Introduction

The two-spotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae), is a destructive pest that causes serious damage to a wide range of crops and its host plants are over 140 families and 1100 species, including field crops, vegetables, fruits, and ornamental plants such as cotton, peach, strawberry, cucumber, soybean, eggplant, etc. (Maleknia et al. 2016; Mollaloo et al. 2017; Najafabadi et al. 2019). Tetranychus urticae uses its mouthparts to penetrate host cells and ingest cell contents (Wang et al. 2016), causing the leaves to lose green quickly until they wither and fall off. The population of T. urticae can be easily expanded because of its short life cycle and high reproductive potential (Saito et al. 2010; Nauen et al. 2001).

Using acaricides is the most common method to control T. urticae in recent years. However, the wide application of acaricides not only enables T. urticae to develop resistance (Brattsten et al. 1986; Van Leeuwen et al. 2010) but also leads to side effects on humans (García-Marí and González-Zamora 1999) and non-target organisms (Croft 1990), as well as the outbreak of secondary pests (Elzen 2001). One of the most used methods to manage resistance development and the conservation of biological agents is reduction of applied concentration (He et al. 2013; Song et al. 2013). Sublethal effects can be very delicate and affect populations at lower concentrations than the traditional ones (Stark and Banks 2003). In some cases, sublethal effects of pesticides can be integrated into pest control (Wang et al.2016). For instance, sublethal concentrations may increase pest developmental duration and reduce adult fecundity and longevity (Wang et al. 2016; Elzen 2001). Sublethal concentrations have also been applied to assess the selectivity of pesticides to beneficial mites (Alinejad et al. 2016, 2020; Bozhgani et al. 2019; Shahbaz et al. 2019). So, it is important to understand the sublethal effects and risks of acaracide application.

B-azolemiteacrylic shows excellent inhibition effects on mitochondrial respiratory chain complexes II, which mainly kills mites through contact and gastric toxicity. It also has quick effect, long duration of efficacy, broad spectrum of pests and low toxicity to non-target organisms such as bees, silkworms, fish and birds, and no interactive resistance to conventional acaricides such as abamectin and cypermethrin. It is safe for crops and environmentally friendly, and can meet the needs of integrated pest control (Song et al. 2017; Gong et al. 2017; Li 2016).

After application in the field, its toxicity will gradually decrease to sublethal doses with the extension of time and the change of environment. In addition to directly killing the target mites, some individuals will survive due to uneven application of the acaricide and other reasons, and suffer sublethal effects. As a result, the structure and size of the mite population will change again, and secondary pests will probably rise to become the primary ones (Quan et al. 2016; Han 2011). Therefore, understanding the sublethal effects of acaricides is key to evaluating their efficacy and acaricide risk management. Besides, there have been no reports on the sublethal effect of B-azolemiteacrylic on T. urticae. In the present study, the LC10 and LC30 of B-azolemiteacrylic were applied to T. urticae to investigate sublethal effects using the life-table method, and the related parameters were analyzed, aiming to evaluate the influence of sublethal effects on the development and reproduction of T. urticae, and to provide practical information for the rational use of B-azolemiteacrylic and comprehensive control of T. urticae in the field.

Material and methods

Mite colony maintenance and host plant

The stock population of T. urticae was originally obtained from Xinglong Mountain, Gansu Province, China, in May 2012, and it is known as a susceptible strain. Mites were reared on bean leaves (Phaseolus vulgaris L.) under acaricide-free conditions in an incubator at 25 ± 1 °C,75 ± 5% RH, and L16:D8 photoperiod.

Acaracide preparation

In this research a commercial formulation of B-azolemiteacrylic was used (SYP-9625, suspension concentrate 30%), produced by Baozhuo, Sinochem Crop Protection Products, China.

