Development of permethrin resistance in Culex quinquefasciatus Say in Kuala Lumpur,Malaysia

The resistance status towards permethrin among the laboratory strain, the permethrin-selected strain and four field strains of Culex quinquefasciatus collected in Kuala Lumpur, Malaysia was determined using three standard laboratory methods: WHO larval bioassay, WHO adult bioassay and biochemical microplate assay. Cx. quinquefasciatus permethrin-selected strain larvae were the least susceptible to permethrin with a resistance ratio of 47.28-folds, whereas all field strain larvae of the same species were tolerant to permethrin with resistance ratios of more than 3-folds. In contrast, in adult stage, the permethrin exposed permethrin-selected strain (resistance ratio = 1.27) was found to be more susceptible to permethrin than all permethrin-exposed field strains (resistance ratios = 2.23–2.48). Complete mortalities for all strains of Cx. quinquefasciatus adults proved the effectiveness of the synergist; piperonyl butoxide (PBO). For the biochemical microplate assay, the reduction of the mean optical density of elevated oxidase activity of three field strains upon exposure to PBO confirmed the association between oxidase activity and permethrin tolerance. On the other hand, irregular patterns of the mean optical density of elevated oxidase activity in the laboratory strain, permethrin-selected strain and Jalan Fletcher strain illustrated the gene variation within these mosquito colonies as well as the involvement of other enzyme activities in the permethrin resistance occurred.     

Correlation between fenvalerate resistance and cytochrome P450-mediated O-demethylation activity in Helicoverpa armigera (Lepidoptera: Noctuidae)

Cytochrome P450 monooxygenases are a major metabolic mechanism responsible for pyrethroid resistance in Helicoverpa armigera (Hübner) from Asia. Cytochrome P450-mediated O-demethylation activity toward p-nitroanisole (PNOD) of individual fourth instars was determined in five strains of H. armigera by using a microplate reader. The four resistant strains of YS, HD, YGF, and YG59 had 6-, 71-, 2540-, and 11,800-fold resistance, respectively, to fenvalerate in comparison with the susceptible BK77 strain. Their mean PNOD activity was 4-, 10-, 24-, and 60-fold, respectively, compared with the BK77 strain. A strong positive correlation (correlation coefficient r = 0.98) between PNOD activity and fenvalerate resistance was found. Of 48 larvae from each strain, only 4% larvae of the susceptible BK77 strain had detectable PNOD activity, whereas 25, 33, 79, and 96% of larvae from the resistant strains YS, HD, YGF, and YG59 exhibited PNOD activity, respectively. There was a clear discrimination of patterns of PNOD frequency distribution between H. armigera strains and their magnitudes of fenvalerate resistance. The PNOD activity can be used as a biochemical marker for monooxygenase-mediated pyrethroid resistance in field populations of H. armigera.     

High-Level Field Resistance to Bacillus sphaericus C3-41 in Culex quinquefasciatus from Southern China

A flowable mosquito-larvicidal formulation of Bacillus sphaericus strain C3-41 has been continuously used for 8 years to control Culex quinquefasciatus larvae in Dongguan, Guangdong Province, China. This formulation had high efficacy against the target larvae during the first 6 years of treatment. However, under this high selection pressure, C. quinquefasciatus showed a significant level of resistance to C3-41 from years seven to eight. The resistance ratio of field-collected larvae at LC 50 was calculated to be 22 672-fold in comparison with the susceptible laboratory colony. Interestingly, no cross-resistance was observed to B. sphaericus strain LP1-G which had the same toxicity against both susceptible and resistant larvae, and B. thuringiensis subsp. israelensis was found to be more active to the latter than the former. After six months treatment with the B. thuringiensis subsp. israelensis formulation in the B. sphaericus resistant population area, the mosquito population recovered its susceptibility to B. sphaericus C3-41, with the resistance ratio of field-collected larvae dropping from 22 672- to 5.67-fold.     

