Chemical control of cattle ticks—resistance problems

Chemical control, by dipping or spraying infested cattle with ixodicidal compounds, is the only practicable method of dealing with the cattle tick problem in the short term. Widespread exposure to ixodicidal compounds, often at subeffective concentrations, has resulted in the selection of resistant strains from tick populations. A situation has now been reached where cattle ticks in widely separated parts of the world have shown a capacity to develop resistance to most of the currently available chemical classes of ixodicide. There is a very real probability that these resistant strains will eventually replace susceptible tick populations, particularly in those areas where the cattle tick problem is already most acute. Research and development of new chemical groups of compounds with high ixodicidal activity and low mammalian toxicity is a matter of some urgency. In this review, the biological background of the tick problem is outlined. A brief history of the development of tick resistance to arsenic, organochlorine, organophosphorus and carbamate compounds is given. Reference is made to the underlying biochemical mechanisms involved. Some of the human factors affecting the development of tick resistance are examined and future prospects are assessed.     

Cross‐resistance relationships between neonicotinoids and pymetrozine in Bemisia tabaci (Hemiptera: Aleyrodidae)

BACKGROUND: Although cross‐resistance between compounds in the same insecticide group is a frequently observed phenomenon, cross‐resistance between groups that differ in structural and functional characteristics can be extremely unpredictable. In the case of controlling the whitefly, Bemisia tabaci Gennadius, neonicotinoids and the pyridine azomethine antifeedant pymetrozine represent independent lines of discovery that should be suited for alternation to avoid prolonged selection for the same resistance mechanism. Reports of an association between responses to neonicotinoids and pymetrozine were investigated by resistance profiling of seven B. tabaci strains and complementary reciprocal selection experiments. RESULTS: All strains demonstrated a consistent correlation between responses to three neonicotinoid compounds: thiamethoxam, imidacloprid and acetamiprid. Responses to neonicotinoids for six field strains clearly correlated with responses to pymetrozine. Reciprocal selection experiments confirmed an unexpected case of intergroup cross‐resistance. A seventh strain exhibited a so far unique phenotype of strong resistance to pymetrozine but full susceptibility to neonicotinoids. Selection experiments confirmed that in this strain the mechanism of pymetrozine resistance is specific and has no implications for neonicotinoids. CONCLUSION: Cross‐resistance between neonicotinoids and pymetrozine in B. tabaci probably reflects the overexpression of a cytochrome‐P450‐dependent monooxygenase capable of metabolising both types of compound in spite of their apparent structural dissimilarity. Given the predominance of this mechanism in B. tabaci, both can contribute to resistance management but should be placed within the same treatment ‘window’. Copyright © 2010 Society of Chemical Industry     

Acaricide resistance and resistance mechanisms in Tetranychus urticae populations from rose greenhouses in the Netherlands

BACKGROUND: Spider mites are important crop pests that rapidly develop resistance to acaricides. To investigate whether acaricide resistance is a threat to greenhouse rose culture in the Netherlands, the susceptibility of 15 strains of Tetranychus urticae was tested to several currently used acaricides, and resistance mechanisms were investigated. RESULTS: Although the observed levels of susceptibility differed between strains and acaricides, resistance was detected in most strains. The activity of detoxifying enzymes was significantly increased in most field‐collected strains, and a number of amino acid substitutions known to be involved in resistance were detected. CONCLUSIONS: Resistance levels to traditional acaricides such as bifenthrin and abamectin were prominent, and might result in control failure under field conditions. Resistance to more recently registered compounds was detected in several populations. Resistance levels were generally unstable in the laboratory without selection pressure. The toxicological, biochemical and genetic data in this study will be essential in devising an efficient resistant management for Dutch rose culture. Copyright © 2011 Society of Chemical Industry     

