Introduction

Abamectin is a 16-membered macrolide ring produced during the fermentation of Streptomyces avermitilis (Lasota & Dybas, 1991). It is the only highly effective and broad-spectrum bio-pesticide used in crop protection and medicine and is consequently widely used to control a variety of pest insects of crops, pest mites, and zoonotic parasites (Bhardwaj et al., 2020; Cui et al., 2018; Lloberas et al., 2013; Verdu et al., 2015). China is the primary abamectin manufacturer globally, and abamectin is also among the most commonly used bio-pesticides in China. Moreover, abamectin has been used in conjunction with the active ingredients of more than 30 pesticides, leading to the registration of over 1,000 mixed formulations (Sun et al., 2020; Xu et al., 2014). In addition to broad-spectrum activity and high-efficiency, abamectin also exhibits low residue potential and is safe for non-target organisms. Abamectin exerts its insecticidal effects by primarily acting on the γ-aminobutyric acid (GABA) receptors of insect neuronal synapses or neuromuscular synapses, thereby interfering with the normal release of GABA and prolonging the opening of GABA-gated chloride channels (Fisher & Mrozik, 1992). The massive influx of chloride ions causes hyperpolarization of nerve cell membrane potentials that then suppress the excitability of nerve membranes, leading to death (Abongwa et al., 2016; Bai and Steven Ogbourne, 2016). The mass production and widespread use of abamectin have highlighted the problem of resistance during the comprehensive management of pest insects and mites (Kwon et al., 2010; Lasota & Dybas, 1991; Xu et al., 2017). Therefore, abamectin resistance has become a focus of global research.

Abamectin is the biogenic pesticide of choice for comprehensively controlling pest mites and other insects in agriculture. Bryobia praetiosa (Acariformes: Tetranychidae) is a major pest of Medicago sativa (alfalfa) and legume plants, causing serious damage to legume production (Jiang, 2007). Pesticide-resistance rapidly develops in B. praetiosa due to its small size, short generation time, fast rate of spread, high reproductive rate, and considerable generation overlap (Zhang et al., 2007). Long-term application of a single pesticide at high doses is bound to induce resistance and re-infestation of pests, in addition to changes in their biology, ecology, and behavior, thereby shortening the service life of pesticides like abamectin (Sun et al., 2001; Ullah et al., 2020). The widespread use of abamectin has led to the development of varying levels of resistance in pest mites, including very high levels of resistance in some areas (Fisher & Mrozik, 1992; Gao et al., 2010; Guo, 1998). In particular, the abamectin-resistant phenotype of most pest mites is controlled by multiple genes conferring natural fitness benefits (Kwon et al., 2010; Wei et al., 2013). Consequently, sensitive strains can hardly recover in short time periods. In addition, abamectin-resistant Panonychus citri has developed significant cross-resistance to emamectin benzoate (Gao et al., 2010; Yin et al., 2016). Similarly, Plutella xylostella has developed high resistance to tebufenozide and cross-resistance to abamectin, indoxacarb, and chlorfenapyr (He & Wang, 2015) As a result, resistance mechanisms have led to chemical pesticides being ineffective in the treatment of B. praetiosa that have abamectin resistance.

The development of abamectin resistance in pests (e.g., spider mite species) is closely linked with increases in the activity of various detoxifying enzymes inside their bodies, which mainly include carboxylesterase (CarEs), glutathione-S-transferases (GSTs) and mixed function oxidase (MFOs). In fact, three detoxifying enzymes CarEs, MFOs and GSTs are very important components correlated with insecticide resistance and their functions were well documented (Han et al., 2016; Pavlidi et al., 2015; Ru et al., 2017; Shen et al., 2014). They can at least partly explain the changes in biological characteristics (prolonged growth period and reduced fecundity) of abamectin resistant strain. The relative fitness cost of abamectin-resistant strains in pests decreased (Deng et al., 2002; Liu & Shen, 2003; Song et al., 2013). The life history characteristics and relative fitness of abamectin-resistant strain of B. praetiosa have not been reported so far. Here, we hypothesized that the increase of abamectin resistance in B. praetiosa was related to the rapid increase of major detoxifying enzymes, and the abamectin resistant strain of B. praetiosa exhibited reproductive disadvantages. In this study, laboratory bioassays, biochemical analyses, and life history analyses were used to systematically investigate the resistance of B. praetiosa to abamectin, the biochemical mechanism of resistance, and altered reproductive and developmental characteristics.

