Evidence of sibling species between two host-associated populations of brown planthopper, N. lugens (stål) (Homoptera: Delphacidea) complex based on morphology and host–plant relationship studies

Morphological and host–plant relationship studies were conducted to differentiate two sympatric populations of brown planthopper (BPH), Nilaparvata lugens, one from rice (Oryza sativa) and the other from Leersia hexandra, a weed grass. In morphometric studies based on esterase activities, an UPGMA dendrogram using 17 quantitative morphological characters, including stridulatory organs (courtship signal-producing organs) between two sympatric populations of N. lugens, one from rice and the other from L. hexandra, a weed grass revealed that both populations were separated from each other. An out-group, N. bakeri, was found to be completely different from the two sympatric populations of N. lugens. Rice plants were best suited for the establishment of the rice-infesting population, and L. hexandra was a favourable host for the Leersia-infesting population. The individuals derived from one host did not thrive on the other host, as shown by a significant reduction in survival and nymphal development, ovipositional preferences, ovipositional response, and egg hatchability. Therefore, morphological and host–plant relationship studies indicate that rice-associated population with high esterase activities and L. heaxandra-associated population with low esterase activities are two closely related sibling species.     

Allozyme variation among biotypes of the brown planthopperNilaparvata lugens in the Philippines

Allozyme variation was studied in threeNilaparvata lugens biotypes infesting specific rice varieties and a biotype infesting a weed grass,Leersia hexandra. Of the 20 enzymes inN. lugens for which activity was noted, 9 were polymorphic. Eleven enzyme loci were monomorphic for the same allele in all biotype populations; the rest were polymorphic for two or more alleles. The mean number of alleles per polymorphic locus was 2.3, while the mean number of alleles per locus was 1.5; heterozygosity ranged from 0.02 to 0.06 (biotype 1 > biotype 3 >Leersia-infesting biotype > biotype 2). Allelic frequency differences were observed in five loci among the four biotypes. However, the coefficient of genetic identity (I) of 0.99+ showed that the four biotype populations were genetically close relatives or merely populations ofN. lugens undergoing genetic differentiation. This work was partly supported by a financial grant received from the Directorate for Technical Cooperation and Humanitarian Aid, Switzerland.     

Susceptibility of malathion-resistant strains of Tribolium castaneum and T. confusum to the insect growth regulators methoprene and hydroprene

A comparison was made of the effects of methoprene (isopropyl 11-methoxy-3, 7, 11-trimethyl-2, 4-dodecadienoate) and hydroprene (ethyl 3, 7, 11-trimethyl dodeca-2, 4 dienoate) on the developmental survival of three malathion-resistant strains of Tribolium castaneum (Herbst) and one of T. confusum J. du Val, with a malathion-susceptible strain of each species. No cross-resistance to either IGR in any of the malathion-resistant strains was detected.

---

Résumé On a fait une comparaison des effets de methoprene (isopropyl 11-methoxy-3, 7, 11-trimethyl-2, 4-dodecadienoate) et hydroprene (ethyl 3, 7, 11-trimethyldodeca-2, 4 dienoate) sur le développement de trois races de Tribolium castaneum résistantes au malathion, et une race de T. confusum résistante au malathion, avec une race de chaque espèce prédisposée au malathion. Aucune résistance croisée a l'un ou l'autre IGR ne fut découverte dans les races résistantes au malathion.

     

The genetic basis of oviposition preference differences between sympatric host races of the brown planthopper (Nilaparvata lugens)

We have previously analysed the genetic architecture of host-associated performance differences between stocks of the brown planthopper, Nilaparvata lugens (Homoptera: Delphacidae), derived from two host plants: cultivated rice variety TN1 and the semi-aquatic weed Leersia hexandra. It has been established that performance is influenced by a small effective number of loci (1 to 3), with dominance in the direction of the rice population. The results reported here show that, like performance, oviposition preference has a simple genetic basis, but in this case there is no evidence of directional dominance, sex linkage, non-allelic interaction or genotype–environment interaction. The simple genetic architecture of host-associated performance and oviposition preference are as expected if there was a sympatric host shift, most probably from Leersia to rice. However, contrary to the theoretical expectation from models of sympatric host-race formation, there was no detectable genetic association between the traits. The fact that individual females tend to distribute their eggs between plants may have helped to promote a host shift despite this lack of association.     