Concentration–response bioassay

Toxicity of pesticides to adults of two-spotted mites was tested using the leaf-dipping method (Meng 2002). A bean leaf was placed on a wet sponge in a Petri dish (7 cm diameter) and was surrounded with wet cotton to prevent the escape of mites. Thirty female adult spider mites were transferred to the leaf and prepared for bioassay. The control bean leaf was dipped with distilled water. B-azolemiteacrylic was diluted with distilled water, and five concentrations were prepared for testing: 0.8, 0.4, 0.2, 0.1 and 0.05 mg L−1. Every bean leaf with 30 adult spider mite females as mentioned above was dipped into each of the five B-azolemiteacrylic solution for 5 s, and then they were put in Petri dishes after blotting the spare pesticides. Concentration–response bioassay, comprising five concentrations and control, was carried out in four replications, with 180 females per replication and total sample size 720 females. The mortality covered the range of 10–90%. The LC50 value has a 95% confidence limit.

The mortality of adult females was recorded after 48 h of applying B-azolemiteacrylic. Mites were considered as dead if they did not show any reaction when touched by a brush. The Petri dishes were stored in a cabinet at 25 ± 1 °C, 75 ± 5% RH, and L16:D8 photoperiod.

Assessment of sublethal effects on F0 and F1 generations

Pre-ovipositional adult females from the stock population were transferred to fresh bean leaf discs (20 mites per 7-cm-diameter disc), each of which placed on wet cotton on a sponge in a Petri dish. After about 30–60 min, the discs were dipped for 5 s in distilled water (control) or B-azolemiteacrylic at LC10 or LC30. The sample size was 600 females. After 48 h, each survived female mite (F0 generation) was carefully moved to a new, fresh bean leaf disc with one adult male, which ensure that the pair could mate. Each concentration included 60 pairs. The females’ longevity and fecundity were recorded every 12 h until death. Eggs (F1 generation) laid by F0 generation were collected and transferred to new leaf discs, and each leaf disc only contained one egg. Each concentration included 60 eggs. Hatching rate and development of F1 generation were observed every 12 h. After they entered the adult stage, the sex ratio of F1 was calculated. Then all the females were subjected to further rearing, each paired with one male in a disc for 1 day. The longevity and fecundity were monitored until all females died.

Statistical analysis

In order to determine the LC values and sublethal concentrations, we used IBM SPSS v.24.0. The data obtained from F1 T. urticae were analyzed by one-way ANOVA followed by Tukey's honestly significant difference (HSD) test. Development duration, longevity, fecundity and demographic parameters of F1 T. urticae individuals were analyzed according to the two-sex life table procedure by using the Bootstrap method with 100,000 resamplings (Chi and Liu 1985; Chi 1988; Huang and Chi 2012). The paired bootstrap test was used to compare differences (Chi 2018). The computer program TWOSEX-MSChart (Chi 2018) was used to analyze the raw data. The survival rate curve was constructed using Kaplan–Meier test in IBM SPSS v.24.0.

Results

Estimation of LC10 and LC30 of B-azolemiteacrylic to Tetranychus urticae

The LC50 values of B-azolemiteacrylic on T. urticae was estimated to be 0.127 mg L−1 based on the leaf-dipping method, and then sublethal concentrations (LC10 and LC30) were calculated to be 0.043 and 0.009 mg L−1, respectively (Table 1).

Table 1 Regression equation of B-azolemiteacrylic treatment for 48 h on Tetranychus urticae
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Sublethal effects of B-azolemiteacrylic on F0 generation

After being treated with B-azolemiteacrylic at sublethal doses LC10 and LC30 for 48 h, the influence on their longevity and oviposition period was recorded. Longevity and oviposition period of adult females were significantly shortened after being treated with LC10 and LC30 of B-azolemiteacrylic (Fig. 1). Compared to the control’s 23.4 days, the longevity was reduced by 13.4% (LC10) and 17.1% (LC30); the oviposition period dropped from 11.46 days (control) to 8.95 days (LC10) and 8.05 days (LC30). Besides, the longevity and oviposition period at LC30 treatment were significantly shorter than at LC10 (Fig. 1).