Materials and methods

Resistance development and toxicity assessment

Susceptible strain (SS) development

B. praetiosa were collected from a pesticide-free field used for alfalfa seed production at the Grass Germplasm Center in the Gansu Province of China. More than 1,000 B. praetiosa individuals were collected and maintained on Vigna unguiculata (cowpea) seedlings with 3–5 leaves in a climate chamber. A sponge disc (about 1 cm thick and 9 cm in diameter) was placed in a 10 cm diameter Petri dish andwas covered with a 10 cm diameter filter paper. Distilled water was then added to the Petri dish to saturate the sponge and cowpea leaves were placed on the filter paper and surrounded by degreasing cotton wetted with distilled water to moisturize the leaves and prevent mites from escaping. The mites were maintained in isolation in an artificial climate chamber (26 ± 1 °C, 50%–60% humidity, and under a photoperiod of 16 h light (2.8 × 103 Lux) to 8 h dark for more than 30 generations, and then used as the SS strain.

Resistant strain (RR) development

Some mites were separated from the SS strain and propagated on cowpea seedlings in a climate chamber until the population density reached a sufficient level (i.e., approximate 5000 susceptible female adults). The mites were then exposed to abamectin at a concentration that killed about 65%–75% of the individuals to emulate a selective pressure. Specifically, the treatment concentration series of avermectin were 0.05, 0.10, 0.20, 0.40, 0.80, 1.60, and 3.20 mg/L, respectively. Mites were exposed by evenly spraying the abamectin solution with a small sprayer on both sides of the leaves until droplets coalesced and began to drip from the leaves. At 24 h after spraying the cowpea seedlings carrying the SS mites, mortality was assessed by transferring the surviving mites onto fresh cowpea seedlings. The mites living on cowpea seedlings were removed for 1–2 d after oviposition. When a generation of eggs developed into mature mites, another abamectin spray treatment was applied. Resistance was then measured in the laboratory every 3–5 generations to calculate the median lethal concentration (LC50) of abamectin against B. praetiosa and to monitor resistance development within the population. The resistance index (RI) indicates the degree of resistance in a population and is calculated by: RI = (LC50 of Fn)/(LC50 of F0), where Fn represents the nth generation of RR selection, and F0 represents the initial generation of RR selection.

Toxicity assessment

Toxicity was assessed using the slide-dip method, as recommended by the Food and Agriculture Organization (FAO, 1980), with some modifications. Healthy adult female mites (aged 3–5 d) were picked with a brush and placed on their backs on a slide with double-sided adhesive tape attached and 30 mites on each slide. Abamectin was dissolved in distilled water (containing 0.01% Triton X-100) to seven concentrations that were then used to determine LC50 values for SS and RR strains. The end of the slide with mites was then immersed in the solution and gently shaken for 5 s, followed by removal and blotting of the residual liquid surrounding the mites with filter paper. Treatment with each concentration was repeated three times (n = 3) and distilled water containing 0.01% Triton X-100 was used as the control. The mites were then maintained in a biochemical incubator for 24 h at (26 ± 1)°C, with (70 ± 5)% relative humidity, and with a photoperiod of 16 h light to 8 h dar. Mortality was then assessed by gently blowing the mites with a pipette bulb and those with immobile legs were recorded as dead. The natural mortality (< 20%) observed in the control were used to count the corrected mortality with Abbott’s formula.