Characterization of a novel esterase conferring insecticide resistance in the mosquito Culex tarsalis

Resistance to the organophosphate insecticide, malathion, in a strain of Culex tarsalis mosquitoes is due to increased activity of a malathion carboxylesterase (MCE). To determine whether resistance was due to a qualitative or quantitative change in the MCE, the enzyme was purified from both malathion-resistant and -susceptible mosquitoes. Enzyme kinetic measurements revealed that the two strains have one MCE in common, but resistant mosquitoes also have a unique MCE which hydrolyses malathion 18 times faster. Interestingly, this MCE does not hydrolyse α-naphthyl acetate, a substrate commonly used to detect increased levels of esterases in other organophosphate-resistant insects. Unlike the over-produced esterase of some related mosquito species, each MCE in C. tarsalis accounts for only a small fraction (0.015%) of the total extractable protein in either strain. Therefore, resistance in these insects is due to the presence of a qualitatively different enzyme, and not to a quantitative increase of a non-specific esterase. This study therefore demonstrates that the underlying biochemical mechanisms of insecticide resistance in one insect cannot necessarily be predicted from those of another, even closely related species. © 1995 Wiley-Liss, Inc.     

Effects of elevated CO2 and transgenic Bt rice on yeast‐like endosymbiote and its host brown planthopper

The effects of elevated CO₂ (750 vs. 375μl/l) on population abundances and metabolism enzyme of AChE and protective enzymes of SOD, POD and CAT in brown planthoppers (BPH) Nilaparvata lugens, and on size and abundances of yeast‐like endosymbiotes (YLES) were studied as BPH fed Bacillus thuringiensis (Bt) rice expressing pure Cry1Ab after successively two generations in open‐top chambers. The results indicated that: (1) Brachypterous and macropterous subpopulations and total population increased with elevated CO₂. Significant increases were found as BPH fed non‐transgenic rice while only significant increase as macropterous‐BPH fed Bt rice. (2) The responses of brachypterous and macropterous‐BPH to Bt rice were different. Brachypterous‐subpopulation significantly decreased (13.6%) while macropterous ones significantly increased (43.8%) as fed Bt rice relative to non‐transgenic rice at elevated CO₂. (3) Elevated CO₂ only significantly inhibited AChE activity as brachypterous‐BPH fed non‐transgenic rice. Significant increases in POD and SOD, and significant decrease in CAT were found as brachypterous‐BPH fed Bt rice, while significant increases in CAT and significant decrease in POD were also observed as fed non‐transgenic rice in elevated CO₂ relative to ambient CO₂. (4) Bt rice significantly inhibited POD and SOD activity at ambient CO₂, while only significantly enhanced SOD activity at elevated CO₂. (5) Elevated CO₂ significantly decreased YLES per mg/head of brachypterous‐BPH females while only significantly decreased YLES per mg/head as brachypterous‐BPH males fed Bt rice. And there were significant differences in YLES width or length between females and males. Elevated CO₂ can markedly affect the symbiosis relationship between YLES and BPH through the bottom‐up forcing on BPH physiological metabolism. And the damage inflicted by BPH on rice, irrespective of the presence of insecticidal genes, is predicted to be higher at elevated CO₂. Furthermore, transgenic Bt rice can also exacerbate emigrating‐macropterous‐BPH occurring especially at elevated CO₂.     

