The effectiveness of organophosphate acaricides on stored product mites interacting in biological control

Three organophosphates (pirimiphos-methyl, chlorpyrifos-methyl, chlorpyrifos) were tested on a laboratory strain of Cheyletus eruditus (Schrank), a predatory mite used for biological control of stored food mites, and on tow species of acaroid mites, Acarus siro L. and Tyrophagus putrescentiae (Schrank). Biological control is often preceded by a chemical treatment with organophosphates and thus it is important to know how the acaricides affect the predators. It was found that chlorpyrifos-methyl was the most toxic organophosphate on C. eruditus. The effectiveness of pirimiphos-methyl and chlorpyrifos was approximately equal and was three times lower than the effectiveness of chlorpyrifos-methyl. The organophosphates were nearly equally effective on both acaroid mites but A. siro was slightly more susceptible than T. putrescentiae. On the basis of these results, the use of pirimiphos-methyl or chlorpyrifos rather than chlorpyrifos-methyl is recommended for protection of empty stores or stored grain against resiliant populations of stored food mites.     

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.     

Toxicity of organophosphorus insecticides to diazinon-reversely-selected,pesticide-pressure-relaxed and field-resistant strains of bulb miteRhizoglyphus robini (Acari: Acaridae)

Some field strains ofRhizoglyphus robini in Taiwan, developed high multi-resistance to organophosphorus insecticides including diazinon, dimethoate, fenitrothion, malathion, methyl parathion, omethoate, parathion and prothiophos. The organophosphorus resistance of bulb mites remained stable when the pesticide pressure was relaxed. Changes of the susceptibility to several organophosphorus insecticides for a strain after relaxation of selection pressure for 3 and 27 generations were compared. Since susceptible bulb mites could not be found in the field, a reverse selection for susceptibility to diazinon was designed and conducted. Although heterogeneity was observed in the progeny of the mixed populations, the susceptibility of these reversely-selected mites increased slowly. A stable and susceptible strain was obtained after 26 generations of selection.Contribution No. 1412 from Taiwan Agricultural Research Institute.     

Resistance to binapacryl and tetradifon, and the genetic background, in fruit tree red spider mite, Panonychus ulmi, from English apple orchards

Assays on eggs of Panonychus ulmi held on leaf discs of Myrobalan plum were used to characterise strains (1) multi-resistant to organophosphates (OPs) and binapacryl (strain BR), (2) multi-resistant to OPs and tetradifon (strain TR), and (3) susceptible to OPs, binapacryl and tetradifon (strain EM). By crosses of EM females and BR males, and of EM females and TR males, and by repeated backcrossing with EM males under selection pressure of the acaricides concerned, two strains were produced in which the specific genes for resistance to binapacryl and tetradifon were isolated in the genome of the EM strain. Reciprocal crosses of the resulting backcrossed strains with EM, assays on the F₁ eggs and the F₂ haploid eggs, confirmed the presence of single major genes for resistance to binapacryl and tetradifon, and the absence of maternal effects. The expression of gene BR was incompletely dominant in response to binapacryl, whilst gene TR was effectively dominant in response to tetradifon. Eggs of the field-derived strain BR showed c. 50× resistance to binapacryl, 8–19 × to dinobuton, but only c. 2× to dinocap and dinoseb. The backcrossed strain was less resistant (26 ×) to binapacryl implying that resistance in strain BR was augmented by the effects of minor genes. Resistance to tetradifon (150–200 × on eggs) involved cross-resistance to tetrasul (23 ×) and to chlorfensulphide (27×); resistance in the field-derived strain appeared to depend on the major gene alone. Resistance mechanisms were highly specific in their effects, so that there was no cross-resistance to unrelated acaricides, including amitraz, dicofol, cyhexatin, formetanate, chlordimeform, and quinomethionate. Fecundity of strain BR was 44% less than that of EM; laboratory and field evidence suggest that multi-resistant mites tend to be less fit than susceptible mites. The practical implications are discussed.     

