Monitoring of diamondback moth (Lepidoptera: Plutellidae) resistance to spinosad, indoxacarb, and emamectin benzoate

Six to nine populations of the diamondback moth, Plutella xylostella (L.), were collected annually from fields of crucifer vegetables in the United States and Mexico from 2001 to 2004 for baseline susceptibility tests and resistance monitoring to spinosad, indoxacarb, and emamectin benzoate. A discriminating concentration for resistance monitoring to indoxacarb and emamectin benzoate was determined based on baseline data in 2001 and was used in the diagnostic assay for each population in 2002-2004 together with a discriminating concentration for spinosad determined previously. Most populations were susceptible to all three insecticides, but a population from Hawaii in 2003 showed high levels of resistance to indoxacarb. Instances of resistance to spinosad occurred in Hawaii (2000), Georgia (2001), and California (2002) as a consequence of a few years of extensive applications in each region. The collaborative monitoring program between university and industry scientists we discuss in this article has provided useful information to both parties as well as growers who use the products. These studies provide a baseline for developing a more effective resistance management program for diamondback moth.     

小菜蛾对九种杀虫剂的抗药性

采用叶片药膜法测定了9种杀虫剂对海南、山东、广东、湖北武汉和襄樊等5个地区小菜蛾Plutella xylostella(L.)田间种群的毒力,和室内相对敏感品系比较。结果显示,5个地区的小菜蛾种群都对氯氰菊酯产生了10倍以上的抗性,广东和山东种群达到30倍以上的抗性。对阿维菌素的抗性山东小菜蛾种群达到135倍,广东种群达到30倍,其他地区均在10倍以下。但是,作用机制与阿维菌素类似的药剂,氟虫腈的抗性5个地区均在5倍或5倍以下。对昆虫生长调节剂定虫隆和氟虫脲的抗性除广东种群分别在10倍和20倍以上外,其他地区均在5倍以下。广东种群对多杀菌素的抗性为7.8倍,其他地区的种群没有产生抗性,多杀菌素和杀虫双均为作用于乙酰胆碱受体的药剂,但是作用的位点不同。杀虫双对5个种群的LC50值均是比较大的(>800 mg/L),尽管没有明显的抗性,不适宜用于小菜蛾田间防治。对呼吸链电子转移抑制剂溴虫腈的抗性均在10倍以下。对辛硫磷的抗性在2~5倍。     

Temperature-related fitness costs of resistance to spinosad in the diamondback moth, Plutella xylostella (Lepidoptera: Plutelidae)

Fitness costs associated with resistance genes expressed in the absence of insecticides affect the evolution of insecticide resistance and the outcome of resistance management programmes. However, measurements of fitness costs may not be straightforward as they vary with environmental conditions. The diamondback moth, Plutella xylostella L. (Lepidoptera: Plutellidae), has developed resistance to spinosad, the first insecticide of the Naturalyte class, after only a few years of field application of this product. In this study, we compared the performance of two homogenous strains of P. xylostella, one susceptible (SS) and the other resistant (RR) to spinosad at an unfavourable, low natural temperature regime, a favourable median-fluctuating temperature regime and an unfavourable high-fluctuating temperature regime. The RR strain showed only marginal fitness cost at the median temperature regime. At the low temperature regime, the RR strain failed to produce any viable offspring, while the SS strain achieved positive population growth. At the high temperature regime, the RR strain showed a 33% decrease in intrinsic rate of increase compared to the SS strain. The results demonstrate that fitness costs of resistance to spinosad are temperature-dependent, increasing in scale at unfavourably low and high temperatures; costs were particularly high at low temperatures. Suggestions for designing effective management programmes are made to delay or avoid development of resistance to spinosad by P. xylostella under different temperature conditions.     