Genetic aspects of resistance to imazalil in Aspergillus nidulans

Mutant strains of Aspergillus nidulans have been isolated which display a low level of resistance to imazalil, a recently developed systemic fungicide. Agar growth tests showed that A. nidulans is about three times as sensitive to imazalil when growing on supplemented minimal medium (SM) as compared with complete medium. This effect was reduced by adding glutamic acid to the SM.Imazalil resistance was found to be based on a multigenic system; 21 single gene mutations define 8 loci which were allocated to 6 different linkage groups. Mutations at different loci lead pleiotropically to one or more of the following properties: hypersensitivity or resistance to acriflavin, cycloheximide and neomycin, resistance to chloramphenicol and fenarimol, and to cold sensitivity. Of 120 cycloheximide-resistant strains isolated, 98 were also imazalil-resistant.Recombination analysis of different imazalil-resistant strains with mutations at three loci resulted in additive effects, giving strains with a high level of resistance to imazalil.The results indicate that imazalil may interfere either with protein synthesis like cycloheximide, chloramphenicol and neomycin or with synthesis or function of cell membranes. Interference with cell membrane synthesis might lead to altered sterol composition, resulting in selective permeability to different compounds.     

Limitations in the exploitation of N-phenylcarbamates and N-phenylformamidoximes to control benzimidazole-resistant Venturia nashicola on Japanese pear

The efficacy of N -phenylcarbamates and N -phenylformamidoximes against benzimidazole-resistant strains of Venturia nashicola was tested experimentally in Japanese pear orchards. Both compounds gave effective control when applied in an orchard where highly benzimidazole-resistant strains were predominant. However, unsatisfactory control was observed in an orchard where the fungal population was dominated by fungal strains with intermediate or weak benzimidazole resistance. Strains resistant to N -phenylcarbamates and N -phenylformamidoximes were widely distributed in Japanese pear orchards, suggesting that effective use of these compounds to control benzimidazole-resistant strains of V. nashicola would not be practicable. Increased sensitivity to N -phenylformamidoximes in highly benzimidazole-resistant isolates was shown to be controlled by a single chromosomal gene, but progenies that were highly resistant to both a benzimidazole fungicide and a N -phenylformamidoximes appeared in crosses between parents resistant to each fungicide alone. Similar strains were also found in pear orchards, and this'double resistance'was shown to be heritable.     

Esterases and resistance to synthetic pyrethroids in the Egyptian cotton leafworm

Esterases hydrolyzing α-naphthyl acetate (α-NA), β-naphthyl acetate (β-NA), and p-nitrophenyl acetate (p-NPA) were investigated colorimetrically in larval homogenates of synthetic pyrethroid susceptible (S) and resistant (R) strains of Spodoptera littoralis (Boised). The hydrolytic activity towards the three substrates in cybolt, decamethrin, and fenvalerate R strains were from 3 to 6.5 times as high as in the S strain. The increase in esterase activity was closely associated with the development of resistance in the R strains. DEF (S,S,S-tributyl phosphorotrithioate) proved to be an inhibitor for all esterases, with a particularly potent action on p-NPA-hydrolyzing enzymes. The inhibitory action was more pronounced in R strains than in the S strain. Pretreatment with DEF increased the toxicity of pyrethroid compounds in the R strains more than in the S strain and hence decreased the levels of resistance in these strains. This is evidence that the esterases contribute to the resistance against synthetic pyrethroids in S. littoralis larvae.     

Acaricide toxicity and resistance in larvae of different strains of Tetranychus urticae and Panonychus ulmi (Acari: Tetranychidae)

The toxicities of eight structurally different acaricidal compounds to six‐legged larvae (first motile stage) of three laboratory strains of the two‐spotted spider mite, Tetranychus urticae, and the European red mite, Panonychus ulmi, were evaluated following spray application. The larvae of five field‐derived strains of T urticae originating from France, Italy, Brazil, California and Florida were also tested for their susceptibilities to discriminating concentrations of several acaricides resulting in 95% mortality when applied to the organophosphate‐resistant laboratory reference strain WI. The spray bioassay used was robust and gave repeatable results with a wide range of acaricidal compounds, irrespective of their mode of action (ovo‐larvicides or primarily acting on motile life stages). Compounds tested were abamectin, azocyclotin, chlorpyrifos, clofentezine, deltamethrin, fenpyroximate, hexythiazox and pyridaben. Larvae of one of the laboratory strains of T urticae, AK, originally collected in Japan in 1996 and maintained without further selection pressure, exhibited 2000‐ and >4000‐fold resistance to the mitochondrial electron transport inhibitors pyridaben and fenpyroximate, respectively. Another strain of T urticae, AU, obtained from Australia and maintained in the laboratory under selection with hexythiazox and clofentezine since 1987 showed >770‐ and >1000‐fold resistance to clofentezine and hexythiazox, respectively. The same resistance pattern was observed against larvae of a laboratory strain of P ulmi, CE, also selected with hexythiazox. Larvae of one of the field‐derived strains of T urticae, BR, showed a lower susceptibility to a number of compounds, whilst the others were susceptible to all compounds except the organophosphates. © 2001 Society of Chemical Industry     