Fig. 1
figure 1

Mean (± SE) longevity and oviposition period (days) of Tetranychus urticae treated with sublethal concentrations of B-azolemiteacrylic. Means capped with a different letter are significantly different (Tukey’s HSD test: Ρ < 0.05)

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The total and daily fecundity of the treated mites were significantly lower than of the control (Table 2). Total fecundity for untreated mites was 76.1 eggs/individual, whereas this was reduced by 30.9% (LC10) and 39.2% (LC30) after treatment. Compared with the control, the daily fecundity of each female dropped by 11.7% (LC10) and 16.6% (LC30). Total and daily fecundity at LC30 treatment were significantly lower than at LC10 (Table 2). The hatching rate (χ2 = 1.604, d.f. = 2, P = 0.45) and sex ratio (χ2 = 1.343, d.f. = 2, P = 0.51) of the F1 generation did not differ among the three treatments.

Table 2 Effects of treatment with two sublethal concentrations of B-azolemiteacrylic on mean (± SE) fecundity parameters of F0 generation of Tetranychus urticae
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Sublethal effects of B-azolemiteacrylic on F1 generation

The larva and adult periods and the average female longevity of the treated mites were significantly shortened (Table 3); at the LC10 treatment they were decreased by 5.4,13.3 and 8.0% respectively, whereas at the LC30 treatment reduction was 11.3, 17.4 and 9.4%, respectively. There were no significant differences in duration of the egg and deutonymph stages among all the treatments (Table 3).

Table 3 Effects of treatment with two sublethal concentrations of B-azolemiteacrylic on mean (± SE) developmental duration (days) of F1 generation of Tetranychus urticae
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The pre-oviposition period of the F1 generation in treatment had no significant difference from that of control, whereas the oviposition period was reduced by 20.1 and 20.7% at LC10 and LC30, respectively (Table 4). The post-oviposition period was significantly prolonged relative to the control, by 6.5% (LC10) and 10.6% (LC30). The total fecundity after LC10 and LC30 treatment was significantly lower than that of the control; it was decreased by 11 and 20.2%, respectively. Compared to the control, the sex ratio of F2 generation was also decreased (Table 4).

Table 4 Effects of treatment with two sublethal concentrations of B-azolemiteacrylic on mean (± SE) developmental duration (days) and fecundity parameters of F1 generation Tetranychus urticae
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The survival curves of F1 generation were similar with that of the control (χ2 = 1.627, d.f. = 2, P = 0.44), all of type I (arched curve) (Fig. 2). The survival rate of both treatments were lower than that of the control except for the egg stage, and treated mites lived shorter than mites of the control group.

Fig. 2
figure 2

Age-specific survival rate (Ix) for F1 generation of Tetranychus urticae treated with sublethal concentrations of B-azolemiteacrylic

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Fecundity (Mx, the average number of females produced by a female mite) earliest at LC30 treatment (on day 13), then at LC10 (day 14) and latest at the control (day 16). The peak was highest for the control, and lowest for the LC30 treated mites (Fig. 3), indicating that the capability of each adult to produce females decreased after being with a sublethal dose of B-azolemiteacrylic.

Fig. 3
figure 3

Age-specific fecundity (mx) for F1 generation of Tetranychus urticae treated with sublethal dosage of B-azolemiteacrylic

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The net reproductive rate (R0) of both treatments was significantly lower than that of the control group—compared to the control, R0 was 33.3% (LC10) and 51.3% (LC30) lower (Table 5), indicating that the B-azolemiteacrylic had a great impact on the fecundity of the F1 generation. Compared with the control group, the mean generation time (T), the intrinsic rate of increase (rm), the finite rate of increase (λ), and the population doubling time for mites treated with both sublethal concentrations of B-azolemiteacrylic did not differ significantly (Table 5).

Table 5 Effects of treatment with two sublethal concentrations of B-azolemiteacrylic on mean (± SE) biological parameters of F1 generation Tetranychus urticae
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Discussion

In the present study, the biological parameters and demographic data related to different generations of T. urticae were investigated by applying sublethal concentrations of B-azolemiteacrylic. In recent years, a number of studies have been conducted for evaluating the lethal and sublethal effects of various pesticide groups such as tetrazine, tetronic acid, pyrazolium, pyrethroid, organophosphate, pyridine azomethines, and neonicotinoid derivatives on two-spotted spider mites, as well as its predatory mites (Hamedi et al. 2010, 2011; Lima et al. 2013; Alinejad et al. 2016; Bozhgani et al. 2018; Havasi et al. 2021). As one of the effective acrylonitrile group acaricides, however, no sublethal effects of B-azolemiteacrylic on biological parameters of T. urticae were known.