Potential for insecticide resistance in populations of Bactrocera dorsalis in Hawaii: spinosad susceptibility and molecular characterization of a gene associated with organophosphate resistance

The potential for populations to become resistant to a particular insecticide treatment regimen is a major issue for all insect pest species. In Hawaii, for example, organophosphate (OP)‐based cover sprays have been the chemical treatment most commonly applied against oriental fruit fly, Bactrocera dorsalis (Hendel) (Diptera: Tephritidae), populations since the 1950s. Moreover, bait spray treatments using spinosad were adopted as a major control tactic in the Hawaii area‐wide fruit fly pest management program beginning in the year 2000. To determine the current level of spinosad and OP tolerance of wild B. dorsalis populations, bioassays were conducted on flies collected from a range of geographic localities within the Hawaiian islands. Adult B. dorsalis flies were tested (1) for the level of susceptibility to spinosad using LC₅₀ diagnostic criteria, and (2) for the presence of alleles of the ace gene previously shown to be associated with OP resistance. Regarding spinosad tolerance, only flies from Puna, the one area lacking prior exposure to spinosad, showed any significant difference compared to controls, and here the difference was only in terms of non‐overlap of 95% fiducial limit values. With respect to OP tolerance, specific mutations in the ace gene associated with resistance to these insecticides were found in only two populations, and in both cases, these alleles occurred at relatively low frequencies. These results suggest that at the present time, populations of B. dorsalis in Hawaii show no evidence for having acquired resistance to the insecticides widely used in control programs.     

Stability of spinosad resistance in Frankliniella occidentalis (Pergande) under laboratory conditions

The stability of spinosad resistance in western flower thrips (WFT), Frankliniella occidentalis (Pergande), populations with differing initial frequencies of resistance was studied in laboratory conditions. The stability of resistance was assessed in bimonthly residual bioassays in five populations with initial frequencies of 100, 75, 50, 25 and 0% of resistant individuals. There were no consistent changes in susceptibility of the susceptible strain after eight months without insecticide pressure. In the resistant strain, very highly resistant to spinosad (RF50>23,000-fold), resistance was maintained up to eight months without further exposure to spinosad. In the absence of any immigration of susceptible genes into the population, resistance was stable. In the case of the population with different initial frequency of resistant thrips, spinosad resistance declined significantly two months later in the absence of selection pressure. With successive generations, these strains did not change significantly in sensitivity. Spinosad resistance in F. occidentalis declined significantly in the absence of selection pressure and the presence of susceptible WFT. These results suggest that spinosad resistance probably is unstable under field conditions, primarily due to the immigration of susceptible WFT. Factors influencing stability or reversion of spinosad resistance are discussed.     

Assessment of insecticide resistance in five insect pests attacking field and vegetable crops in Nicaragua

Field populations of Hypothenemus hampei (Ferrari), Plutella xylostella (L.), Spodoptera exigua (Hübner), Helicoverpa zea (Boddie) and Bemisia tabaci (Gennadius) were tested for resistance to several insecticides commonly used in Nicariagua. Assays were conducted to estimate the LD50s or LC50s and the corresponding resistance ratios. A diagnostic concentration was used to discriminate between susceptible and resistant strains of H. hampei. The tests with >6,000 H. hampei adults collected from six different sites indicate the absence of resistance to endosulfan. Resistance to cypermethrin, deltamethrin, chlorfluazuron, thiocyclam, and methamidophos was documented in six field populations of P. xylostella. High levels of resistance to cypermethrin and deltamethrin, but moderate levels of resistance to chlorpyriphos and methomyl, were also documented in two field populations of S. exigua. Moderate levels of resistance to cypermethrin, deltamethrin and chlorpyriphos were also documented in three field populations of H. zea. Moderate to high levels of resistance to bifenthrin, methamidophos and endosulfan were documented in four field populations of B. tabaci. The presence of significant correlations between LD50s or LC50s suggests the occurrence of cross-resistance or simultaneous selection for resistance by different insecticides with different modes of action. Our data could not differentiate between these two possibilities. Because insecticides will continue being used in Nicaragua, a resistance management program is urgently needed. The implementation of integrated pest management tactics must be accompanied by specific regulations for pesticide registration. In the future, pesticide registration regulations in Nicaragua should include periodic resistance monitoring. The mechanisms to cover the costs of resistance monitoring and resistance management should also be established.     