Analysis of midgut proteinases from Bacillus thuringiensis-susceptible and -resistant Heliothis virescens (Lepidoptera: Noctuidae)

Insects with altered proteinases can avoid intoxication by Bacillus thuringiensis (Bt) toxins. Therefore, proteinase activities from gut extracts of Bt-susceptible (YDK) and -resistant (YHD2-B, CXC and KCBhyb) Heliothis virescens strains were compared. The overall pH of gut extracts from YDK and CXC were statistically similar (9.56 and 9.62, respectively), while the pH of extracts from KCBhyb and YHD2-B were significantly more alkaline (9.81 and 10.0, respectively). Gut extracts from YHD2-B and CXC larvae processed Cry1Ac and Cry2Aa protoxin slower than extracts from YDK larvae, suggesting that differences in proteolysis contribute to resistance in these strains. Casein zymogram analysis of gut extracts revealed both qualitative and quantitative differences in caseinolytic activities among all strains, but the overall caseinolytic activity of YHD2-B gut extract was lower. Kinetic microplate assays with a trypsin substrate (l-BApNA) demonstrated that proteinases in YDK gut extract had increased alkaline pH optima compared to resistant strains YHD2-B, CXC and KCBhyb. Gut extracts from YHD2-B had reduced trypsin-like activity, and activity blots indicated that YHD2-B had lost a trypsin-like proteinase activity. In assays with a chymotrypsin substrate (SAAPFpNA), enzymes from all Bt-resistant strains had increased pH optima, especially those from KCBhyb. Activity blots indicated that CXC had lost a chymotrypsin-like proteinase activity. Because serine proteinases are a critical component of Bt toxin mode of action, these differences may contribute to decreased toxicity in the Bt-resistant strains.     

A strain of Bacillus sphaericus causes slower development of resistance in Culex quinquefasciatus

Two field-collected Culex quinquefasciatus colonies were subjected to selection pressure by three strains of Bacillus sphaericus, C3-41, 2362, and IAB59, under laboratory conditions. After 13 and 18 generations of exposure to high concentrations of C3-41 and IAB59, a field-collected low-level-resistant colony developed >144,000- and 46.3-fold resistance to strains C3-41 and IAB59, respectively. A field-collected susceptible colony was selected with 2362 and IAB59 for 46 and 12 generations and attained >162,000- and 5.7-fold resistance to the two agents, respectively. The pattern of resistance evolution in mosquitoes depended on continuous selection pressure, and the stronger the selection pressure, the more quickly resistance developed. The resistant colonies obtained after selection with B. sphaericus C3-41 and 2362 showed very high levels of cross-resistance to B. sphaericus 2362 and C3-41, respectively, but they displayed only low-level cross-resistance to IAB59. On the other hand, the IAB59-selected colonies had high cross-resistance to both strains C3-41 and 2362. Additionally, the slower evolution of resistance against strain IAB59 may be explained by the presence of another larvicidal factor. This is in agreement with the nontoxicity of the cloned and purified binary toxin (Bin1) of IAB59 for 2362-resistant larvae. We also verified that all the B. sphaericus-selected colonies showed no cross-resistance to Bacillus thuringiensis subsp. israelensis, suggesting that it would be a promising alternative in managing resistance to B. sphaericus in C. quinquefasciatus larvae.     

Dermacentor variabilis: resistance to ticks acquired by mast cell-deficient and other strains of mice

The acquisition of resistance to ticks was monitored in mice of six different strains. Mice were subjected to repeated infestations with Dermacentor variabilis larvae, different skin sites being used for each successive infestation. In the third and fourth infestations, resistance was expressed in three strains of mice (WBB6F1-W/Wv, WBB6F1-+/+, and CFW), as demonstrated by significant reductions in percentages of larvae engorging and in mean weights of fed larvae. Both WBB6F1-W/Wv mice, which are mast cell-sufficient strain attained significantly higher levels of resistance. It is suggested that mast cells may play a relatively minor role in the mechanisms of resistance in this strain of mice. C57B1 mice also expressed tick resistance in their third and fourth infestations as measured by reduced percentages of engorged larvae, but not by reduced mean larval weights. Possibly, the mechanisms of tick resistance in this strain differ from those in other strains. Two other mouse strains (C3H-HeJ and C3H-HeSn) remained relatively susceptible to tick feeding throughout five infestations. In secondary infestations of all strains tested, no resistance was evident. Instead, enhanced feeding of larvae appeared to occur. A new objective measurement of tick resistance is the mean weights of detached, unengorged larvae taken from resistant animals at the end of the infestation period. These were found to be consistently less than those from susceptible animals.     