OBSERVATIONS ON THE CONTROL OF MITE INFESTATIONS IN STORED WHEAT BY CHEYLETUS SPP. (ACARINA, CHEYLETIDAE)

An account is given of the control of the grain mite Acarus siro L. (= Tyroglyphus farinae L.) by the predatory mite Cheyletus sp. in imported wheat stored in bags or in bulk. After 12–18 months of storage Cheyletus sp. had developed in 75% of the parcels of wheat and in 67% of these it became dominant over other mites. This usually occurred in late summer for bagged wheat. Control of A. siro by Cheyletus sp. compared favourably with that achieved by fumigation with methyl bromide and on occasion was superior. Both development and dominance of Cheyletus sp. were adversely affected by earlier fumigation but there was some evidence that spraying wheat with γ-BHC favoured the development of this species and, in some circumstances, its dominance. Dominance by Cheyletus sp. in bulk grain often occurred in the winter months when the surface moisture content of the wheat was at its highest.
The development of Cheyletus eruditus Schr. is described in a bulk grain store where dense populations of A. siro and Glycyphagus destructor Schr. were eliminated by the predator. The numbers of C. eruditus in wheat tended to increase faster when the surface layers were disturbed than when the grain was undisturbed. Nevertheless, dominance over the other mites tended to occur first in undisturbed grain.     

European monitoring of resistance to insecticides in Myzus persicae and Aphis gossypii (Hemiptera: Aphididae) with special reference to imidacloprid

The susceptibility to several insecticides of 16 and 8 strains of Myzus persicae Sulzer and Aphis gossypii Glover, respectively, received from different European countries in 2001 was investigated. Most of the strains were derived from places known for their aphid resistance problems to conventional insecticides before imidacloprid was introduced. In many regions and agronomic cropping systems imidacloprid has been an essential part of aphid control strategies for a decade, and therefore the susceptibility of aphid populations to imidacloprid using FAO-dip tests and diagnostic concentrations in a leaf-dip bioassay was checked. Additional insecticides tested were cyfluthrin (chemical class: pyrethroid), pirimicarb (carbamate), methamidophos and oxydemeton-methyl (organophosphates). Diagnostic concentrations (LC99-values of reference strains) for each insecticide were established by dose response analysis using a new leaf-disc dip bioassay format in 6-well tissue culture plates. Virtually no resistance to imidacloprid in any of the field-derived populations of M. persicae and A. gossypii was detected. In contrast, strong resistance was found to pirimicarb and oxydemeton-methyl, and to a lesser extent also to cyfluthrin. Two strains of A. gossypii exhibited reduced susceptibility to imidacloprid when tested directly after collection. However, after maintaining them for six weeks in the laboratory, the aphids were as susceptible as the reference strain. The diagnostic concentration of methamidophos did not reveal any resistance in M. persicae, but did so in four strains of A. gossypii.     

安徽省城市居民储藏物中孳生粉螨群落组成及多样性研究

从安徽省15个城市居民储藏物中采集了48种样本,采用水膜镜检法和电热集螨法进行粉螨的分离,并进行螨种的鉴定及计数。检获粉螨27种,隶属于7科19属。其平均孳生密度为28.65±7.6只/g,物种丰富度指数为2.70,物种多样性指数为2.62,物种均匀度指数为0.89。其群落组成较为多样化,粉螨污染储藏物的情况严重。     

Insecticide susceptibility status of Aedes aegypti (Diptera: Culicidae) from Townsville

Insecticide susceptibility of a 1995 Townsville Aedes aegypti (L.) population was investigated in the laboratory according to World Health Organization guidelines. Using baseline data from two Townsville populations (1955 and 1989), larval bioassays detected significant increases in susceptibility to synthetic pyrethroids and malathion, and significant reductions in susceptibility to most organophosphates and propoxur. Adult bioassays detected significant resistance to bendiocarb and DDT. Comparison of larval data with the international reference ROCK strain showed substantial resistance to have developed to malathion and fenthion. Further analysis revealed the presence of distinct substrains in the baseline 1989 population, which displayed varying levels of temephos susceptibility. We concluded that susceptibility investigations should assess mosquito populations collected from many sites within an area rather than taking a single population from one site, and that the 1989 Ae. aegypti colony would be unsuitable for use as a susceptible reference population.     