Seasonal changes of methamidophos susceptibility and biochemical properties in Plutella xylostella (Lepidoptera: Yponomeutidae) and its parasitoid Cotesia plutellae (Hymenoptera: Braconidae)

Methamidophos resistance and acetylcholinesterase (AChE) insensitivity to methamidophos, dichlorvos, and carbofuran were determined in the field populations of Plutella xylostella (L.) (Lepidoptera: Yponomeutidae) and its parasitoid Cotesia plutellae Kurdjumov (Hymenoptera: Braconidae) collected from the corresponding hosts between October 1998 and December 2003 in Fuzhou and Minhou, Fijian, China. Resistance levels to methamidophos and AChE insensitivity to the three insecticides in the two species of insects were high during autumn and spring and low during summer. Resistance to methamidophos was 15.3- and 12.6-fold higher in resistant F0 parents of P. xylostella and C. plutellae than in their susceptible F11 progeny, respectively. The bimolecular rate constant (k(i)) values of AChE to methamidophos, dichlorvos, and carbofuran were 4.6-, 6.3-, and 7.7-fold higher in F11 progeny of P. xylostella, and 3.7-, 4.5-, and 3.7-fold higher in F11 progeny of C. plutellae than those in their F0 parents, respectively. Compared with susceptible F11 progeny, the resistance ratios for methamidophos were 4.2-29.8 and 3.8-13.1 in 21 field populations of P. xylostella and C. plutellae, respectively. The k(i) values of AChE to methamidophos, dichlorvos, and carbofuran were 2.0-21.6-, 3.6-9.5-, and 2.6-9.2-fold higher in F11 progeny of P. xylostella, and 1.8-7.6-, 1.9-4.6-, and 2.2-7.6-fold higher in F11 progeny of C. plutellae than those in 21 field populations, respectively. Significant correlative variations of methamidophos resistance as well as significant correlative variations of k(i) values of AChE to insecticides between the two species of insects also were found in space and time. The k(i) values of AChE to insecticides in C. plutellae were far higher than those in P. xylostella. There were no obvious differences in the Km and Vmax of AChE between F0 parents and F11 progeny of P. xylostella and C. plutellae, respectively. But carboxylesterase activity was 1.6-fold higher in F0 parents of C. plutellae than in F11 progeny, and glutathione S-transferase activity was 1.5-fold higher in F0 parents of P. xylostella than in F11 progeny. The results suggested that the AChE insensitivity to insecticides might play the most important role in methamidophos resistance in the two species of insects. From these results, a spatial and temporal correlative evolution of methamidophos resistance and insensitive AChE was found to exist between P. xylostella and C. plutellae.     

西花蓟马对多杀菌素的抗性汰选和遗传方式

在室内进行了西花蓟马Frankliniella occidentalis(Pergande)种群对多杀菌素的抗性筛选和抗性遗传方式分析。经过2年多的汰选,抗性水平相比较初始种群提高了30倍,与同时测定的敏感品系相比抗性达到80.8倍。抗性遗传方式分析结果表明,正、反交后代显性度分别为0.51和0.43,二者差异不显著,说明西花蓟马对多杀菌素的抗性为常染色体、不完全显性遗传;剂量对数死亡机率值曲线分析结果显示回交后代在死亡率50%处,自交后代在死亡率25%和75%处未出现明显平坡,回交和自交后代实测值卡方检验进一步证实抗性为多基因控制。     

南京地区小菜蛾的抗药性检测及初步分析

利用浸叶法对南京郊区小菜蛾Plutella xylostella的抗药性进行了监测,发现其对拟除虫菊酯类药剂的抗性较高,而对氟虫腈、辛硫磷、毒死蜱、多杀菌素和虫酰肼依然处于敏感阶段。利用抗性小菜蛾测试发现,氧化胡椒基丁醚（增效醚,简称PBO）对拟除虫菊酯具有显著的增效作用。室内敏感性恢复实验表明,在不接触药剂的条件下,小菜蛾对拟除虫菊酯的抗性迅速下降,繁殖10代以后,抗性维持在低抗水平。因此, 小菜蛾的抗药性治理,应充分利用不同药剂的轮换使用,避免单一使用某一品种,以延缓抗性的发展,同时可以利用PBO增强拟除虫菊酯的防治效果,保证这类药剂在小菜蛾防治中的作用。     