New developments in insecticide resistance in the glasshouse whitefly (Trialeurodes vaporariorum) and the two-spotted spider mite (Tetranychus urticae) in the UK

A recent survey of insecticide resistance in two of the most problematic pests in UK glasshouses revealed some new developments. At least some individuals in all UK samples of Trialeurodes vaporariorum that were tested resisted the insect growth regulator (IGR) buprofezin. The most strongly resistant strains were unaffected by the field application rate of this compound, and even samples from populations that had never been exposed to buprofezin contained individuals that survived the highest concentration applied (10,000 mg litre-1). The field rate of buprofezin was shown to select for resistance through vapour action alone. The benzophenylurea teflubenzuron, an unrelated IGR, was cross-resisted by buprofezin-resistant individuals. There was no evidence of resistance to imidacloprid, but all T vaporariorum strains tested, regardless of origin, exhibited a high innate tolerance to nicotine, when compared with another whitefly species, Bemisia tabaci. Marked resistance to fenbutatin oxide and tebufenpyrad was found in single glasshouse populations of Tetranychus urticae, but these compounds and abamectin appeared to remain highly effective against all other strains collected.     

Monitoring of insecticide resistance in damson hop aphid,Phorodon humuli Schrank (Hemiptera: Aphididae) from German hop gardens

The damson hop aphid, Phorodon humuli (Schrank) (Hemiptera: Aphididae), is one of the most important sucking pests of many hop-growing areas world-wide. In this study we determined the efficacy of several insecticides against strains collected throughout the year 2001. All strains were collected in different hop gardens in the Hallertau (Bavaria), Germany, the largest hop-growing area of the world. First of all we established a leaf dip bioassay, carried out using six-well tissue culture plates and appropriate for monitoring susceptibility against imidacloprid, oxydemeton-methyl, cyfluthrin, amitraz, pymetrozine and pirimicarb. Four of these compounds, imidacloprid, cyfluthrin, pymetrozine and amitraz, are currently registered for the control of sucking pests in German hop gardens and are useful against P. humuli. The leaf-dip bioassay system turned out to be very reliable and robust. Ten P. humuli strains were collected in May 2001 and maintained in the laboratory to assess their resistance to the different insecticides in comparison with two laboratory reference strains (H2 and H5). Using diagnostic concentrations, resistance monitoring for imidacloprid and cyfluthrin was investigated during July and August 2001 on 53 populations from 30 sites around the Hallertau, an area of ca 2500 km2. Resistance to diagnostic concentrations (LC95 for reference strains) of imidacloprid, amitraz and pymetrozine was not detected in any strain received in 2001, but late-season (August) populations seemed to respond more heterogeneously than those collected mid-season (July). Overall composite mean mortalities to diagnostic concentrations of imidacloprid (13 mg litre(-1)) in collections from May, July and August were 95 (+/-2.5), 98 (+/-2.3) and 87 (+/-5.9)%, respectively. Moderate resistance to pyrethroids was observed in all strains collected in May and August using a diagnostic concentrations of 10 mg litre(-1) cyfluthrin (LC95 of the susceptible reference strain H5). Slight to moderate resistance to diagnostic concentrations of oxydemeton-methyl and pirimicarb was observed in some, but not all, strains collected early season. The results are discussed in terms of the implemention of hop aphid resistance management strategies in German hop-cultivation areas.     

