae) with chemical insecticides owing to the rapid development of resistance. The aim of this study was to investigate the establishment of resistance and the underlying mechanisms to major insecticide classes with classical bioassays and known biochemical resistance markers. RESULTS: During a 2005–2007 survey, 53 Q biotype populations were collected. Application history records showed extensive use of neonicotinoids, organophosphates, carbamates and pyrethroids. High resistance levels were identified in the majority of populations (>80%) for imidacloprid (RF: 38–1958×) and α‐cypermethrin (RF: 30–600×). Low resistance levels (RF < 12) were observed for pirimiphos‐methyl. A strong correlation between resistance to imidacloprid and the number of applications with neonicotinoids was observed. Significant correlations were observed between COE and P450‐dependent monoxygenase activity with resistance to α‐cypermethrin and imidacloprid respectively. A propoxur‐based AChE diagnostic test indicated that iAChE was widespread in most populations. Resistance levels for α‐cypermethrin were increased when compared with a previous survey (2002–2003). Differentiation of LC₅₀ values between localities was observed for imidacloprid only. CONCLUSION: Bemisia tabaci resistance evolved differently in each of the three insecticides studied. Imidacloprid resistance seems less established and less persistent than α‐cypermethrin resistance. The low resistance levels for pirimiphos‐methyl suggest absence of cross‐resistance with other organophosphates or carbamates used. Copyright © 2008 Society of Chemical Industry     

Lack of fitness costs associated with pyriproxyfen resistance in the B biotype of Bemisia tabaci

BACKGROUND: The insect growth regulator pyriproxyfen has provided effective control of the whitefly Bemisia tabaci Gennadius in many countries. Here, whether or not fitness costs were associated with pyriproxyfen resistance in a laboratory‐selected resistant strain (QC02‐R) of the B biotype was determined. RESULTS: Mortality caused by pyriproxyfen and fitness traits over time were measured in unselected and selected hybrid strains, which were created by crossing individuals of the resistant strain with individuals of a susceptible strain. Fitness costs were not associated with resistance in QC02‐R, as mortality caused by pyriproxyfen did not increase over time in unselected hybrid strains and fitness traits were similar in unselected and selected hybrid strains. Using a new method to examine the inheritance of resistance, based on data from fitness cost experiments, it was estimated that pyriproxyfen resistance is controlled by two loci in the QC02‐R strain. CONCLUSION: The lack of fitness costs associated with pyriproxyfen resistance could promote the evolution of resistance in field populations with similar traits to QC02‐R. Copyright © 2008 Society of Chemical Industry     

Cross‐resistance study and biochemical mechanisms of thiamethoxam resistance in B‐biotype Bemisia tabaci (Hemiptera: Aleyrodidae)

BACKGROUND: B‐biotype Bemisia tabaci (Gennadius) has invaded China over the past two decades. To understand the risks and to determine possible mechanisms of resistance to thiamethoxam in B. tabaci, a resistant strain was selected in the laboratory. Cross‐resistance and the biochemical mechanisms of thiamethoxam resistance were investigated in the present study. RESULTS: A 66.3‐fold thiamethoxam‐resistant B. tabaci strain (TH‐R) was established after selection for 36 generations. Compared with the susceptible strain (TH‐S), the selected TH‐R strain showed obvious cross‐resistance to imidacloprid (47.3‐fold), acetamiprid (35.8‐fold), nitenpyram (9.99‐fold), abamectin (5.33‐fold) and carbosulfan (4.43‐fold). No cross‐resistance to fipronil, chlorpyrifos or deltamethrin was seen. Piperonyl butoxide (PBO) and triphenyl phosphate (TPP) exhibited significant synergism on thiamethoxam effects in the TH‐R strain (3.14‐ and 2.37‐fold respectively). However, diethyl maleate (DEM) did not act synergistically with thiamethoxam. Biochemical assays showed that cytochrome P450 monooxygenase activities increased 1.21‐ and 1.68‐fold respectively, and carboxylesterase activity increased 2.96‐fold in the TH‐R strain. However, no difference was observed for glutathione S‐transferase between the two strains. CONCLUSION: B‐biotype B. tabaci develops resistance to thiamethoxam. Cytochrome P450 monooxygenase and carboxylesterase appear to be responsible for the resistance. Reasonable resistance management that avoids the use of cross‐resistance insecticides may delay the development of resistance to thiamethoxam in this species. Copyright © 2009 Society of Chemical Industry     