寄主植物对斜纹夜蛾酯酶和乙酰胆碱酯酶活性的影响

利用生物测定与生物化学的方法,就取食不同寄主植物的2个斜纹夜蛾[Spodoptera litura(Fab.)]品系对丙溴磷和灭多威的敏感性及其体内的酯酶与乙酰胆碱酯酶的活性变化进行了研究。结果表明,取食不同寄主植物的斜纹夜蛾对丙溴磷和灭多威的敏感性不同。在敏感品系中,取食不同食料的斜纹夜蛾后代对灭多威的敏感性顺序为:烟草<棉花<大豆<人工饲料;对丙溴磷而言,敏感性顺序则为:棉花<烟草<大豆<人工饲料。而在田间抗性品系中,对丙溴磷的敏感性顺序为:棉花<烟草<人工饲料<大豆;对灭多威的敏感性顺序为:棉花<烟草<大豆<人工饲料。此外,在田间抗性和敏感品系中,取食不同食料的斜纹夜蛾体内的酯酶活性之间均存在显著差异,对其乙酰胆碱酯酶的活性也有程度不同的诱导效应,但并不会引起乙酰胆碱酯酶的质变。     

Enzymatic diagnosis of resistance to deltamethrin in diapausing larvae of the codling moth,Cydia pomonella (L.)

The resistance to deltamethrin was evaluated in diapausing larvae of 14 field populations of the codling moth (Cydia pomonella L.) From the main French orchard areas using biological assays. Resistance to deltamethrin was compared to mixed-function-oxidase (mfo) activity measured at the individual level through ethoxycoumarin-O-deethylase (ECOD) activity using a fluorescence microplate reader. The larvae were collected in corrugated paper band traps in the autumn of 1995. Analysis was previously performed on two laboratory strains, one susceptible and one resistant to deltamethrin, in order to characterize the changes in resistance during diapause development. Resistance to deltamethrin as well as the ECOD activity were stable during the diapause, and ECOD activity was always significantly lower in the susceptible strain than in the resistant one. The ECOD activity was significantly correlated to the frequency of resistant moths in the field populations. This strongly suggested the involvement of the mfo system in the resistance to deltamethrin in these populations. The intra-strain variabilities in ECOD activity of both laboratory and field resistant insects indicated that other resistance mechanisms might also be involved. Further investigations on these mechanisms are needed in order to develop complete diagnostic methods and to define suitable control strategies against each resistant population. Arch. Insect Biochem. Physiol. 39:55–64, 1998. © 1998 Wiley-Liss, Inc.     

A point mutation in the acetylcholinesterase-1 gene is associated with chlorpyrifos resistance in the plant bug Apolygus lucorum

Control of Chinese Apolygus lucorum relies heavily on organophosphate insecticides. Here we describe resistance to the organophosphate chlorpyrifos in an A. lucorum strain, BZ-R, which was developed from a field-collected strain (BZ) by selection with chlorpyrifos in the laboratory. BZ-R showed 21–58 fold resistance to chlorpyrifos compared with the laboratory reference strain LSF and another susceptible strain, BZ-S, derived from BZ. BZ-R also showed several fold resistance to two other organophosphates and a carbamate. No synergism of chlorpyrifos by metabolic enzyme inhibitors nor any increase in detoxifying enzyme activities were observed in BZ-R. No sequence differences in acetylcholinesterase-2 were found to be associated with the resistance but the frequency of an alanine to serine substitution at position 216 of acetylcholinesterase-1 was 100% in BZ-R, ∼21–23% in SLF and BZ, and 0% in BZ-S. A single generation treatment of chlorpyrifos on the BZ strain also increased its frequency of the serine substitution to 64%. Recombinantly expressed acetylcholinesterase-1 carrying the serine substitution was about five fold less sensitive to inhibition by chlorpyrifos oxon than the wild-type enzyme. Quantitative real-time PCR found no differences in ace1 or ace2 expression levels among the strains tested. Thus the chlorpyrifos resistance is strongly associated with the serine substituted acetylcholinesterase-1. An equivalent substitution has been found to confer resistance to many organophosphate and carbamate insecticides in four other insect species.     