Our study indicated that when treated by B-azolemiteacrylic at LC30, the protonymph stage was significantly prolonged, and the larvae stage, adult stage and average life span were shortened. In addition, the oviposition period, fecundity and sex ratio from mites of the F1 generation treated at LC10 and LC30 were also decreased. These results corresponded with those of Havasi et al. (2018), in which the experimental concentration of diflovidazin played a negative role during all pre-adult developmental stages such as the egg, larva, protonymph, and deutonymph among males. Regarding females, no significant difference was observed between the immature stages for all the tested concentrations, except in egg and protonymph stages. Similar results were also seen in other investigations (Fan 2015; Tian 2017; Gao 2018). On the contrary, an increase in the concentration caused a significant difference during immature stages of T. urticae in males and females when treated by sublethal concentrations of bifenazate (Li et al. 2017). This might be caused by a different working mechanism of the two agents.

The results of the present study indicated the sublethal concentration had a certain inhibitory influence on the population growth of F0 generation, which was specifically displayed in decreases of longevity, oviposition period, fecundity and hatching rate, sex ratio of the next generation; the higher the concentration, the greater the degree in reduction. Negative sublethal effects of a variety of acaricides on, for instance, fecundity, life span, and oviposition period of pest mites have been reported by many researchers (Yong et al. 2011; Tao and Wu 2006; Xin et al. 2019; Li et al. 2016; Bozhgani et al. 2019; Havasi et al. 2020). Our results were consistent with those of Alinejad et al. (2015), in which a significant decrease happened in longevity after being treated with sublethal concentrations of fenazaquin. Similarly, a significant decrease occurred in the longevity for mites treated with azadirachtin at 64 and 128 ppm (Martínez-Villar et al. 2005), the reduction in fecundity was shown after treatment with a sublethal dose of spiromesifen (Marcic 2005). Reduction of the oviposition period can decrease the next-generation population size. Shortening of the life span would not only restrain fecundity, but also lower the potential damage caused by pest mites to their hosts.

Life-table parameters play a vital role in the comprehensive evaluation of the controlling effect of pesticides against mites. It is recommended to evaluate the sublethal effect of agents on target pests with the instantaneous rate of increase (r) or intrinsic rate of increase (rm) of the population, and conduct a comprehensive study with the life table technology (Stark and Wennergren 1995, Stark and Banks 2003). In this study, the net reproductive rate (R0) following the treatment of females from F0 generation with sublethal concentrations of B-azolemiteacrylic was significantly lower than that of the control group, but the intrinsic rate of increase (rm) and finite rate of increase (λ) were not significantly different from the control. The results were congruent with those of Wang et al. (2014a, b) and Marcic (2007), in which the sublethal doses of bifenthrin (LC10 and LC25) and spirodiclofen (6, 12, 24, 48, and 96 mg L−1) were examined on the two-spotted spider mite, respectively. Similar results about the effect of triflumuron on T. urticae were also seen in the study of Sáenz-de-Cabezón et al. (2006).

Based on the results of the present study, the exposure to sublethal concentrations of B-azolemiteacrylic had a negative effect on biological parameters of T. urticae (i.e., lower R0). B-azolemiteacrylic sublethal doses could effectively inhibit the developmental rates of F0 and F1 populations of T. urticae, and the higher the concentration, the stronger the inhibition effect. Besides, no proliferation effect was found in T. urticae population, which suggests that T. urticae may not easily develop resistance to B-azolemiteacrylic. This advantage is of positive significance to the formulation of integrated management strategies for T. urticae. Consequently, it is recommended that applying B-azolemiteacrylic at lower rates could lead to effective control of T. urticae. Nevertheless, most of the similar experiments including ours carried out under laboratory conditions may not be fully representative of a natural field, because environmental complexity, different plants and other natural characteristics cannot be 100% replicated in a small room. Further experiments carried out under greenhouse and field conditions are therefore needed.