北京地区蔬菜烟粉虱种群动态及其对烟碱类杀虫剂的抗药性监测

为了明确北京地区蔬菜烟粉虱的种群发展动态及对常用烟碱类杀虫剂的抗药性,为烟粉虱防治适期的确定及高效杀虫剂的选择提供基础资料及科学指导,2009年调查了北京地区春茬和秋茬蔬菜烟粉虱的种群动态,并监测了该地区2009-2011年度烟粉虱种群对3种烟碱类杀虫剂的抗药性。结果表明,春茬蔬菜烟粉虱在5月中旬种群快速上升,持续到春茬拉秧;秋茬蔬菜烟粉虱在塑料棚内9月上旬种群数量即开始上升,露地出现时间较晚,10月中旬后种群数量下降;塑料棚内种群数量明显高于露地蔬菜烟粉虱数量。烟粉虱2009年对吡虫啉和噻虫嗪的抗药性很高,2010年和2011年抗性程度显著下降,LC50处于133.94～251.16mg/L,属中等抗性水平;而对新型杀虫剂烯啶虫胺的抗药性在近3年内持续升高,抗性倍数由6.68倍升至83.62倍,即由低抗性水平升至高抗性水平。上述调查结果表明,北京地区蔬菜烟粉虱的防治应掌握春季5月中旬前、秋季9月中旬前种群处于低密度时进行。烟粉虱对新型杀虫剂烯啶虫胺的抗药性在近三年内快速上升,生产中应注意药剂的轮换使用。     

Insecticide resistance status of Bemisia tabaci Q‐biotype in south‐eastern Spain

BACKGROUND: Bemisia tabaci Gennadius Q‐biotype has readily developed resistance to numerous insecticide classes. Studies in the Mediterranean area are needed to clarify the resistance status and cross‐resistance patterns in this invasive whitefly biotype. The levels of resistance in nymphs of seven strains of B. tabaci Q‐biotype from south‐eastern Spain to representative insecticides were determined. RESULTS: Six populations had low to moderate levels of resistance to azadirachtin (0.2‐ to 7‐fold), buprofezin (11‐ to 59‐fold), imidacloprid (1‐ to 15‐fold), methomyl (3‐ to 55‐fold), pyridaben (0.9‐ to 9‐fold), pyriproxyfen (0.7‐ to 15‐fold) and spiromesifen (1‐ to 7‐fold), when compared with a contemporary Spanish Q‐biotype reference population (LC₅₀ = 2.7, 8.7, 15.2, 19.9, 0.34, 20.9 and 1.1 mg L^?1 respectively). A single population collected from a greenhouse subject to intensive insecticide use exhibited generally higher resistance levels to the same array of compounds (31‐, 1164‐, 3‐, 52‐, 9‐, 19‐ and 3‐fold respectively). Pyridaben and spiromesifen were extremely effective against nymphs of all strains, with LC₅₀ values significantly below recommended application rates. CONCLUSION: In contrast to previous reports, high rates of efficacy exist for numerous insecticide classes against B. tabaci Q‐biotype populations in these intensive agricultural regions of south‐eastern Spain. This probably reflects the recent and significant reductions in exposure that have resulted from a wider uptake of IPM technologies and strategies. However, the continued presence of resistance genes also suggests that a reversion to levels of high insecticide exposure could result in a rapid selection for resistance. Copyright © 2009 Society of Chemical Industry     

Effectiveness of thiamethoxam and imidacloprid seed treatments against Bemisia tabaci (Hemiptera: Aleyrodidae) on cotton