Enzyme activity assays

Sample preparation

Each enzyme source, including carboxylesterases (CarEs), glutathione S-transferases (GSTs) and mixed-function oxidases (MFOs), was prepared from 200 adult female mites. The mites were homogenized at 4 °C with 1.5 mL PBS (0.04 mol/L, pH 7.0) for CarEs and GSTs, and 1.5 mL PBS (0.1 mol/L, pH 7.8) for MFOs. After the homogenates were centrifuged at 10,000 g for 10 min at 4 °C, the supernatants were used as enzyme source for measuring the activities of CarEs, GSTs, and MFOs.

The total protein content in the supernatant was determined using the Coomassie brilliant blue G-250 protein assay proposed by Grintzalis et al. (2015). Briefly, the CBB reagent is prepared as concentrated by dissolving 60 mg CBB in 100 ml 2 N HCl by stirring for 40 min and removing any undissolved particulates either by filtering. Dilute this reagent 2 × with 2 N HCl [designated as CBB 2 N:2 N HCl reagent] right before use. Prepare a series of BSA standard solutions (in ddH2O) with protein concentration from BSA (1—40 μg/ mL). 200 µL of each supernatant in a microplate with 50 μL CBB 2 N:2 N HCl (1:1) reagent. As reagent blank use 200 µL ddH2O in place of the protein solution. As reagent blank use 200 µL ddH2O in place of the protein solution. After 5–10 min of incubation at room temperature, and measuring OD value at 610 nm. The OD value was converted to protein concentration based on the standard curve.

Enzyme activity assays

CarEs activity was measured as described by Shen et al (2014). Briefly, alpha-naphthyl acetate (100 μL, 3 × 10−4 mol/L, containing 1 × 10−4 mol/L physostigmine) was used as substrate in a reaction with 75 μL of the enzyme solution for 10 min at 30 °C. Then, 25 μL of a chromogenic solution (1% fast blue B in water: 5% sodium dodecyl sulfate, 2:5) was added, and the OD value was measured with a microplate reader at 600 nm. In the control reaction, 75 μL of 0.04 mol/L pH 7.0 PBS buffer was used instead of the enzyme solution. The final OD value was calculated by subtracting the value of the control from the value of the enzyme sample. The spectrophotometric data was then converted into specific enzyme activity [μmol/(μg·min)] based on an α-naphthol standard curve and enzyme concentrations.

GSTs activity was measured as described by Clark et al (1984). Briefly, 240 μL of 66 mmol/L pH 7.0 PBS was added to samples, followed by 10 μL of CDNB (0.03 mmol/L) and 30 μL of GSH (50 mmol/L), which were then reacted with 20 μL of the GST samples at 37 °C. The resulting change in OD value was measured at 340 nm with a microplate reader. Measurements were made at 30 s intervals for 5 min. The control group samples were treated with 20 μL of 66 mmol/L PBS instead of the enzyme solution. The specific activity of the GSTs μmol/(μg·min) was then calculated following Shen et al., (2014) as follows:

$$\begin{array}{c}\mathrm{GSTs}\;\mathrm{activity}\left(\mathrm\mu\mathrm m\mathrm o\mathrm l/\min\right)=\Delta{\mathrm{OD}}_{340}\upsilon/\mathrm\varepsilon\;\mathrm L\\\mathrm{GSTs}\;\mathrm{specific}\;\mathrm{activity}=\mathrm{GST}\;\mathrm{activity}/\mathrm{GST}\;\mathrm{concentration}\end{array}$$

where ΔOD340 is the change in absorbance per minute (ΔOD340/min), υ is the enzyme reaction system, ε is the extinction coefficient of the product [0.0096 L/(μmol·cm), and L is the length of light path (1 cm).