抗有机磷三带喙库蚊的酯酶研究

本文主要利用聚丙烯酰胺凝胶电泳及离体酶学技术研究抗有机磷三带喙库蚊(Culex tritaeniorhynchus Giles)与各种水解酶系的关系.结果表明:三带喙库蚊抗性品系的羧酸酯酶活性明显高于敏感品系.此外,酯酶同功酶谱显示抗性品系有一染色很深的高活性羧酸酯酶带E₄,而胆碱酸酶活性测定结果表明抗性与敏感品系无明显差异.因此本实验证明,该蚊对有机磷杀虫剂之所以产生高度抗性.其主要原因与羧酸酯酶的活力增加有关.这和我们生物测定结果完全一致.     

Altered Glycosylation of 63- and 68-kilodalton microvillar proteins in Heliothis virescens correlates with reduced Cry1 toxin binding,decreased pore formation,and increased resistance to Bacillus thuringiensis Cry1 toxins

The binding and pore formation abilities of Cry1A and Cry1Fa Bacillus thuringiensis toxins were analyzed by using brush border membrane vesicles (BBMV) prepared from sensitive (YDK) and resistant (YHD2) strains of Heliothis virescens. 125I-labeled Cry1Aa, Cry1Ab, and Cry1Ac toxins did not bind to BBMV from the resistant YHD2 strain, while specific binding to sensitive YDK vesicles was observed. Binding assays revealed a reduction in Cry1Fa binding to BBMV from resistant larvae compared to Cry1Fa binding to BBMV from sensitive larvae. In agreement with this reduction in binding, neither Cry1A nor Cry1Fa toxin altered the permeability of membrane vesicles from resistant larvae, as measured by a light-scattering assay. Ligand blotting experiments performed with BBMV and 125I-Cry1Ac did not differentiate sensitive larvae from resistant larvae. Iodination of BBMV surface proteins suggested that putative toxin-binding proteins were exposed on the surface of the BBMV from resistant insects. BBMV protein blots probed with the N-acetylgalactosamine-specific lectin soybean agglutinin (SBA) revealed altered glycosylation of 63- and 68-kDa glycoproteins but not altered glycosylation of known Cry1 toxin-binding proteins in YHD2 BBMV. The F1 progeny of crosses between sensitive and resistant insects were similar to the sensitive strain when they were tested by toxin-binding assays, light-scattering assays, and lectin blotting with SBA. These results are evidence that a dramatic reduction in toxin binding is responsible for the increased resistance and cross-resistance to Cry1 toxins observed in the YHD2 strain of H. virescens and that this trait correlates with altered glycosylation of specific brush border membrane glycoproteins.     

Genetic variation of trypsin and chymotrypsin inhibitors in pigeonpea [Cajanus cajan (L.) Millsp.] and its wild relatives

Variation in the trypsin inhibitors (TIs) and the chymotrypsin inhibitors (CIs) among 69 pigeonpea [Cajanus cajan (L.) Millsp.] strains from a wide geographical distribution and among 17 accessions representing seven wild Cajanus species was studied by electrophoretic banding pattern comparisons and by spectrophotometric activity assays. The TI and CI electrophoretic migration patterns among the pigeonpea strains were highly uniform but varied in the inhibitor band intensities. The migration patterns of the inhibitors in the wild Cajanus species were highly species specific. The mean TI activity of pigeonpea strains (2279 units) was significantly higher than that of the wild Cajanus species (1407 units). However, the mean CI activity in the pigeonpea strains (62 units) was much lower than that in the wild species (162 units). Kenya 2 and ICP 9151 were the lowest and the highest, respectively, in both the TI and CI activities among all the pigeonpea strains used in this study. A highly-significant positive correlation was observed between the TI and CI activities. The Bowman-Birk type inhibitors with both TI and CI activities were identified in all the pigeonpea strains and also in the accessions of all the wild species except C. volubilis (Blanco) Blanco. The C. volubilis accession ICPW 169 was found to be null for both CI bands and CI activity. Environment, strain, and environment x strain interaction showed highly-significant effects on both the TI and CI activities. Growing the pigeonpea strains at a different environment from their area of adaptation increased TI and CI activities and also altered the maturity period.     

Microplate assay of glutathione s-transferase activity for resistance detection in single-mosquito triturates

1. Optimum conditions are described for a simple, rapid microplate assay that measures glutathione s-transferase (GST) activity accurately and precisely in small portions of single mosquito homogenates. 2. Up to 10 assay replicates were possible for individual adults and larvae. Concentration of GST activity in the head/thorax region allows blood-fed mosquitoes with abdomens removed to be used in assays. 3. The method allows the use of GST activity as a biochemical character in comparative studies of populations. 4. The microplate assay detects elevated GST activities associated with DDT resistance in Anopheles arabiensis.     