BACKGROUND: Bemisia tabaci (Gennadius) biotype B is one of the most important pests on cotton around the world. Laboratory, greenhouse and field experiments were conducted to determine the efficacy of thiamethoxam and imidacloprid seed treatments against B. tabaci on cotton. RESULTS: Under laboratory conditions, the two treatments caused whitefly adult mortality, reduced oviposition and increased mortality of nymphs at 10, 20, 30 and 40 days after germination (DAG). The longer the adults fed on plants from treated seeds, the higher the mortality. The two treatments did not have any effect on eggs. The efficacy of the treated seeds against B. tabaci gradually decreased from 10 to 40 DAG, being the lowest at 40 DAG. In laboratory experiments, the efficacies between the two treatments were similar. In greenhouse experiments, the two treatments were equally effective with lower numbers of whiteflies than untreated controls. With both treatments the concentrations of the active ingredient were gradually reduced with aging of the plants and from the bottom to the top leaves of the plants. Numbers of live whiteflies were well correlated with the dosage of active ingredients. Under field conditions, the seeds treated with both insecticides exhibited similar efficacy against B. tabaci for up to ～2 months. CONCLUSION: Cotton seeds treated with imidacloprid and thiamethoxam were effective against B. tabaci for up to 45 days under laboratory and greenhouse conditions, and up to ～2 months under field conditions. Use of imidacloprid‐ and thiamethoxam‐treated seeds can be an important alternative for management of whiteflies on cotton. Copyright © 2010 Society of Chemical Industry     

Cross‐resistance,inheritance and biochemical mechanisms of imidacloprid resistance in B‐biotype Bemisia tabaci

BACKGROUND: The B‐type Bemisia tabaci (Gennadius) has become established in many regions in China, and neonicotinoids are extensively used to control this pest. Imidacloprid resistance in a laboratory‐selected strain of B‐type B. tabaci was characterised in order to provide the basis for recommending resistance management tactics. RESULTS: The NJ‐Imi strain of B‐type B. tabaci was selected from the NJ strain with imidacloprid for 30 generations. The NJ‐Imi strain exhibited 490‐fold resistance to imidacloprid, high levels of cross‐resistance to three other neonicotinoids, low levels of cross‐resistance to monosultap, cartap and spinosad, but no cross‐resistance to abamectin and cypermethrin. Imidacloprid resistance in the NJ‐Imi strain was autosomal and semi‐dominant. It is shown that enhanced detoxification mediated by cytochrome‐P450‐dependent monooxygenases contributes to imidacloprid resistance to some extent in the NJ‐Imi strain. Results from synergist bioassays and cross‐resistance patterns indicated that target‐site insensitivity may be involved in imidacloprid resistance in the NJ‐Imi strain of B. tabaci. CONCLUSION: Although oxidative detoxification mediated by P450 monooxygenases is involved in imidacloprid resistance in the NJ‐Imi strain of B‐type B. tabaci, target‐site modification as an additional resistance mechanism cannot be ruled out. Considering the high risk of cross‐resistance, neonicotinoids should be regarded as a single group when implementing an insecticide rotation scheme in B. tabaci control. Copyright © 2009 Society of Chemical Industry     

Biotype and insecticide resistance status of the whitefly Bemisia tabaci from China

BACKGROUND: Resistance to numerous insecticide classes in Bemisia tabaci Gennadius has impaired field control efficacy in south‐eastern China. The biotype and resistance status of B. tabaci collected from these areas was investigated. RESULTS: Two different biotypes of B. tabaci (B‐biotype and Q‐biotype) were detected in south‐eastern China, and the samples collected from geographical regions showed a prevalence of the Q‐biotype and the coexistence of B‐ and Q‐biotypes in some regions. Moderate to high levels of resistance to two neonicotinoids were established in both biotypes (28–1900‐fold to imidacloprid, 29–1200‐fold to thiamethoxam). Medium to high levels of resistance to alpha‐cypermethrin (22–610‐fold) were also detected in both biotypes. Four out of 12 populations had low to medium levels of resistance to fipronil (10–25‐fold). Four out of 12 populations showed low levels of resistance to spinosad (5.7–6.4‐fold). All populations tested were susceptible to abamectin. CONCLUSION: The Q‐biotype B. tabaci is supplanting the B‐biotype which used to be ubiquitous in China. Field populations of both B‐ and Q‐biotypes of B. tabaci have developed high levels of resistance to imidacloprid and thiamethoxam. Abamectin is the most effective insecticide against adult B. tabaci from all populations. Copyright © 2010 Society of Chemical Industry     

The presence of Rickettsia is associated with increased susceptibility of Bemisia tabaci (Homoptera: Aleyrodidae) to insecticides