MFOs activity was measured as described by Kim et al (2004), with slight modifications. The supernatant was filtered through glass-wool and centrifuged at 100,000 g for 75 min. The resulting pellet of microsomes was resuspended in the homogenization buffer for examining. Using p-nitroanisole (0.05 mol/L in acetone) as substrate, under the condition of NADPH and oxygen as electronic donors, MFOs catalyzed O-demethylating action producing p-nitrophenol. The reaction was terminated by hydrochloric acid (1 mol/L), and then extracted using chloroform and 0.5 mol/L NaOH solution and measured at 400 nm. The specific activity was calculated according to the standard curve of nitrophenol and protein concentration of enzyme source [μmol/(mg·30 min)].

All experiments were repeated for three times, and the average value was obtained from the three repeated data.

Determination of kinetic constants

Following methods described by Wilkinson, kinetic constants were determined by diluting the substrate to six concentrations, followed by reacting them with a fixed volume of enzyme solution. The Km and Vmax values were then calculated by linear regression using a Lineweaver–Burk double reciprocal plot (Shen et al., 2014).

Construction of a life table parameter for resistant and susceptible strains

A cowpea seedling with 3–5 leaves was transplanted into a pot and each leaf was transferred with an adult female mite that was removed after oviposition. When the eggs hatched on the leaf, one larva was retained, and the remaining larvae and eggs were removed. The base of the leaf was wrapped with wetdegreasing cotton to moisturize the leaf and prevent mites from escaping. The pot was then placed in a porcelain plate, covered with gauze, and placed in a climate chamber at 26 °C, with 50% – 60% humidity, and with a photoperiod of 16 h: 8 h, L: D. Pesticides were not used during the maintenance of the 80 – 100 individuals in each of the RR and SS groups. The development of each mite was recorded daily, and the number of eggs laid per female per day was recorded until their death.

Statistical analyses

The concentration response data was analyzed by using probit analysis (Finney, 1971) with POLO software to determine the LC50 values, their standard errors, slopes and 95% fiducial limits. Chi-square tests were used to evaluate the reliability of the regression equation for resistance and the LC50. Resistance index (RI) = LC50 of the RR strain/LC50 of the SS strain. For the activity of three enzymes, mean activity values ± SE (n = 3) between SS and RR strains.

According to the life history table constructed for the resistant and sensitive populations (Deng et al., 2003), the number of female individuals tested for SS and RR strains was 53 and 62, which were used to estimate the development duration and reproductive parameters of each female. the intrinsic growth rate (rm), net reproductive rate (R0), finite rate of increase (λ), average generation time (T), and population doubling time (Dt) were calculated by the following equations, respectively (Deng et al., 2002, 2003; Wang et al., 2014):

$$\begin{array}{c}R_0=\Sigma\;L_xM_x\\\mathrm T=\Sigma\;X\;L_xM_x/\Sigma\;L_xM_x\\\begin{array}{c}r_m=\ln R_0/T\\\lambda=\exp{(r}_m)\\D\mathrm t=\ln2/r_m\end{array}\end{array}$$

where X is the time interval (d), Lx is the survival rate of any individual during X, Mx is the average number of eggs laid per female during X, and Dt is the population doubling time. For the RR strain, the percentage of decrease in the duration of each developmental period = (duration of developmental period of SS − duration of developmental period of RR strain)/duration of developmental period of SS strain × 100%. Relative fitness was determined by rm and R0 values.

All data differences between SS and RR strains were analyzed by three independent samples (n = 3) t-test with a P < 0.05 or P < 0.01.