Development and stability of insecticide resistance in the leafminer Liriomyza trifolii (Diptera: Agromyzidae) to cyromazine, abamectin, and spinosad

Three populations of the leafminer, Liriomyza trifolii (Burgess), were collected from commercial ornamental production greenhouses in the United States and tested for susceptibility to three commercial insecticides. A leaf dip bioassay of leaves containing young (1-2-d-old) larvae was used. Based on larval mortality and compared with a susceptible laboratory reference colony, the three strains varied in spectrum and level of resistance to the insecticides. CA-1, collected from Gerbera daisy, was moderately resistant to cyromazine (18.1-fold) and abamectin (22.0-fold), but highly resistant to spinosad (> 188-fold). CA-2, collected from chrysanthemums, was not resistant to abamectin, had a low level of resistance to cyromazine (8.2-fold), but was extremely resistant to spinosad (1,192-fold). GA-1, collected from chrysanthemums, had very low levels of resistance to cyromazine (5.4-fold) and spinosad (1.9-fold) but was moderately resistant to abamectin (30.6-fold). When reared in the absence of insecticide selection pressure, all three strains reverted to approximately the level of the reference strain. The CA-1 strain reverted in nine generations to cyromazine; however, the lowest levels of abamectin and spinosad resistance reverted to was 3.1-fold at F8 and 3.2 at the F10, respectively. The CA-2 strain reverted in five generations to both cyromazine and spinosad. GA-1 reverted in five generations to abamectin. Based on the results, resistance to these three insecticides was unstable. Additionally, there was no cross-resistance among these three insecticides.     

Integrative model for binding of Bacillus thuringiensis toxins in susceptible and resistant larvae of the diamondback moth (Plutella xylostella)

Insecticidal crystal proteins from Bacillus thuringiensis in sprays and transgenic crops are extremely useful for environmentally sound pest management, but their long-term efficacy is threatened by evolution of resistance by target pests. The diamondback moth (Plutella xylostella) is the first insect to evolve resistance to B. thuringiensis in open-field populations. The only known mechanism of resistance to B. thuringiensis in the diamondback moth is reduced binding of toxin to midgut binding sites. In the present work we analyzed competitive binding of B. thuringiensis toxins Cry1Aa, Cry1Ab, Cry1Ac, and Cry1F to brush border membrane vesicles from larval midguts in a susceptible strain and in resistant strains from the Philippines, Hawaii, and Pennsylvania. Based on the results, we propose a model for binding of B. thuringiensis crystal proteins in susceptible larvae with two binding sites for Cry1Aa, one of which is shared with Cry1Ab, Cry1Ac, and Cry1F. Our results show that the common binding site is altered in each of the three resistant strains. In the strain from the Philippines, the alteration reduced binding of Cry1Ab but did not affect binding of the other crystal proteins. In the resistant strains from Hawaii and Pennsylvania, the alteration affected binding of Cry1Aa, Cry1Ab, Cry1Ac, and Cry1F. Previously reported evidence that a single mutation can confer resistance to Cry1Ab, Cry1Ac, and Cry1F corresponds to expectations based on the binding model. However, the following two other observations do not: the mutation in the Philippines strain affected binding of only Cry1Ab, and one mutation was sufficient for resistance to Cry1Aa. The imperfect correspondence between the model and observations suggests that reduced binding is not the only mechanism of resistance in the diamondback moth and that some, but not all, patterns of resistance and cross-resistance can be predicted correctly from the results of competitive binding analyses of susceptible strains.     