Mapping of a broad-spectrum brown planthopper resistance gene, <Emphasis Type="Italic">Bph3</Emphasis>, on rice chromosome 6

The brown planthopper (BPH) is one of the most destructive insect pests of rice in Thailand. We performed a cluster analysis that revealed the existence of four groups corresponding to the variation of virulence against BPH resistance genes in 45 BPH populations collected in Thailand. Rice cultivars Rathu Heenati and PTB33, which carry Bph3, showed a broad-spectrum resistance against all BPH populations used in this study. The resistant gene Bph3 has been extensively studied and used in rice breeding programs against BPH; however, the chromosomal location of Bph3 in the rice genome has not yet been determined. In this study, a simple sequence repeat (SSR) analysis was performed to identify and localize the Bph3 gene derived from cvs. Rathu Heenati and PTB33. For mapping of the Bph3 locus, we developed two backcross populations, BC₁F₂ and BC₃F₂, from crosses of PTB33 × RD6 and Rathu Heenati × KDML105, respectively, and evaluated these for BPH resistance. Thirty-six polymorphic SSR markers on chromosomes 4, 6 and 10 were used to survey 15 resistant (R) and 15 susceptible (S) individuals from the backcross populations. One SSR marker, RM190, on chromosome 6 was associated with resistance and susceptibility in both backcross populations. Additional SSR markers surrounding the RM190 locus were also examined to define the location of Bph3. Based on the linkage analysis of 208 BC₁F₂ and 333 BC₃F₂ individuals, we were able to map the Bph3 locus between two flanking SSR markers, RM589 and RM588, on the short arm of chromosome 6 within 0.9 and 1.4 cM, respectively. This study confirms both the location of Bph3 and the allelic relationship between Bph3 and bph4 on chromosome 6 that have been previously reported. The tightly linked SSR markers will facilitate marker-assisted gene pyramiding and provide the basis for map-based cloning of the resistant gene.     

Comparisons of malathion susceptibility, target sensitivity, and detoxification enzyme activity in nine field populations of Oxya chinensis (Orthoptera: Acrididae)

The malathion susceptibility, acetylcholinesterase (AChE) sensitivity, and the activity of selected detoxification enzymes including general esterase (EST) and glutathione S-transferase (GST) were compared among field populations of the grasshopper Oxya chinensis (Thunberg) (Orthoptera: Acrididae) collected from nine regions of China. Bioassay results showed that these populations had various levels of the susceptibility to malathion with the LDo values ranging from 1.4- to 22.6-fold compared with the most susceptible population (Xiangyuan or XY). The Jinnan (JN) population seemed to be malathion resistant (22.6-fold), whereas other populations exhibited 1.4- to 6.8-fold reduced malathion susceptibility with a rank order of Changan > Baodi > Hanzhong > Xinxiang > Yinchuan > Beidagang > Jinyuan. It seemed that the observed malathion resistance in the JN population was attributed to at least two resistance mechanisms, including increased EST activity (2.2-fold) and reduced sensitivity of AChE to inhibition by malaoxon (4.6-fold) compared with those of the XY population. In contrast, differential malathion susceptibilities in other populations may be due to increased activities of certain detoxification enzymes (e.g., EST and GST), reduced sensitivity of AChE, or other factors, which were not consistent across the populations examined. Such differential susceptibilities to malathion were likely due to different population habitats (e.g., grasslands, rice [Oryza sativa L.]-producing regions) with very different insecticide application histories and pest management practices.     

the differentiation of amino acid requirements in three host‐related populations of the brown planthopper,nilaparvata lugens (stål)*

Abstract In order to understand the differentiation of amino acid requirments in host‐related populations of the brown planthopper (BPH), Nilaparvata lugens (Stål), we established three BPH populations by separately maintaining them on three rice varieties, i.e. TN1 (common susceptible control), Mudgo (containing resistant gene Bph 1) and ASD7 (containing resistant gene bph2) for over 30 generations, and then reared them on 20 holidic diets, each with distinct overall amino acids and ratio of essential amino acid (EAA) to nonessential amino acid (NEAA). The emergence rate, brachypter rate, nymphal duration and weight of newly molted adults were evaluated and compared. The results showed significant difference among the three populations in the effect of amino acid variation on BPH performance, ascending in an order of Mudgo population ASD7 population > TN1 pupulation. The results also indicated that the required optimum concentrations of overall amino acids by BPH populations on Mudgo and ASD7 were 4.0% ‐4.8% and 4.0%, respectively, higher than that of TN1 population (2.4% ‐ 3.2%). In addition, it was found that Mudgo and ASD7 populations were more sensitive to the concentration of EAAs than TN1 population. We propose that there is substantial differentiation in responses to dietary amino acid conditions among different host‐associated BPH populations and that such differentiation may be closely related to the induced virulence shift on resistant rice varieties.     