Results

Selection of RR strain

Abamectin resistance developed slowly in B. praetiosa, with a 17.13—fold increase in resistance after 18 generations of selection. While the resistance of the F20 and F24 generations increased rapidly, the resistance index increased by 26.09 times and 30.11 times, respectively (Table 1). Based on the b value from the resistance calculation for each generation, there were two stages in the development of abamectin resistance in B. praetiosa. The first stage occurred in the F0 – F15 generations, in which the b value gradually decreased from 2.3011 to 0.7154, indicating the emergence and increased abundance of resistant heterozygotes in the RR strain, along with increased heterogeneity in the resistant strain. The second stage occurred in the F15 – F24 generations, wherein the b value gradually increased from 0.7154 to 3.0895, indicating that the resistant heterozygotes of the RR strain increasingly evolved into resistant homozygotes, leading to decreased heterogeneity of the resistant strain (Kwon et al., 2010). The RI and b values in the last two measurements exhibited little differences because the RI values of F20 and F24 were 26.09 and 30.11, while the b values were 3.0874 and 3.3295, respectively. Therefore, RR selection was stopped at the F24 generation.

Table 1 The selection of resistance to abamectin in Bryobia praetiosa
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Specific activities of detoxification enzymes in the RR and SS strains

The specific activities of CarEs, GSTs, and MFOs in the RR strain reached 1.6425, 1.7442, and 0.3106 μmol/(μg\(\cdot\)min), respectively (Fig. 1). The activities of CarEs, GSTs, and MFOs in the RR strain were 1.13-, 1.75- and 4.02-fold, respectively higher than those of the SS strain, respectively. GSTs and MFOs protein concentrations were also significantly different between RR and SS strains (P < 0.01), suggesting that the main resistance mechanism of abamectin may be the increase of GST and MFO activities.

Fig. 1
figure 1

Specific activities of detoxification enzymes between avermectin-resistant strain (RR) and susceptible strain (SS) of Bryobia praetiosa. Data are expressed as means ± SE from three independent experiments (n = 3). **P < 0.01 for the same enzyme is significantly different between SS and RR strains by t-test

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The kinetic constants of detoxification enzymes were compared between the RR and SS strains. The Vmax values of CarEs, GSTs, and MFOs in the RR strain were significantly different from those of the SS strain, with 1.57-, 2.34-, and 2.76-fold higher values, respectively (Fig. 2). The Km values of CarEs and GSTs in the RR strain were significantly different from those of the SS strain (P < 0.05), with 1.62- and 1.58-fold higher values, respectively; while The Km value of MFOs from the RR strain was significantly different (P < 0.01) and 2.58-fold higher than that of the SS strain (Fig. 3), indicating that the affinity for the substrate p-nitroanisole was significantly reduced and that MFOs underwent quantitative and qualitative changes upon resistance development.

Fig. 2
figure 2

Comparison of the Vmax values of detoxification enzymes between avermectin-resistant strain (RR) and susceptible strain (SS) of Bryobia praetiosa. Data are expressed as means ± SE from three independent experiments (n = 3). *P < 0.05 and **P < 0.01 for the same enzyme are significantly different between SS and RR strains by t-test, respectively

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Fig. 3
figure 3

Comparison of the Km values of detoxification enzymes between avermectin-resistant strain (RR) and susceptible strain (SS) of Bryobia praetiosa. Data are expressed as means ± SE from five independent experiments (n = 3). *P < 0.05 and **P < 0.01 for the same enzyme are significantly different between SS and RR strains by t-test, respectively

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Duration of developmental periods in the RR and SS strains

The durations of all periods in the RR strain were significantly shortened (P < 0.05) compared to the SS strain, except for the egg period. Specifically, the larval, pre-nymph, lat-nymph, pre-oviposition, and oviposition periods were shortened by 23.11%, 16.16%, 20.12%, 21.82%, and 19.40%, respectively. The average longevity of RR strain was also 28.09% shorter than that of SS strain (Table 2).

Table 2 Comparison of developmental duration of various mite stages between abamectin -resistance strain (RR) and susceptible strain (SS) of Bryobia praetiosa
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Fertility of RR and SS strains

The RR strain exhibited 33.99% and 34.14% decreases in the average number of eggs laid per female per day and the average number of eggs laid per female, respectively, in addition to a 19.40% decrease in the oviposition period duration relative to that of the SS strain (Table 3). In addition, the hatch rate of eggs laid by RR females was significantly lower than that of SS females (P < 0.05).The results indicated that the RR strain was more disadvantageous in reproductive characteristics than the SS strain.