Baseline responses of Alphitobius diaperinus (Coleoptera: Tenebrionidae) to spinosad, and susceptibility of broiler populations in Eastern and Southern Australia

Spinosad was proposed as a potential chemical for control of lesser mealworm, Alphitobius diaperinus (Panzer) (Coleoptera: Tenebrionidae), in Australian broiler houses after the detection of strong cyfluthrin resistance in many beetle populations. In 2004-2006, spinosad susceptibility of 13 beetle populations from eastern and southern Australian broiler houses and a cyfluthrin/fenitrothion-resistant reference population was determined using topical application, and was compared with the susceptibility of an insecticide-susceptible reference population. Comparisons of dose-response curves and baseline data showed that all populations, including the insecticide-susceptible population, were roughly equivalent in their response to spinosad, indicating no preexisting spinosad resistance. Two field populations, including the resistant reference population, which had confirmed cyfluthrin/fenitrothion-resistance, showed no cross-resistance to spinosad. There was no significant correlation between beetle weight and LC9.9. A discriminating concentration of 3% spinosad was set to separate resistant and susceptible individuals. Considering the levels of spinosad resistance that have been recorded in other insect pests, the sustained future usefulness of spinosad as a broiler house treatment will rely on effective integrated beetle management programs combined with carefully planned chemical use strategies.     

Field trials of CpGV virus isolates overcoming resistance to CpGV-M

The Cydia pomonella granulovirus (CpGV) has been used for many years as biological agent for codling moth control in apple orchards. Resistance to the Mexican strain of CpGV was detected in orchards in Germany, France and Italy. A laboratory insect colony was started from insects collected in a French resistant orchard. It was named RGV. Various virus isolates were identified as active against this resistant insect colony. Field tests were carried out in 2007 to test if the two virus isolates CpGV-I12 and NPP-R1 were effective in the field. Although these virus isolates were not able to reduce insect caused fruit damages, they significantly reduced the overwintering insect populations. NPP-R1 was subjected to eight passages on RGV larvae (NPP-R1.8) that improved its biological activity on RGV larvae. 2008 field trials were set up to test this improved virus strain, compared to CpGV-I12 and Madex plus active on RGV. These tests confirmed the ability to control both in susceptible and resistant insect populations.     

High stability and no fitness costs of the resistance of codling moth to Cydia pomonella granulovirus (CpGV-M)

Resistance against the biocontrol agent Cydia pomonella granulovirus (CpGV-M) was previously observed in field populations of codling moth (CM, C. pomonella) in South-West Germany. Incidental observations in a laboratory reared field colony (CpR) indicated that this resistance is rather stable, even in genetically heterogeneous CM colonies consisting of both susceptible and resistant individuals. To test this hypothesis, the resistance level of CpR that was 1000times less susceptible to CpGV-M was followed for more than 60 generations of rearing. Even without virus selection pressure, the high level of resistance, expressed as median lethal concentration, remained stable for more than 30 generations and declined only by a factor of 10 after 60 generations. When cohorts of the F32 and F56 generations of the same colony were selected to CpGV-M for five and two generations, respectively, the resistance level increased to factor of >1,000,000 compared to a susceptible control colony. Laboratory reared colonies of CpR, did not exhibit any measurable fitness costs under laboratory conditions in terms of fecundity and fertility. Resistance testing of seven selected codling moth field populations collected between 2003 and 2008 in commercial orchards in Germany that were repeatedly sprayed with CpGV products gave evidence of different levels of resistance and a more than 20-fold increase of the resistance in 1-3years when selection by CpGV-M was continued. A maximum 1,000,000-fold level of resistance to CpGV-M that could be induced in the laboratory under virus pressure had been also observed in one field population. The high stability of resistance observed in the genetically heterogenous colony CpR indicates that resistance to CpGV-M is not very costly.     