Metabolic activities of metronidazole-sensitive and -resistant strains of Helicobacter pylori: repression of pyruvate oxidoreductase and expression of isocitrate lyase activity correlate with resistance.

In this study, we compared metronidazole (Mtz)-sensitive and -resistant strains of Helicobacter pylori for metabolic differences that might correlate with drug resistance. Included in this study was an isogenic Mtz(r) strain, HP1107, that was constructed by transforming genomic DNA from Mtz(r) strain HP439 into Mtz(s) strain HP500. Enzyme activities were also measured for Mtz(r) strains grown in the presence or absence of 18 micrograms of metronidazole per ml (ca. one-half of the MIC). These studies confirmed the presence of the Embden-Meyerhof-Parnas, Entner-Doudoroff, and pentose pathways. H. pylori strains expressed enzymatic activities indicative of a complete and active Krebs cycle. All strains expressed pyruvate oxidoreductase (POR) and alpha-ketoglutarate oxidoreductase (KOR) as measured with the redox-active dye benzyl viologen (30 to 96 nmol/min/mg of protein for POR and 30 nmol/min/mg of protein for KOR). When grown in the presence of Mtz at > or = 3.5 micrograms/ml, Mtz(r) strains expressed no detectable POR or KOR activity. The apparent repression of POR and KOR activities by Mtz affected bacterial growth as manifest by extended lag periods and growth yield reductions of > 30%. A dose-dependent relationship was demonstrated between the metronidazole concentration in the growth medium and the specific activity of POR measured in bacterial cell extracts. The observed repression was not due to inactivation of POR by Mtz. In addition to repression of POR and KOR activities, growth in the presence of Mtz also led to decreases in the activities of various Krebs cycle enzymes, including aconitase, isocitrate dehydrogenase and succinate dehydrogenase. All of the Mtz(r) strains examined expressed isocitrate lyase and malate synthase activities indicative of the glyoxylate bypass. No isocitrate lyase activity was detected in Mtz(s) strain HP500. Isocitrate lyase activity was expressed by HP500 following transformation to Mtz resistance (Mtz(r) strain HP1107) with DNA from an Mtz(r) strain. The results of this study suggest that Mtz resistance may be a recessive trait, possibly involving inactivation of a regulatory gene, that results in constitutive expression of isocitrate lyase. Repression of POR and KOR activities in response to low levels of Mtz may be a general response of H. pylori strains to Mtz, but only resistant strains manage to survive via activation of compensatory metabolic pathways.     

A Strain of Bacillus sphaericus Causes Slower Development of Resistance in Culex quinquefasciatus

Two field-collected Culex quinquefasciatus colonies were subjected to selection pressure by three strains of Bacillus sphaericus, C3-41, 2362, and IAB59, under laboratory conditions. After 13 and 18 generations of exposure to high concentrations of C3-41 and IAB59, a field-collected low-level-resistant colony developed >144,000- and 46.3-fold resistance to strains C3-41 and IAB59, respectively. A field-collected susceptible colony was selected with 2362 and IAB59 for 46 and 12 generations and attained >162,000- and 5.7-fold resistance to the two agents, respectively. The pattern of resistance evolution in mosquitoes depended on continuous selection pressure, and the stronger the selection pressure, the more quickly resistance developed. The resistant colonies obtained after selection with B. sphaericus C3-41 and 2362 showed very high levels of cross-resistance to B. sphaericus 2362 and C3-41, respectively, but they displayed only low-level cross-resistance to IAB59. On the other hand, the IAB59-selected colonies had high cross-resistance to both strains C3-41 and 2362. Additionally, the slower evolution of resistance against strain IAB59 may be explained by the presence of another larvicidal factor. This is in agreement with the nontoxicity of the cloned and purified binary toxin (Bin1) of IAB59 for 2362-resistant larvae. We also verified that all the B. sphaericus-selected colonies showed no cross-resistance to Bacillus thuringiensis subsp. israelensis, suggesting that it would be a promising alternative in managing resistance to B. sphaericus in C. quinquefasciatus larvae.     