Imidacloprid resistance and its mechanisms in field populations of brown planthopper, Nilaparvata lugens Stål in China

Imidacloprid is a key insecticide universally used for control of brown planthopper (BPH), and its resistance had been studied previously with laboratory selected strains. When the failure in field control happened in China in 2005, imidacloprid resistance in field populations of BPH (AQ, NJ, GL and WJ) was monitored and studied. The results demonstrated that the BPH of field populations had developed moderate to high level of resistance to imidacloprid. This resistance is attributed mainly to the enhanced P450 monooxygenases detoxification and could be enhanced in the same growing season if the insecticide was sprayed over and over. Further studies revealed that imidacloprid resistant hoppers did not show cross-resistance to all the neonicotinoid insecticides and high level of imidacloprid resistance in BPH was very unstable. Thus, efficient substitute neonicotinoids could be selected and “window control” could be implemented in resistance management. For checking the resistance mutation previously reported in laboratory selected strains, new corresponding target subunit genes were cloned and sequenced, but no mutations were found associated consistently with resistance.     

Monitoring resistance to diflubenzuron and deltamethrin in French codling moth populations (Cydia pomonella)

French populations of codling moth have developed resistance to several insecticide classes. The susceptibility of susceptible and resistant laboratory strains to diflubenzuron and deltamethrin was evaluated using different exposure methods against various life stages. The tarsal contact method for adults was found to be an appropriate method for testing neurotoxic compounds such as deltamethrin. Insect growth inhibitors, like diflubenzuron, need to be analysed on juvenile instars. Monitoring methodologies were developed and evaluated (a) on neonates obtained by crossing field‐collected males with virgin females from the susceptible laboratory strain so as to overcome the problem of reduced fertility of wild females and (b) on diapausing larvae. All 36 populations analysed exhibited significant levels of resistance to both diflubenzuron and deltamethrin. Resistance to the two compounds was linked, regardless of the cultivation methods used in the monitored area. The monitoring methodologies will be implemented to evaluate the evolution of resistance according to the resistance management strategies that have been adopted. © 2000 Society of Chemical Industry     

Development of resistance and cross-resistance in fenvalerate and deltamethrin selected strains of Spodoptera littoralis (Boisd.)

Resistances of 33- and 27-fold to the pyrethroids fenvalerate and deltamethrin, respectively, have been induced in larvae of the Egyptian cotton leafworm Spodoptera littoralis (Boisd.) by selection of two subcolonies of a field population in the laboratory. Selected strains showed relatively high levels of resistance (11- to 36-fold) to all the tested pyrethroids, moderate levels of cross-resistance (3- to 13-fold) to DDT and the carbamate aminocarb, and a slight increase in tolerance levels to endrin and lindane. The pyrethroid-resistant strains, by contrast, manifested no cross-resistance to the organophosphorus compounds that were tested. Moreover, a negative crossresistance was observed in the pyrethroid-resistant strains to dicrotophos, methamidophos and the carbamate methomyl.     

海南省芒果可可球二孢对多菌灵的敏感性及菌株适合度研究

针对中国海南地区芒果蒂腐病致病菌可可球二孢对多菌灵的抗性水平及菌株的适合度进行了研究。采用区分剂量法,测定了2016年从海南省芒果园采集、分离的90株可可球二孢对多菌灵的敏感性,并对抗性及敏感菌株在菌丝生长、致病力、渗透压及相对渗率等方面进行了比较。结果表明:海南地区芒果可可球二孢对多菌灵的抗性频率为65.56%,且以高抗菌株为主（8个中抗菌株,51个高抗菌株）;该病菌对芒果危害甚大,其中具强致病力的菌株达65株,占72.22%。高抗菌株的菌丝生长优于敏感菌株,存在极显著差异（P0.01）;抗性菌株的致病力与敏感菌株无显著差异;敏感菌株的相对渗率高于抗性菌株,表明敏感菌株细胞膜的透性较高,胞内电解质渗出较多。研究表明,海南地区芒果可可球二孢对多菌灵的抗性水平较高,且抗性菌株适合度高于敏感菌株,易形成优势群体。     

Recovery of fungicide-resistant <Emphasis Type="Italic">Botrytis</Emphasis> isolates from strawberry nursery plants