Assessment of insecticide resistance after the outbreak of diamondback moth (Lepidoptera: Plutellidae) in California in 1997

During an outbreak of the diamondback moth, Plutella xylostella (L.), in California in 1997, nine populations were collected from the major broccoli areas throughout the state. Populations were assayed for their susceptibility to currently used materials (Bacillus thuringiensis subsp. kurstaki, permethrin, and methomyl) and to newer materials that had not yet been commercially used in California (spinosad, emamectin benzoate, and chlorfenapyr). For the currently used insecticides, elevated levels of resistance were seen only with permethrin and seven of the nine populations had tolerance ratios (TR) of > 100. With the newer chemistries, TR values were all < 15. To compare potential cross-tolerance, TR values of the currently used insecticides were compared with TR values of the newer insecticides. There were significant relationships found between: methomyl and emamectin benzoate, methomyl and spinosad, and permethrin and spinosad. Further biochemical studies are needed to confirm the actual mechanisms that lead to these relationships and field tests are needed to determine what impact, if any, such TR levels would have on control in the field. These data indicate that resistance to at least one of the commonly used insecticides (permethrin) may have played a role in the outbreak during 1997. However, other factors may have been at least equally important. The winter of 1996-1997 was warmer than normal, and during the period from February through August of 1997 the amount of rainfall was < 50% of normal. Hot and dry conditions are known to be conducive to outbreaks of P. xylostella. These data add to an overall knowledge about the geographic variation of resistance in P. xylostella populations within the United States. They also serve as a baseline for monitoring changes in susceptibility to these newer insecticides and can also help explain the occurrence of outbreaks caused by factors other than insecticide resistance.     

Development of resistance to spinosad in oriental fruit fly (Diptera: Tephritidae) in laboratory selection and cross-resistance

In this study, we assessed the potential for the development of resistance to the insecticide spinosad in a laboratory colony of the oriental fruit fly, Bactrocera dorsalis (Hendel) (Diptera: Tephritidae). Resistance was selected by using topical applications of spinosad. After eight generations of selection, the LD50 of the selected line was 408 times greater compared with that of the untreated parental colony. This spinosad-resistant line did not exhibit cross-resistance to 10 other insecticides tested, including six organophosphates (naled, trichlorfon, fenitrothion. fenthion, formothion, and malathion) one carbamate (methomyl), and three pyrethroids (cyfluthrin, cypermethrin, and fenvalerate). However, using lines previously selected for resistance to these same insecticides, two of the 10 lines tested (naled- and malathion-resistant) did show some cross-resistance to spinosad. Also, oriental fruit flies from different field collections where naled and malathion have been used for control purposes displayed some resistance to spinosad. In addition, the effects of direct ingestion of spinosad through dietary supplementation also were tested. Overall, the laboratory resistance and cross-resistance data developed in this study provide new information that will be useful for managing the development of resistance when spinosad is used to control B. dorsalis in the field.     

Lethal and sub‐lethal effects of spinosad on the life‐history traits of army worm,Spodoptera litura (Fabricius) (Lepidoptera: Noctuidae), and its fitness cost of resistance

Spodoptera litura is one of the most destructive polyphagous insect pests, with more than 120 host‐plant species. In our present study, a field‐collected population of S. litura when selected with spinosad for 11 consecutive generations resulted in the development of 3921‐fold resistance to spinosad as compared to the susceptible strain. The spinosad‐resistant strain of S. litura had a relatively high fitness cost (0.17) as compared to the susceptible strain. Furthermore, the lethal and sub‐lethal effects of different concentrations of spinosad were checked on the susceptible strain at different levels; i.e., LC₄₀, LC₃₀, LC₂₀ and LC₁₀, which revealed that the impact of spinosad on the life‐history traits of S. litura increased with the increase in concentration of spinosad. A significant impact of spinosad was recorded on the larval duration, pre‐pupal weight, pupal duration, pupal weight, reproductive potential and adult emergence. The outcomes of the current research clearly indicate that fitness cost of spinosad and its sub‐lethal effects have a significant impact on population dynamics of S. litura, for which it can be incorporated in integrated pest management.     