ESTERASE PHENOTYPES IN NATURAL POPULATIONS OF CULEX PIPIENS PALLENS (DIPTERA: CULICIDAE) AND THEIR VARIATIONS UNDER INSECTICIDE SELECTION*

Abstract Three populations of Culex pipiens pallens Coquillett, from Hangzhou, Linhai and Jinhua, were determined for their probability of resistant esterase phenotypes. Further, each population was selected with dipterex, chlorpyrifos, temephos and malathion at a given dose of 60%‐70% mortality and was determined for the variation of resistant esterase phenotypes in its offspring. The results indicated that esterase B₁ phenotype was dominant in three populations although different natural populations have a different probability distribution of esterase B phenotypes. Insecticide selection showed that various esterase loci seemed to have different selective advantages under different insecticide pressures. The esterase B₁ phenotype was selected under temephos, and the esterase B₂ phenotype was selected under dipterex and chlorpyrifos. Except for the esterase B₁ phenotype, the heterozygote of B₁/B₂ phenotypes was detected under malathion pressure. These results are discussed in relation to the selective advantages of esterase genes to OP pressure and mosquito control strategies.     

Comparative spikelet morphology ofOryza schlechteri Pilger and related species ofLeersia andOryza (Poaceae)

Oryza schlechteri has only recently been rediscovered in Papua New Guinea. This species is quite distinct from otherOryza species but resembles closely related genusLeersia in its small panicle and profusely spreading habit. On the basis of spikelet morphology that has previously been used to distinguishLeersia andOryza species,O. schlechteri was compared withO. brachyantha. O. longiglumis, O. sativa, L. perrieri, L. tisseranti, andL. hexandra to clarify its generic position. Results corroborate that the presence of a sterile lemma and a striated spikelet epidermal (abaxial) surface lacking siliceous triads inO. schlechteri allies this species with otherOryza species rather than withLeersia.     

Characterization of esterases in malathion-resistant and susceptible strains of the pteromalid parasitoid Anisopteromalus calandrae

General esterase, malathion-specific carboxylesterase, phosphotriesterase, glutathione S-transferase, cytochrome P-450-dependent monooxygenase activity, and target site sensitivity were compared in malathion-resistant (R) and malathion-susceptible (S) strains of the parasitoid Anisopteromalus calandrae (Howard) (Hymenoptera: Pteromalidae). Activity against -naphthyl acetate was not significantly different in male and female wasps for either strain. General esterase activity ranged from 1.2-fold to 2.5-fold higher in the R strain compared with the S strain, but these differences between strains were not consistent. Based on V_max/K_m ratios estimated for a number of analogs of four substrates (-naphthyl acetate, β-naphthyl acetate, 4-methylumbelliferyl acetate, and p-nitrophenyl acetate) there was no evidence that general esterase activity was elevated or reduced in the R strain. Malathion-specific carboxylesterase (MCE) activity, determined by using 2,3-¹⁴C-malathion as substrate, was 10- to 30-fold higher in the R strain compared with that in the S strain. The MCE has a pH optima at about pH 7, is cytosolic, and is labile upon storage at −80°C. MCE activity could be recovered from native 10% PAGE gels and IEF–PAGE gels (pI=5.2), but the peak of MCE activity also contained the major peak of activity against -naphthyl acetate. There was no evidence for major involvement of phosphotriesterase, glutathione S-transferase, monooxygenase, or altered acetylcholinesterase in the resistance. These data suggest that an increased activity of a MCE in the R strain is the probable major mechanism conferring resistance to malathion in A. calandrae. This study provides the first characterization of a biochemical resistance mechanism in a parasitoid with a high level of resistance to an organophosphate insecticide.     