Differences in susceptibility and physiological fitness of Mexican field Trichoplusia ni strains exposed to Bacillus thuringiensis

The use of different commercial Bacillus thuringiensis (Bt) products in the Bajio guanajuatense area in Mexico began 12 yr ago, and resistance to Bt in this area has been reported for Plutella xylostella (L.) The current study provides a baseline response and resistance potential to Bt in field and laboratory strains of Bajio Trichoplusia ni (Hübner). Differences in susceptibility to Bt among T. ni populations were observed. T. ni neonates collected in Romita, Guanajuato, were more susceptible to Bt than those collected in Salvatierra or San Luis de la Paz, Guanajuato. After five generations of exposure to XenTari in the laboratory, decreased susceptibility was found only in the Salvatierra insects, with an LC50 that was 2.1-fold greater than that of a Mexican laboratory strain. The XenTari-selected San Luis de la Paz strain was from 16- to 87-fold more resistant to CrylA protoxins than U.S. (US) and Mexican laboratory strains. Although CrylAb is not a component of XenTari, this strain also was significantly less susceptible to CrylAb toxin compared with a US strain, with a resistance ratio of 40.4. The larval weights and lengths, pupal lengths, and percentage of pupation were significantly lower for the Salvatierra strain than for all other strains. The relationship of T. ni susceptibilities to Bt Cry toxins and protoxins after several generations of exposure to XenTari and its similarity to P. xylostella behavior.     

Variation in susceptibility to Bacillus thuringiensis toxins among unselected strains of Plutella xylostella

So far, the only insect that has evolved resistance in the field to Bacillus thuringiensis toxins is the diamondback moth (Plutella xylostella). Documentation and analysis of resistant strains rely on comparisons with laboratory strains that have not been exposed to B. thuringiensis toxins. Previously published reports show considerable variation among laboratories in responses of unselected laboratory strains to B. thuringiensis toxins. Because different laboratories have used different unselected strains, such variation could be caused by differences in bioassay methods among laboratories, genetic differences among unselected strains, or both. Here we tested three unselected strains against five B. thuringiensis toxins (Cry1Aa, Cry1Ab, Cry1Ac, Cry1Ca, and Cry1Da) using two bioassay methods. Tests of the LAB-V strain from The Netherlands in different laboratories using different bioassay methods yielded only minor differences in results. In contrast, side-by-side comparisons revealed major genetic differences in susceptibility between strains. Compared with the LAB-V strain, the ROTH strain from England was 17- to 170-fold more susceptible to Cry1Aa and Cry1Ac, respectively, whereas the LAB-PS strain from Hawaii was 8-fold more susceptible to Cry1Ab and 13-fold more susceptible to Cry1Da and did not differ significantly from the LAB-V strain in response to Cry1Aa, Cry1Ac, or Cry1Ca. The relative potencies of toxins were similar among LAB-V, ROTH, and LAB-PS, with Cry1Ab and Cry1Ac being most toxic and Cry1Da being least toxic. Therefore, before choosing a standard reference strain upon which to base comparisons, it is highly advisable to perform an analysis of variation in susceptibility among field and laboratory populations.     

Differential levels of mite infestation of wheat and barley in Czech grain stores

Abstract  While mites are able to utilize numerous food sources, the suitability of the food strongly influences population growth. The different suitabilities of various stored agricultural products will thus affect the level of infestation. In this study, we compared field mite infestation rates in two stored cereals: wheat and barley. We analyzed mite abundance, frequency and species composition in samples of grain obtained from 79 selected Czech grain stores. Stored barley seemed to be more vulnerable to mite attack than wheat, as we consistently found more infested samples, more species and higher mean and median mite abundance per sample in barley as compared to wheat. The mean mite abundance per sample were 55 and 506 individuals for wheat and barley, respectively. In barley, 10% of samples exceeded allergen risk threshold (i.e., 1 000 individuals per kg of grain). Altogether, 25 species were identified from approximately 35 000 individuals. The most frequently identified species were the same in wheat and barley, that is, Tydeus interruptus Sig Thor, Acarus siro L., Tarsonemus granarius Lindquist, Lepidoglyphus destructor (Schrank) and Tyrophagus putrescentiae (Schrank). Based on principal components analysis, we found a closer association of T. interruptus , T. putrescentiae , L. destructor and Cheyletus eruditus (Schrank) with barley samples, corresponding to the high frequency and abundance values of these mites. The probable reasons for the higher infestation, especially mite abundance in barley, are discussed in relation to the higher proportion of crushed parts, which may release favorable nutrient sources and amplify the abundance values.     