In a four-year survey of strawberry nursery plants, 340 Botrytis isolates were collected and examined for fungicide resistance. High percentages of isolates with resistance to members of all fungicide classes registered on strawberries in Germany were found, i.e. trifloxystrobin (90.3%), boscalid (53.8%), cyprodinil (41.5%), fludioxonil (28.2%) and fenhexamid (15.6%). Of these isolates, 10.3% possessed multiple resistance to all compounds. At 5.0%, resistance to the recently registered fungicide fluopyram was low. Dutch nursery material harboured significantly higher proportions of isolates with resistance to any or all fungicides than plants produced in Germany. These data point to nursery plants as a possible route of introducing fungicide-resistant Botrytis strains into commercial strawberry fields.     

Metabolism of imidacloprid and DDT by P450 CYP6G1 expressed in cell cultures of Nicotiana tabacum suggests detoxification of these insecticides in Cyp6g1-overexpressing strains of Drosophila melanogaster, leading to resistance

BACKGROUND: With the worldwide use of insecticides, an increasing number of pest insect species have evolved target-site or metabolism-based resistance towards some of these compounds. The resulting decreased efficacy of pesticides threatens human welfare by its impact on crop safety and further disease transmission. Environmental concentrations of some insecticides are so high that even natural populations of non-target, non-pest organisms such as the fruit fly Drosophila melanogaster Meig. have been selected for resistance. Cyp6g1-overexpressing strains of D. melanogaster are resistant to a wide range of chemically diverse insecticides, including DDT and imidacloprid. However, up to now there has been no evidence that the CYP6G1 enzyme metabolises any of these compounds. RESULTS: Here it is shown, by heterologous expression in cell suspension cultures of Nicotiana tabacum L. (tobacco), that CYP6G1 is capable of converting DDT (20 microg per cell culture assay) by dechlorination to DDD (18% of applied amount in 48 h), and imidacloprid (400 microg) mainly by hydroxylation to 4-hydroxyimidacloprid and 5-hydroxyimidacloprid (58 and 19% respectively in 48 h). CONCLUSION: Thus, the gap between the supposed resistance gene Cyp6g1 and the observed resistance phenomenon was closed by the evidence that CYP6G1 is capable of metabolising at least two insecticides.     

杀虫剂室内选育亚洲玉米螟抗药性及交互抗性的研究

用4种杀虫剂及1种混合剂以点滴法分别逐代处理敏感性亚洲玉米螟五龄幼虫,以选育其抗药性。至30代,该虫对速灭杀丁及呋喃丹产生_的抗性分别达40.3及30.2倍,而后经5—6代正常饲养,其抗性虽有所衰退,但仍明显,可认为已成为抗性品系;而用甲基对硫磷、甲基对硫磷+速灭杀丁混合剂及辛硫磷(24代)处理,其抗性仅分别达1.8、3.4及4.0倍,由于获得抗性不明显,将此三种试虫称之选育品系。抗性品系对多种药剂存在着明显的交互抗性;而选育品系对其选育剂的抗性虽不明显,但它却对另外的有些药剂产生了抗性。     

A High Multi-drug Resistance to Chemically Unrelated Oomycete Fungicides in Phytophthora infestans

Mutants of Phytophthora infestans with high resistance to the amidocarbamates iprovalicarb and benthiavalicarb and to the cyanoimidazole cyazofamid were isolated after UV-mutagenesis and selection on media containing one of the above fungicides. In vitro fungitoxicity tests showed that all resistant strains presented a highly reduced sensitivity to both cyazofamid and to the amidocarbamates. Cross-resistance studies with other oomycete fungicides from different chemical groups showed that the mutation(s) for resistance to iprovalicarb (IPV), benthiavalicarb (BVC) and cyazofamid (CZF) also greatly reduced the sensitivity of mutant strains to the phenylamide metalaxyl, acetamide cymoxanil, morpholine dimethomorph, benzamide zoxamide and to chlorothalonil. A lower reduction of sensitivity of mutant strains to the strobilurins azoxystrobin, kresoxim-methyl, pyraclostrobin and trifloxystrobin, azolones famoxadone and fenamidone and to antimycin A was observed. A resistance correlation was not apparent for the dithiocarbamate propineb and phenylpyridinamine fluazinam. Studies of fitness parameters in the wild-type and mutant strains of P. infestans showed that most resistant isolates had significantly reduced sporulation and sporangial germination, but not in the differentiation of sporangia into zoospores. Pathogenicity tests on tomato seedlings showed that most resistant isolates were significantly less pathogenic compared to the wild-type parent strain. However, experiments on the stability of the resistant phenotypes did not show a reduction in resistance when the mutants were grown for more than eleven generations on inhibitor-free medium. This is believed to be the first report of high level multi-drug resistance in fungal pathogens to chemically unrelated fungicides inhibiting different sites of cellular pathway.     