Biochemical genetics of malathion resistance in the smaller brown planthopper, Laodelphax striatellus

The esterases hydrolyzing β-naphthyl acetate of the smaller brown planthopper were separated into nine different bands by thin layer electrophoresis. Malathion-resistant planthoppers showed remarkably higher activity of the E₇ band than the susceptible ones. Inheritance of malathion resistance was closely correlated with that of esterase activity in HE, LE and their F₁, F₂ and backcross offspring. The E₇ band of F₁ individuals showed activity middle between HE and LE parent strains. F₂ offspring segregated in ratios of 1.07 : 2.05 : 0.88 for the low, middle and high activity of the E₇ band, respectively, and backcross offspring in ratios of 0.95 : 1.05 or 1.0 : 1.0 for low and middle activity, respectively. It was indicated that esterase activity of the E₇ band was controlled by a co-dominant factor on the autosomal chromosome(s). Most of the individuals surviving from contact with doses of malathion high enough to cause 100% mortality in the susceptible strain showed middle activity of the E₇ band. The suggestion is made that esterase activity of the E₇ band and malathion resistance depend on the same factor.

---

Biochemische genetik der malathion-resistenz bei der kleinen braunen spornzikade, Laodelphax striatellus

Zusammenfassung Esterase hydrolysierende β-Naphthylacetat-Extrakte aus der kleinen braunen Spornzikade wurden mittels Dünnschicht-Elektrophorese in 9 verschiedene Banden zerlegt. Malathionresistente Zikaden zeigten in der E₇-Bande eine bemerkenswert h?here Aktivit?t als die anf?lligen. Die Vererbung der Malathion-Resistenz war bei den St?mmen HE und LE sowie ihren F₁-, F₂-, und Rückkreuzungs-Nachkommenschaften eng mit der Esterase-Aktivit?t verbunden. Die E₇-Individuen zeigten eine intermedi?re Aktivit?t zwischen denen der HE- und LE-Eltern-St?mme. F₂-Nachkommen waren im Verh?ltnis von 1.07 : 2.05 : 0.88 mit niedriger, mittlerer und hoher Aktivit?t der E₇-Bande aufgespalten, und Rückkreuzungs-Nachkommen entsprechend im Verh?ltnis von 0.95 : 1.05 oder 1.0 : 1.0 mit niederer und mittlerer Aktivit?t. Es bestehen Hinweise, da? die Esterase-Aktivit?t der E₇-Bande von einem co-dominanten Faktor im autosomalen Chromosom kontrolliert wird. Die meisten Individuen, die nach einem Kontakt mit Malathion-Dosen überlebten, welche bei empfindlichen St?mmen genügen, um 100% ige Mortalit?t zu bewirken, zeigten eine mittlere Aktivit?t der E₇Bande. Es besteht die Vermutung, da? die Esterase-Aktivit?t der E₇-Bande und die Malathion-Resistenz vom gleichen Faktor abh?ngen.

     

Interpopulation Crosses, Inheritance Study, and Genetic Variability in the Brown Planthopper Complex, Nilaparvata lugens (Homoptera: Delphacidae)

Studies on hybridization, inheritance, and population genetics of brown planthoppers that infest rice and weeds were undertaken using starch gel electrophoresis to determine whether the weed-infesting population represents a biological race or a species. F₁ and F₂ generations were produced by crosses between parental insects from the two populations with little indication of hybrid sterility. Gpi, Mdh, and Idh loci were inherited in a simple Mendelian fashion in families of two sympatric populations. Sixteen populations of Nilaparvata spp. from eight locations were collected. The Mdh, Idh, Pgm, Gpi, 6Pgd, and Acp loci were polymorphic. The N. lugens of rice with high esterase activity were clustered into a group and characterized by the presence of alleles Gpi ¹¹⁰ and Gpi ¹²⁰ , whereas N. lugens from weeds with low esterase activity were clustered into another group and characterized by Gpi ¹⁰⁰ and Gpi ⁹⁰ . There was a lack of heterozygotes between the common alleles of the two populations. This means that the two groups of individuals belong to different gene pools.     