Susceptibility of different European populations of Tribolium confusum (coleoptera: Tenebrionidae) to five diatomaceous earth formulations

Laboratory bioassays were conducted to evaluate the susceptibility of seven populations (strains) of the confused flour beetle, Tribolium confusum Jacquelin du Val (Coleoptera: Tenebrionidae), to five commercially available diatomaceous earth (DE) formulations: Insecto, Protect-It, Protector, PyriSec, and SilicoSec. These DEs were applied on wheat, Triticum durum Desf., at two dose rates, 500 and 1000 ppm. The six beetle strains were obtained from Denmark, United Kingdom, Greece, Germany, Italy, and Portugal, whereas a seventh strain came from the Greek strain after laboratory selection with SilicoSec-treated wheat for six generations. Adults of the above-mentioned strains were exposed for 7 d to wheat treated with each DE formulation, and mortality was assessed after exposure. For all DE-dose combinations, significant differences were noted in mortality levels among strains. Generally, the strains from Denmark, United Kingdom, and Germany were the most susceptible to the DEs used, whereas the strain from Portugal was the least susceptible. No significant differences were noted in susceptibility level between the initial Greek strain and the laboratory-selected Greek strain, with the exception of Protector, where the selected strain was significantly more tolerant than the initial strain for both dose rates tested.     

Differences in Biological Characteristics in Organophosphorus-Resistant Strains of the Phytoseiid Mite Typhlodromus Pyri

Strains of Typhlodromus pyri were collected from orchards in SW England, where populations differed in their response to organophosphorus insecticides compared with strains collected in the SE. Biological characteristics of these strains were compared with those in OP-resistant strains collected from the SE and OP- susceptible strains. There were significant differences in size of eggs and adult mites from the different strains, but these differences did not appear to be related to resistance status of the mites. Female OP-susceptible mites had a longer development time than other strains. Over all strains total development time for female mites was 0.8 day less than for males. There were significant differences between strains for pre-oviposition period, but not oviposition or post-oviposition periods. There were significant differences in total numbers of eggs produced between strains, with a standard OP-resistant strain producing the most and an OP-susceptible strain the fewest eggs. There were significant differences in male longevity between strains, but this did not appear to be related to resistance status. This study highlights the fact that strains may differ in biological characteristics that contribute to fitness regardless of their pesticide resistance status.     

Susceptibility and detoxifying enzyme activity in two spider mite species (Acari: Tetranychidae) after selection with three insecticides

Changes in the susceptibility and detoxifying enzyme activity were measured in laboratory strains of Banks grass mite, Oligonychus pratensis (Banks), and twospotted spider mite, Tetranychus urticae Koch, that were repeatedly exposed to three insecticides. Three strains of each mite species were exposed to one of two pyrethroids, bifenthrin, and lambda-cyhalothrin, or an organophosphate, dimethoate, for 10 selection cycles at the LC60 for each insecticide. A reference or nonselected strain of each mite species was not exposed to insecticides. After 10 cycles of exposure, susceptibility to the corresponding insecticides, bifenthrin, lambda-cyhalothrin, and dimethoate, decreased 4.5-, 5.9-, and 289.2-fold, respectively, relative to the reference strain in the respective O. pratensis strains, and 14.8-, 5.7-, and 104.7-fold, respectively, relative to the reference strain in the respective T. urticae strains. In the bifenthrin-exposed O. pratensis strain, there was a 88.9-fold cross-resistance to dimethoate. In the dimethoate-exposed T. urticae strain, there was a 15.9-fold cross-resistance to bifenthrin. These results suggest that there may be cross-resistance between dimethoate and bifenthrin. The reduced susceptibility to dimethoate remained stable for three months in the absence of selection pressure in both mites. The decrease in susceptibility in the O. pratensis strains exposed to bifenthrin, lambda-cyhalothrin, and dimethoate was associated with a 4.7-, 3.0-, and 3.6-fold increase in general esterase activity, respectively. The decrease in susceptibility in the T. urticae strains exposed to bifenthrin and lambda-cyhalothrin was associated with a 1.3- and 1.1-fold increase in general esterase activity, respectively. The mean general esterase activity was significantly higher in the pyrethroid-exposed O. pratensis and T. urticae strains than in the nonselected strain. There was no significant increase in esterase activity in the dimethoate-exposed T. urticae strain. The decrease in susceptibility to insecticides was also associated with reduced glutathione S-transferase 1-chloro-2, 4-dinitrobenzene conjugation activity, but this did not appear to be related to changes in insecticide susceptibility. These results suggest that in these mites, the general esterases may play a role in conferring resistance to pyrethroids. However, some other untested mechanism, such as target site insensitivity, must be involved in conferring dimethoate resistance.     