Table 3 Comparison of fecundity between avermectin-resistant strain (RR) and susceptible strain (SS) of Bryobia praetiosa
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Life table parameters and relative fitness of RR and SS strains

The development and fertility parameters of the RR and SS strains of B. praetions were systematically observed by life table technique at 26 ℃ in the laboratory (Table 4). Significant differences were observed between RR and SS strains for the intrinsic rate of increase (rm), net reproduction rate (R0), finite rate of increase (λ), average generation time (T), and and population doubling time (Dt) parameters (P < 0.05), which reflect development and fertility characteristics of B. praetiosa. The results showed that RR strain was more disadvantageous than SS strain. In addition, the relative fitness of RR individuals was only 0.49-fold that of SS strain, and there is also a significant degree of fitness defect, indicating that the resistant strain had obvious relative fitness defects.

Table 4 Demographic parameters and relative fitness of the abamectin-resistant strain (RR) and the susceptible strain (SS) in Bryobia praetiosa
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Discussion

Abamectin is globally recognized as an important, environmentally friendly biogenic pesticide, with abamectin resistance also receiving much attention. In this study, abamectin was used for intensive selection of B. praetiosa in the laboratory, with an extremely slow increase in resistance observed during the early stage of selection, but much faster development in the later stages comprising the F15 to F24 generations. The abamectin LC50 of the RR individuals sharply increased from 0.9987 mg/L to 30.11 mg/L, with the RI increasing from 8.13 in F15 to 30.11 in F24, consistent with the development of resistance against commonly used pesticides. Resistance developed in the laboratory usually involves multiple genes, possibly due to only a few surviving individuals being retained after each treatment (Kwon et al., 2010). Consequently, the accumulation of minimally important genes across generations eventually leads to resistance controlled by multiple genes (Liu and Shen, 2003; Tang et al., 1990), indicating that further selection of high-level pesticides would lead to higher resistance of pests. Therefore, the rotation of commonly used pesticides or the use of mixed pesticides should be considered in the application of abamectin to delay the development of increased abamectin resistance in B. praetiosa.

The most intensively studied mechanism of pesticide resistance involves biochemical metabolic pathways. It is generally believed that the development of pesticide resistance in pests like mites is closely related to enhanced detoxifying enzyme activities. Although many studies have investigated the effects of abamectin on detoxification enzymes of insects and mites, inconsistent conclusions have been drawn (Kwon et al., 2010; Pavlidi et al., 2015; Ru et al., 2017). For example, Wu et al. (2001) observed that the resistance of Plutella xylostella to abamectin was mainly caused by altered CarEs activity. Similarly, Han et al. (2016) found that increased CarEs activity of Apis ceranacerana was one of the main reasons underlying its resistance to abamectin. In addition, another study demonstrated that the activities of GSTs and MFOs played the dominant role in the metabolism of abamectin in Tetrany chusurticae, but the influence of CarEs less (Ru et al., 2017). In the present study, we also observed that GSTs and MFOs in the RR strain played a major role in abamectin metabolism, while CarEs exhibited little effects. These results confirm the conclusion by Zhao et al. (2003) that enhanced MFOs and GSTs activities are the main reason underlying abamectin resistance in Tetrany chusurticae. In particular, the most significant increase in enzymatic activity was observed for MFOs, and we speculate that this may be closely related to the methoxy group (-OCH3) of abamectin. Taken together, these observations suggest that resistance caused by biochemical metabolic pathways is closely associated with particular pest mite species characteristics, pesticide chemical structures, resistance levels, food quality, and environmental conditions (Shen et al., 2014; Sun et al., 2020; Verdu et al., 2015).