稻褐飞虱致害性遗传与性别的关联性研究

以单雌蜜露排泄量和体重增量作为致害性指标,研究了稻褐飞虱Nilaparvata lugens (Stål)的致害性遗传与性别的关联性。褐飞虱弱致害种群（TN1种群）与强致害种群（Mudgo种群或ASD7种群）（已分别用水稻品种TN1、Mudgo和ASD7强迫饲养82代）在抗虫品种水稻上的致害力差异明显,适宜作为遗传分析的亲本材料。当用蜜露排泄量作为致害性指标时,两种处理（TN1种群与Mudgo种群,TN1种群与ASD7种群）正、反杂交的F₁代雌成虫平均蜜露排泄量存在极显著差异,由此可以推断正交F₁代群体与反交F₁代群体致害性存在明显差别。用体重增量作为致害性指标进行实验,得到的结论与上述研究完全一致。以上的结果可推测稻褐飞虱致害性可能属于伴性遗传。     

The Genetic Basis of Malathion Resistance in Housefly (Musca domestica L.) Strains From Turkey

Organophosphate insecticide (parathion/diazinon) resistance in housefly (Musca domestica L.) is associated with the change in carboxylesterase activity. The product of MdE7 gene is probably playing a role in detoxification of xenobiotic esters. In our research, we have isolated, cloned and sequenced the MdE7 gene from five different Turkish housefly strains. High doses of malathion (600 g/fly) were applied in a laboratory environment for one year to Ceyhan1, Ceyhan2, Adana, and Ankara strains while no insecticide treatment was performed in the laboratory to Kirazli strain. Trp²⁵¹ Ser substitution was found in the product of MdE7 gene in all malathion-resistant and Kirazli stocks. In addition, we checked the malathion carboxylesterase (MCE), percent remaining activities in acetylcholinesterase (AChE), glutathion-S-transferase (GST), and general esterase activities in all five strains used in this study. In comparing with universal standard sensitive control WHO, a high level of MCE and GST activities were observed while lower level of general esterase activities was detected in the tested strains. In addition, a higher percent remaining activities in AChE than WHO susceptible strain were observed in all malathion-resistant strains.     

Alterations in esterases are associated with malathion resistance in Habrobracon hebetor (Hymenoptera: Braconidae)

Biochemical mechanisms of malathion resistance were investigated in a malathion-resistant strain of the parasitoid Habrobracon hebetor Say collected from a farm storage in Kansas. General esterase activities were significantly lower in the resistant strain compared with those in a susceptible strain. However, no significant differences were found in activities of malathion specific carboxylesterase (MCE), glutathione S-transferase and cytochrome P450 dependent O-demethylase activities, cytochrome P450 contents, and sensitivity of acetylcholinesterase to inhibition by malaoxon between the 2 strains. Because MCE was not elevated in the resistant strain, the weak malathion resistance in H. hebetor may result from a different mechanism compared with that hypothesized for some insect species in which reduced general esterase activity is accompanied by an elevated MCE. Decreased esterase activity in the resistant strain suggested that null alleles of some esterases were associated with the resistance. Indeed, E1 and E2, major esterases in the susceptible strain, were not present in the resistant strain on polyacrylamide gels that were stained for esterase activity using the model substrate 1-naphthyl acetate. In contrast, the activity of esterase E3 on the gels was much higher in the resistant strain as compared with that of the susceptible strain. These findings indicate that malathion resistance in H. hebetor is associated with both an increased activity of the esterase E3 and null alleles of the esterases E1 and E2.     

Bt rice expressing Cry1Ab does not stimulate an outbreak of its non-target herbivore, Nilaparvata lugens