阜阳地区居室环境中粉螨的群落组成和多样性

目的调查安徽省阜阳地区居室环境中粉螨的群落组成和多样性。方法于2009年7月,选择100户家庭进行样本采集,每居室环境中取4个采样点,对采集到的地面灰尘、床面灰尘、家具灰尘及衣物灰尘进行粉螨的分离、鉴定、计数和数据分析。结果共检获粉螨3609只,隶属于6科15属,共19种。对4类居室灰尘中粉螨群落多样性分析表明:它们的丰富度指数为0.89~2.09,多样性指数为1.28~1.83,均匀度指数为0.65~0.71。结论粉螨群落的结构及多样性与其生境条件直接相关;阜阳地区人居环境粉螨孳生密度较高(48.5%),应加强防螨措施。     

Susceptibility/resistance to mouse hepatitis virus strain 3 and macrophage procoagulant activity are genetically linked and controlled by two non-H-2-linked genes

The mode of inheritance of susceptibility/resistance to mouse hepatitis strain 3 (MHV-3) was determined by typing the set of AXB/BXA recombinant inbred (RI) strains derived from resistant A/J (A) and susceptible C57BL/6J (B) progenitors for susceptibility to infection as determined by the severity of liver pathology. The strain distribution pattern for susceptibility showed a discontinuous variation: one strain was fully resistant (A-like), four strains were fully susceptible (B-like), and 16 strains showed an intermediate degree of susceptibility. The fully susceptible strains developed fulminant hepatitis and died; the fully resistant strain developed no liver disease, whereas a range of disease ranging from mild focal hepatitis to widespread hepatocellular necrosis was seen in the semisusceptible strains. This SDP best fits the two-recessive-gene model of inheritance, and neither of these two loci is linked to the H-2 complex. Macrophage procoagulant activity (PCA) segregated among the RI strains in a strain distribution pattern identical to that of susceptibility/resistance. PCA levels were greater than sevenfold elevated in fully susceptible RI mice and fourfold elevated in semisusceptible mice with no increase in resistant mice. These observations suggest genetic linkage of susceptibility/resistance to MHV-3 infection and macrophage PCA.     

Variation in Susceptibility to Bacillus thuringiensis Toxins among Unselected Strains of Plutella xylostella

So far, the only insect that has evolved resistance in the field to Bacillus thuringiensis toxins is the diamondback moth (Plutella xylostella). Documentation and analysis of resistant strains rely on comparisons with laboratory strains that have not been exposed to B. thuringiensis toxins. Previously published reports show considerable variation among laboratories in responses of unselected laboratory strains to B. thuringiensis toxins. Because different laboratories have used different unselected strains, such variation could be caused by differences in bioassay methods among laboratories, genetic differences among unselected strains, or both. Here we tested three unselected strains against five B. thuringiensis toxins (Cry1Aa, Cry1Ab, Cry1Ac, Cry1Ca, and Cry1Da) using two bioassay methods. Tests of the LAB-V strain from The Netherlands in different laboratories using different bioassay methods yielded only minor differences in results. In contrast, side-by-side comparisons revealed major genetic differences in susceptibility between strains. Compared with the LAB-V strain, the ROTH strain from England was 17- to 170-fold more susceptible to Cry1Aa and Cry1Ac, respectively, whereas the LAB-PS strain from Hawaii was 8-fold more susceptible to Cry1Ab and 13-fold more susceptible to Cry1Da and did not differ significantly from the LAB-V strain in response to Cry1Aa, Cry1Ac, or Cry1Ca. The relative potencies of toxins were similar among LAB-V, ROTH, and LAB-PS, with Cry1Ab and Cry1Ac being most toxic and Cry1Da being least toxic. Therefore, before choosing a standard reference strain upon which to base comparisons, it is highly advisable to perform an analysis of variation in susceptibility among field and laboratory populations.     