In addition to the enzymes involved in pesticide resistance, the effects of chemical pesticides on pest biological development and fecundity may partially explain the rapid development of pesticide resistance; this topic has become an important research focus area (Bhardwaj et al., 2020; Kwon et al., 2010; Gao et al., 2010; He and Wang, 2015). Abamectin significantly impacts life history parameters including the duration of different developmental periods and the fecundity of the pest. Song et al.(2013) evaluated the effect of abamectin at sublethal dosages on the developmental duration of adult Neoseiulus fallacis in the laboratory, observing longer pre-oviposition periods, but shorter oviposition and post-oviposition periods, although the average lifespan was not significantly affected. Our investigation demonstrated that lifestyle periods were all shortened in the RR strain relative to the SS strain including the larval, pre-nymph, lat-nymph, pre-oviposition, and oviposition periods, as did the average lifespan of individuals; however, this was not observed for the egg period. Moreover, the resistant strain exhibited accelerated development and considerably overlapping generations, indicating that abamectin can stimulate the development of B. praetiosa, consistent with previous studies (Deng et al., 2003). Further comparison of RR and SS fecundity revealed that RR strain had less eggs, shorter oviposition periods, and significantly lower fecundity, thus supporting the hypothesis of reproductive disadvantage in resistant pests (Chen et al., 2015).

Relative fitness comprises the genetic response and adaptation (growth, development, and reproduction) of insect pests to environmental stresses (e.g., pesticides), and determines pest adaptive behaviors under the selective pressures exerted by pesticides. Han et al.(2011) reported that an abamectin-resistant population of P. xylostella exhibited reproductive disadvantages relative to a susceptible strain. Further, Song et al.(2013) observed that abamectin at sublethal doses decreased the rm and R0 parameters of N. fallacis, but increased Dt and T values. A comparison of the relative fitness between RR and SS individuals in this study revealed lower fecundity in the RR strain. Thus, the alleles associated with resistance in strains are often closely related to reproductive disadvantages. Nevertheless, there is disagreement regarding the indicators used to evaluate relative fitness (Guo et al., 1998). Roush and Plapp (1982) evaluated the fecundity of RR strain and observed that the average developmental duration was more important than fecundity, concluding that the developmental duration considerably influenced RR reproduction. Further, Tang et al.(1990) observed that both R0 and T were important indicators of relative fitness and both should be considered. The present study also demonstrated a significant difference in R0 and T values between RR and SS strains, and the difference in rm was also significant. Consequently, R0 should be used as an indicator of biological fitness when investigating resistant populations, as it provides a better representation of fecundity and viability for resistant pest insects stressed by pesticides (Wei et al., 2013). In this study, RR strain exhibited defects in growth, development, and reproduction. Further investigation of the fecundity of resistant populations and the underlying mechanism that confers resistance will help guide the comprehensive management of pest mites.

Conclusions

In this study, we systematically investigated abamectin resistance, the underlying biochemical mechanisms of resistance, and the reproductive/developmental characteristics of B. praetiosa with laboratory bioassays, biochemical analyses, and life history analyses. The studies demonstrated that the activities of GSTs and MFOs in the RR strain were significantly different from those in the SS strain, with the most significant increase observed for MFOs. Measurement of the kinetic constants of detoxification enzymes also suggested that the Km and Vmax values of GSTs and MFOs of the RR strain significantly differed from those in the SS strain. Further analysis of life history parameters showed that the developmental duration and lifespan of RR strain were significantly shortened, while egg numbers were considerably lower relative to SS strain. Specifically, the R0, λ, and Dt parameters for RR strain were significantly lower than those of SS strain. In conclusion, increased abamectin resistance in B. praetiosa is associated with a rapid increase in MFOs activity, and GSTs play a role in the development of abamectin resistance, with RR strain demonstrating concomitant reproductive disadvantages. These findings provide a theoretical basis for delaying the occurrence and progression of natural abamectin resistance and for guiding the scientific and rational use of abamectin as a biopesticide.