In this study, the non-target effects of Bt rice “KMD2” expressing a Cry1Ab protein on the performance of the brown planthopper (BPH), Nilaparvata lugens, over multiple generations were evaluated under laboratory and field conditions. In the laboratory, BPH was reared to observe the impact of the Bt rice as compared to its parental non-Bt cultivar Xiushui 11, while the population dynamics and oviposition performance of BPH were investigated in the field. The survival of BPH nymphs fed Bt and non-Bt rice did not differ significantly. The nymph developmental duration of BPH was significantly delayed by the Bt rice by comparison with the non-Bt rice for the 1st and 2nd but not the 4th generation. Most importantly, the fecundity of BPH on the Bt rice was significantly decreased in every generation when compared with the non-Bt rice. In the field investigations, the population density of BPH nymphs was significantly lower in the Bt rice field. However, the temporal pattern of population dynamics of BPH adults was similar between the Bt and non-Bt rice, presumably due to migratory interference of the adults. In the Bt rice field, the percentage of tillers with eggs and the number of eggs per tiller were also significantly lower from tillering to mature stage. Additionally, Cry1Ab protein could not be detected in guts from single BPH adults. In general, our results suggest that the Bt rice “KMD2” could not stimulate an outbreak of BPH.     

Insecticide resistance and detoxifying enzyme activity in the principal bancroftian filariasis vector, Culex quinquefasciatus, in northeastern India

The insecticide resistance status of Culex quinquefasciatus Say (Diptera: Culicidae) to DDT and deltamethrin across army cantonments and neighbouring villages in northeastern India was investigated. In India, DDT is still the insecticide of choice for public health programmes. In military stations, pyrethroids, especially deltamethrins, are used for insecticide‐treated nets (ITNs). Recent information on the levels of resistance to DDT and deltamethrin in mosquito populations of northeastern India is scare. Continued monitoring of insecticide resistance status, identification of the underlying mechanisms of resistance in local mosquito populations and the establishment of a baseline data bank of this information are of prime importance. Insecticide susceptibility assays were performed on wild‐caught adult female Cx. quinquefasciatus mosquitoes to the discriminating doses recommended by the World Health Organisation (WHO) to DDT (4%) and deltamethrin (0.05%). Across all study sites, mortality as a result of DDT varied from 11.9 to 50.0%, as compared with 91.2% in the susceptible laboratory strain (S‐Lab), indicating that Cx. quinquefasciatus is resistant to DDT. The species was found to be 100% susceptible to deltamethrin in all study sites except Benganajuli and Rikamari. Knock‐down times (KDT) in response to deltamethrin varied significantly between study sites (P < 0.01) from 8.3 to 17.8 min for KDT₅₀ and 37.4 to 69.5 min for KDT₉₀. All populations exceeded the threshold level of alpha‐esterase, beta‐esterase and glutathion S‐transferase (GST) established for the S‐Lab susceptible strain, and all populations had 100% elevated esterase and GST activity, except Missamari and Solmara. Beta‐esterase activity in Field Unit II (96.9%) was less than in any of the other populations. Benganajuli had the highest activity level for all the enzymes tested. There was a significant correlation between all enzyme activity levels and insecticide resistance phenotype by populations (P < 0.05). The results presented here provide the first report and baseline information of the insecticide resistance status of Cx. quinquefasciatus in northeastern India, and associated information about biochemical mechanisms that are essential for monitoring the development of insecticide resistance in the area.     

Insecticide resistance and malathion carboxylesterase in the sheep blowfly,Lucilia cuprina

Resistance to the organophosphorus insecticide malathion in genetically related strains of the Australian sheep blowflyLucilia curprina was examined. Separate lines of blowflies were established by homozygosis of the fourth chromosome of the parental RM strain. Both the RM and the derived resistant (der-R) strains are approximately 100 times more resistant to malathion than the related susceptible der-S strain, resistance being correlated with a 45- to 50-fold increase in a malathion carboxylesterase (MCE) activity. MCE has a pH optimum ranging between 6.6 and 8.0 and is strongly inhibited by the carboxylesterase inhibitors triphenyl phosphate, paraoxon, and diiospropylfluorophosphate. Subcellular fractionation revealed that MCE was localized predominantly to the cytosol and mitochondria in both resistant and susceptible blowflies. A single MCE was purified to homogeneity from RM blowflies. It has a pI of 5.5, is a monomer of 60.5 kDa, and hydrolyzes malathion with aV _max of 755 nmol/min/mg protein and aK _m of 11.0 µM. L. cuprina have thus evolved a remarkable MCE which is faster and more efficient at hydrolyzing a specific insecticide than any other insect esterase yet described.