Susceptibility of laboratory and field strains of four stored-product insect species to spinosad

Two field strains of the Indianmeal moth, Plodia interpunctella (Hübner); red flour beetle, Tribolium castaneum (Herbst); and lesser grain borer, Rhyzopertha dominica (F.), and one field strain of the rusty grain beetle, Cryptolestes ferrugineus (Stephens), were collected from hard red winter wheat stored on farms in northeastern Kansas. Fifty eggs of P. interpunctella and 25 beetle adults of each species were exposed to 100 g of untreated wheat or wheat treated with various rates of spinosad, to determine susceptibility of the field and corresponding insecticide-susceptible laboratory strains. Mortality of beetle adults and P. interpunctella larvae was assessed after 7 and 21 d postinfestation, respectively. Field strains of P. interpunctella, C. ferrugineus, and T. castaneum were less susceptible to spinosad than the corresponding laboratory strains. The LD50 and LD95 values for P. interpunctella and C. ferrugineus field strains were 1.7-2.5 times greater than values for corresponding laboratory strains. Adults of both laboratory and field strains of T. castaneum were tolerant to spinosad, resulting in <88% mortality at 8 mg/kg. The LD50 and LD95 values for the field strains of T. castaneum were 2.0-7.5 times greater compared with similar values for the laboratory strain. The field and laboratory strains of R. dominica were highly susceptible to spinosad, and one of the field strains was relatively less susceptible to spinosad than the laboratory strain. Our results confirm a range of biological variability in field populations, which is consistent with findings for other compounds, and underscores the need to adopt resistance management programs with stored grain insect pests. The baseline data generated on the susceptibility of the four insect species to spinosad will be useful for monitoring resistance development and for setting field rates.     

Genetic, biochemical, and physiological characterization of spinosad resistance in Plutella xylostella (Lepidoptera: Plutellidae)

Bioassays (at generation G2) with a newly collected field population (designated MN) of Plutella xylostella (L.) (Lepidoptera: Plutellidae) from Multan, Pakistan, indicated resistance to spinosad, indoxacarb, deltamethrin, abamectin, and acetamiprid. At G2 the field-derived population was divided into two subpopulations, one was selected (G2 to G11) with spinosad (Spino-SEL), whereas the second was left unselected (UNSEL). A significant reduction in the resistance ratio for each compound was observed in UNSEL at G12, indicating that the observed resistance to each insecticide was unstable. For Spino-SEL, bioassays at G12 found that selection with spinosad gave a resistance ratio of 283 compared with MN at G2. The resistance to indoxacarb and acetamiprid in the Spino-SEL population increased to 13- and 67-fold, respectively, compared with MN at G2. The toxicity of deltamethrin to Spino-SEL was similar to its toxicity to the MN population at G2. This suggests that spinosad selection maintained the otherwise unstable resistance to the compound. In contrast, resistance to abamectin decreased significantly from G2 to G12 in Spino-SEL. Logit regression analysis of F1 reciprocal crosses between Spino-SEL and the susceptible Lab-UK indicated that resistance to spinosad was inherited as an autosomal, incompletely recessive trait. The spinosad resistance allele significantly delays the developmental time, reduced pupal weight, number of eggs laid, and number of eggs hatched compared with Lab-UK. Further analysis suggests Spino-SEL exhibited a significantly lower intrinsic rate of population increase (r(m)) to all other populations tested.     

不同地理种群小菜蛾对甲胺磷的抗药性

采用浸叶法研究了福州建新镇、福建农林大学校园内、闽侯县上街镇和闽侯县甘蔗镇等地田间小菜蛾种群对甲胺磷的抗性水平及抗性机制。结果表明,与敏感小菜蛾相比,上述4个田间小菜蛾种群已对甲胺磷产生了34-82倍的抗性水平。敏感小菜蛾乙酰胆碱酯酶(AChE)的Ki值是田间抗性小菜蛾AChE的Ki值的17.3-39.1倍。不同小菜蛾种群对甲胺磷的抗性水平也存在着差异,小菜蛾种群AChE对甲胺磷的敏感性大小与其对甲胺磷的抗性水平高低显著相关。