Insecticide susceptibility and resistance of Culex tritaeniorhynchus (Diptera: Culicidae) larvae collected from Gwangju,Republic of Korea

The susceptibility of Culex tritaeniorhynchus collected from Gwangju, Jeollabuk Province, Republic of Korea (ROK) to insecticides was evaluated under laboratory conditions using ten insecticides (7 pyrethroids and 3 organophosphates) that are currently applied by local public health centers in the ROK. Based on the values of median lethal concentration (LC₅₀), Cx. tritaeniorhynchus larvae were most susceptible to chlorpyrifos (0.006 ppm), fenitrothion (0.022 ppm), fenthion (0.035 ppm) and bifenthrin (0.038 ppm), and were least susceptible to esbiol (1.722 ppm). In comparative resistance tests, the resistance ratios (RRs) of seven insecticides were compared among each other using two strains of Cx. tritaeniorhynchus that were collected from the same locality during 1992 and 2010. Culex tritaeniorhynchus demonstrated significantly increased RRs to pyrethroids over time, while demonstrating decreased RRs among the organophosphates. Among the pyrethroids, permethrin had the highest RR values of 182.1‐ and 833.3‐fold differences, followed by etofenprox with RRs of 138.4‐ and 224.1‐fold differences in values of LC₅₀ and concentration that produced 90% mortality (LC₉₀), respectively. Culex tritaeniorhynchus strains demonstrated the least amount of change in susceptibility to the organophosphates, chlorpyrifos, fenitrothion and fenthion with 0.020‐, 0.019‐ and 0.001‐fold differences in resistance ratios (RRLC₅₀), respectively.     

Genetic analysis of resistance to azinphosmethyl in the pear psylla Cacopsylla pyri

Laboratory selection with azinphosmethyl had little effect on the chemical resistance in Cacopsylla pyri strains, in comparison to the original wild populations. The resistance ratio relative to a susceptible strain varied from 10- to 40-fold depending on the generation studied. Crosses between two resistant strains and the susceptible strain show resistance to be autosomally inherited and semi-dominant in expression. Backcrosses between F1 and the susceptible strain were unable to distinguish unabigously between monogenic and polygenic inheritance. In the majority of experiments, however, the overall dose-response relationship for backcross progeny was consistent with a single gene hypothesis. Additional bioassays showed azinphosmethyl-resistant strain to cross-resistant monocrotophos and phosmet, but not carbamates, pyrethroids, amitraz or the organophosphates chlorpyrifos and mevinphos.     

Monooxygenase Activity in Methidathion Resistant and Susceptible Populations of Amblyseius womersleyi (Acari: Phytoseiidae)

The purpose of this study was to evaluate the cytochrome P450-dependent monooxygenase activities in methidathion resistant and susceptible strains of Amblyseius womersleyi Schicha. Artificial laboratory selections for resistance and susceptibility to methidathion were performed in an organophosphate resistant strain of A. womersleyi (Kanaya strain). Selections for susceptibility were also performed in a susceptible strain of this predaceous mite (Ishigaki Strain). After the selection process, the LC₅₀ of methidathion for the selected strains of A. womersleyi were 816 mg/l (Kanaya R), 4.61 mg/l (Kanaya S) and 1.59 mg/l (Ishigaki S). The monooxygenase activities were determined biochemically by the O-deethylation of 7-ethoxycoumarin (7-EC). The monooxygenase activity in adult females of Kanaya R strain (51.1 pmol/30 min/mg protein) was 3.60- and 5.42-fold higher than the activity observed for Kanaya S and Ishigaki S strains, respectively. Significant correlation between monooxygenase activity and LC₅₀ (mg/l) of methidathion was observed analyzing 16 populations of A. womersleyi with different susceptibilities to methidathion. Monooxygenase activity was also evaluated in different life stages (egg, larva, protonymph, deutonymph and adult) of A. womersleyi. The lowest activity was observed for the larval stage, which presented the highest susceptibility to methidathion. Protonymph, deutonymph and adult presented the highest monooxygenase activities. These stages were the most tolerant to methidathion. Monooxygenase activities of the Kanaya R strain were higher than of the Kanaya S strain in all developmental stages. The present study can be helpful for the implementation of a program involving release of insecticide-resistant populations of A. womersleyi in the field. The monooxygenase activity determination is easier and quicker than the estimation of LC₅₀, requiring fewer mites.