Pyrethroid and DDT cross-resistance in Aedes aegypti is correlated with novel mutations in the voltage-gated sodium channel gene

Samples of the dengue vector mosquito Aedes aegypti (L.) (Diptera: Culicidae) were collected from 13 localities between 1995 and 1998. Two laboratory strains, Bora (French Polynesia) and AEAE, were both susceptible to DDT and permethrin; all other strains, except Larentuka (Indonesia) and Bouaké (Ivory Coast), contained individual fourth-instar larvae resistant to permethrin. Ten strains were subjected to a range of biochemical assays. Many strains had elevated carboxylesterase activity compared to the Bora strain; this was particularly high in the Indonesian strains Salatiga and Semarang, and in the Guyane strain (Cayenne). Monooxygenase levels were increased in the Salatiga and Paea (Polynesia) strains, and reduced in the two Thai strains (Mae Kaza, Mae Kud) and the Larentuka strain. Glutathione S-transferase activity was elevated in the Guyane strain. All other enzyme profiles were similar to the susceptible strain. The presence of both DDT and pyrethroid resistance in the Semarang, Belem (Brazil) and Long Hoa (Vietnam) strains suggested the presence of a knock-down resistant (kdr)-type resistance mechanism. Part of the S6 hydrophobic segment of domain II of the voltage-gated sodium channel gene was obtained by RT-PCR and sequenced from several insects from all 13 field strains. Four novel mutations were identified. Three strains contained identical amino acid substitutions at two positions, two strains shared a different substitution, and one strain was homozygous for a fourth alteration. The leucine to phenylalanine substitution that confers nerve insensitivity to pyrethroids in a range of other resistant insects was absent. Direct neurophysiological assays on individual larvae from three strains with these mutations demonstrated reduced nerve sensitivity to permethrin or lambda cyhalothrin inhibition compared to the susceptible strains.     

Toxicity,inheritance and biochemistry of clofentezine resistance in Tetranychus urticae

Abstract This study evaluates the toxicological and biochemical response of two‐spotted spider mites to clofentezine selection pressure. The mortality rate of Tetranychus urticae in adult females depends on increased clofentezine concentration and clofentezine was found to be effective against females. The resistance rate of the CUM strain selected 12 times once per generation with clofentezine was increased from 1.28‐ to 105.27‐fold. The interaction of some synergists with clofentezine was analyzed in the clofentezine‐resistant CLOF 12 strain. Synergists had no effect on clofentezine toxicity. The clofentezine‐resistant CLOF 12 strain showed resistance against chlorpyrifos, abamectin, propargite, fenpyroximate and amitraz. The modes of inheritance of resistance to clofentezine were found to be incompletely dominant and not sex‐linked. Esterase enzyme activity was detected both by gel electrophoresis and microplate reader methods, while glutathione S‐transferase (GST) and monooxygenase (P450) activity were detected only by the microplate reader method. During the selection period the esterase, the GST and the P450 enzymes activities were raised from 7.69, 7.09 and 0.003 3 to 18.40, 13.11 and 0.003 7 milli‐optical density/min/mg proteins, respectively. An increase was observed in the band intensity of esterases and esterase enzymes may play a role in clofentezine resistance in T. urticae.     

Histopathological Effects and Growth Reduction in a Susceptible and a Resistant Strain of Heliothis virescens (Lepidoptera: Noctuidae) Caused by Sublethal Doses of Pure Cry1A Crystal Proteins from Bacillus thuringiensis

Two strains of the tobacco budworm Heliothis virescens, one selected in the laboratory for resistance to Cry1Ac crystal protein from Bacillus thuringiensis (for which the mechanism of resistance was not associated with reduced binding) and an unselected control strain, were exposed to sublethal doses of pure Cry1A crystal proteins. A histopathological study was conducted to determine the epithelial damage caused by ingestion of Cry1Ac. Tissue sections of the midgut were obtained after 20, 40 and 60 min of toxin ingestion. Histopathological changes were observed primarily in columnar cells and were time-dependent. However, essentially the same level of damage was observed in the two strains. Toxin feeding tests with Cry1Ac and Cry1Ab, indicated that the toxins retarded growth and inhibited food intake of susceptible larvae, but did not significantly affect larvae of the resistant strain. Since the histopathological damage was similar in both strains, it appears that resistant larvae could repair (or substitute) more readily the damaged cells.