谷子(粟)白发病抗病性研究Ⅲ.粟优异种质对白发病生理小种抗谱的测定

以来自粟不同生态区（东北平原、华北平原、内蒙古高原和黄土高原）２２份优异种质进行对白发病菌〔Ｓｃｌｅｒｏｓｐｏｒａｇｒａｍｉｎｉｃｏｌａ（Ｓａｃ．）Ｓｃｈｒｏｔ．〕２０个生理小种６８个菌株抗谱的测定，同时用各生态区菌株对本区粟品种进行抗性关系的研究，其中抗５０％小种的品种有３３２、龙谷２５变、张农１２、大红袍、大同北郊、七月黄、大青苗、紫杆黄谷和西城白９个，具有广谱抗性而在本生态区亦表现抗病性强的品种有３３２、龙谷２５变、张农１２等，选出的广谱抗性品种将为粟抗白发病育种、优良品种的合理布局及抗性机理的深化研究提供了基础材料。     

Various mechanisms responsible for permethrin metabolic resistance in seven field-collected strains of the German cockroach from Iran, Blattella germanica (L.) (Dictyoptera: Blattellidae)

Insecticide resistance in the German cockroach can be mediated by a number of mechanisms, the most common being enhanced enzymatic metabolism. Seven field-collected strains of German cockroach, Blattella germanica (L.) with various levels of resistance to pyrethroids, five out of which were also cross-resistant to DDT were used in this study. The investigation of possible mechanisms responsible for permethrin resistance was carried out using the synergists PBO, DEF and DMC and biochemical assays, including general esterases, glutathione S-transeferases and monooxygenases assays, using an automated microtitre plate reader. PBO and DEF, the inhibitors of cytochrome p450 monooxygenases and general esterases, respectively, affected permethrin resistance to varying degrees depending on the strain. DDT resistance in five strains were not completely eliminated by the synergist DMC, an inhibitor of glutathione S-transferase enzymes, suggesting that a further non-metabolic resistance mechanism such as kdr-type may be present. This suggestion was further supported by GST assay data, where a little elevation in GST activity was detected in only two strains. The synergist data supported by biochemical assays implicated that cytochrome p450 monooxygenases or hydrolases are involved in permethrin resistance in some strains. However, these results implicated both enhanced oxidative and hydrolytic metabolism of permethrin as resistance mechanism in the other strains. The results of synergist and biochemical studies implicated that all the field-collected permethrin resistant strains have developed diverse mechanisms of resistance, although these strains have been collected from the same geographic area. The change in resistance ratios of some strains by using PBO or DEF is discussed. It is of interest to note that because resistance to permethrin was not completely eliminated by DEF and PBO, it is likely that one or more additional mechanisms are involved in permethrin resistance in every strain studied.     

谷子(粟)白发病抗病性研究Ⅲ.粟优异种质对白发病生理小种抗谱的测定

 以来自粟不同生态区(东北平原、华北平原、内蒙古高原和黄土高原)22份优异种质进行对白发病菌[Sclerospora graminicola(Sacc.) Schrot.]20个生理小种68个菌株抗谱的测定,同时用各生态区菌株对本区粟品种进行抗性关系的研究,其中抗50%小种的品种有332、龙谷25变、张农12、大红袍、大同北郊、七月黄、大青苗、紫杆黄谷和西城白9个,具有广谱抗性而在本生态区亦表现抗病性强的品种有332、龙谷25变、张农12等,选出的广谱抗性品种将为粟抗白发病育种、优良品种的合理布局及抗性机理的深化研究提供了基础材料。