BACKGROUND: The presence of certain symbiotic microorganisms may be associated with insecticide resistance in insects. The authors compared the susceptibility of two isofemale lines, Rickettsia-plus and Rickettsia-free, of the sweet potato whitefly Bemisia tabaci (Gennadius) (Homoptera: Aleyrodidae) to major insecticides from different chemical groups, including imidacloprid, acetamiprid, thiamethoxam, pyriproxyfen, spiromesifen and diafenthiuron. RESULTS: While the Rickettsia-plus and Rickettsia-free lines showed no differences in their susceptibility to imidacloprid and diafenthiuron, higher susceptibility of the Rickettsia-plus line to acetamiprid, thiamethoxam, spiromesifen and especially pyriproxyfen was observed. LC(90) values indicated that the Rickettsia-free line was 15-fold more resistant to pyriproxyfen than the Rickettsia-plus line. CONCLUSION: Findings indicate that the infection status of B. tabaci populations by Rickettsia is an important consideration that should be taken into account when performing resistance monitoring studies, and may help in understanding the dynamics of B. tabaci resistance, symbiont-pest associations in agricultural systems and the biological impact of Rickettsia on whitefly biology.     

Reversal of resistance to pyriproxyfen in the Q biotype of Bemisia tabaci (Hemiptera: Aleyrodidae)

Pyriproxyfen, a juvenile hormone (JH) mimic, is a biorational insecticide that disrupts insect development. It is one of the principal insecticides being used to control Bemisia tabaci (Gennadius) on cotton, and has many environmentally positive attributes that make it compatible with integrated pest management (IPM) programs. In Israel, a high level of resistance to pyriproxyfen has been observed in several isolated regions. Here, tests were conducted to establish whether temporal refuges from exposure to pyriproxyfen could be useful for restoring the effectiveness of the compound. Resistance was found to decrease by a factor of 8 when exposure to pyriproxyfen was ceased for 13 generations. Reversal of resistance was accompanied with increased biotic fitness of the revertant colony. By incorporating experimental estimates of nymph survival, sex ratio, fecundity, egg hatching rate and developmental time, the seasonal cost per generation for resistant insects was estimated to be 25%. A genetic simulation model, optimized by empirical data from bioassays, predicted fitness cost per generation of 19% for resistant homozygous (RR) females and hemizygous (R) males, and produced rates of reversal similar to the experimental results. The model also predicted that, even after 5 years ( approximately 55 generations) without pyriproxyfen treatments, the frequency of the resistance allele (R) will still remain high (0.02). It is therefore concluded, on the basis of experimental and modeling results, that the effectiveness of temporal refuges for reversing development of resistance to pyriproxyfen in B. tabaci may be limited.     

The role of B‐type esterases in conferring insecticide resistance in the tobacco whitefly,Bemisia tabaci (Genn)

Separation of non‐specific esterases on electrophoretic gels has played a key role in distinguishing between races or biotypes of the tobacco whitefly, Bemisia tabaci. One intensively staining esterase in particular (termed E_0.14) has assumed significance as a diagnostic of B‐type whiteflies (aka Bemisia argentifolii), despite any knowledge of its biological function. In this study, a whitefly strain (B‐Null) homozygous for a null allele at the E_0.14 locus that had been isolated from a B‐type population was used to demonstrate a significant role for E_0.14 in resistance of B‐type populations to pyrethroids but not to organophosphates (OPs). Bioassays with pyrethroids, following pre‐treatment with sub‐lethal doses of the OP profenofos (to inhibit esterase activity), coupled with metabolism studies with radiolabelled permethrin, supported the conclusion that pyrethroid resistance in a range of B‐type strains expressing E_0.14 was primarily due to increased ester hydrolysis. In the same strains, OP resistance appeared to be predominantly conferred by a modification to the target‐site enzyme acetylcholinesterase. © 2000 Society of Chemical Industry     

宁夏地区烟粉虱对5种常用杀虫剂的抗药性

采用浸叶法分别测定了宁夏8个地区烟粉虱成虫田间种群对4类5种杀虫剂的抗性.结果 表明,供试烟粉虱种群对高效氯氰菊酯已产生了中等至极高水平抗性,抗性倍数为33.31～227.98倍;对烯啶虫胺产生了中等至高水平抗性,抗性倍数为12.14～69.33倍;对毒死蜱和吡虫啉产生了低至中等水平抗性,抗性倍数分别为7.63～38....