Traditional estimating only by measuring the lethal effect of acaricides may underestimate the total effects of acaricides on the pest mites. In order to investigate the sublethal effect of bifenazate on life history and population parameters of the two-spotted spider mite, Tetranychus urticae Koch, the newly emerged females were treated with two lethal concentrations of bifenazate: LC10 (4.92 µg/mL) and LC20 (8.77 µg/mL). Subsequently, the development and fecundity of the progeny generations were observed. Compared to the control, exposure to the 10% lethal concentrations (LC10) and LC20 of bifenazate severely affected the parental generation of T. urticae, including survival rate (reduced 9% and 13%), oviposition period (reduced 77.6% and 83.1%), fecundity per female (decreased 89.2% and 76.9%) and longevity (decreased 79.2% and 83.1%). Besides, the population parameters of the progeny generation from the treated females were also investigated. The results showed that the progeny generation had lower intrinsic rate of increase (rm) and finite rate of increase (λ), longer mean generation time (Tc) compared to the control. The results suggested that the sublethal effects of bifenazate on population growth of T. urticae were significant, and the results of this study could be used as a guide for the rational use of bifenazate in the field for better managing pest mites.
The two-spotted spider mite, Tetranychus urticae Koch, which infests more than 1100 species of host plants, is the most important polyphagous spider mite and a vital pest in temperate regions for many crops (Hamedi et al. 2011; Amini et al. 2016; Migeon & Dorkeld 2016). Although biological control of T. urticae has been proven to be successful in many crops growing in greenhouses, acaricides have always played the central role in its control in field crops, owing to their lower cost and strong effects (Sanderson & Zhang 1995; Van Leeuwen et al. 2010; Jafari et al. 2016). When applied to the field, pesticides not only bring direct acute lethal effect on insects and mites, but also affect the life history traits of individuals surviving through the pesticide treatments. It has been reported that exposure to the sublethal dose of spiromesifen reduced the fecundity or fertility of T. urticae (Marcic et al. 2010), and exposure to the sublethal concentration of clofentezine decreased the hatchability of eggs produced by the surviving females of T. viennensis (Li et al. 2006). In addition, exposure to the sublethal concentration of acaricides also affected the population parameters of tetranychid mites and their phytoseiid predators (Hamedi et al. 2009; He et al. 2011; Pakyari & Enkegaard 2015).
Bifenazate belongs to the group of hydrazine derivatives (Van Leeuwen et al. 2010), and is being used worldwide for control of spider mites on several crop systems (Dekeyser 2005; Van Leeuwen et al., 2015). Recent studies reported that the resistance of T. urticae to bifenazate is tightly linked to the mutations in the mitochondrial cytochrome b (cytb) and the Q0 site of cytb complex III of the electron transport chain (Van Leeuwen et al. 2008; Van Nieuwenhuyse et al. 2009). Reciprocal crosses between the susceptible and resistant mites showed that the resistance was only inherited maternally (non-Mendelian), supporting a hypothesis called mitochondrial control (Van Leeuwen et al. 2006).
Life table, in ecology, is a table for simply and intuitively reflecting the population survival and death process (Chi 1988). It is suggested that life table analysis is the best method to evaluate the lethal and sublethal effects of an acaricide (Kim et al. 2006; Mohammadi et al. 2016). The current study was focused on the sublethal effects of bifenazate on the females of pre-ovipositional stage of the two-spotted spider mite, as well as the population parameters of their progeny. Our results can serve as a reference to determine the rational use of bifenazate as an effective acaricide to manage the two-spotted spider mite in the field.
Materials and methods
The two-spotted spider mite was introduced from the Key Laboratory of Grassland Ecosystem of Education Ministry, College of Grassland Science, Gansu Agricultural University in 2011. After introduction to our laboratory, the mites were reared on cowpea Vigna unguiculata in a climate chamber under 25 ± 1 °C, 75 ± 5 % RH, and a photoperiod of L:D = 14:10 h. Before this experiment, the mites had never contacted with bifenazate or any other acaricides.
Concentration response bioassay
Bifenazate, commercial formulation Acramite® (suspension concentrate, 43%, Chemtura, USA) used in this study was purchased from Beijing NewGreen Environ-Tech. Co., Ltd. The toxicity of bifenazate against females of T. urticae was determined using leaf-residues method (Hamedi et al. 2010; Mahmoudvand et al. 2011). Based on the recommended concentration in the field by its manufacturer, bifenazate were diluted to six concentration gradients (430, 215, 107.5, 53.75, 26.88, and 13.44 µg·a.i./mL) with distilled water. The cowpea leaf discs (3 cm in diameter) were placed into the diluted liquid for 5 s and then allowed them to be naturally dried. The leaf discs treated by distilled water only served as control. Then the leaf discs were placed in Petri dish (8.5 cm in diameter × 2 cm in height, with the sponge and a layer of absorbent cotton inside). Thirty mated female mites of pre-oviposition period (each newly emerged female was paired with a male adult for 12 h) were transferred on the surface of each leaf disc. A bioassay with six concentrations of bifenazate and a control was replicated five times. After 24 h, the mortality of the tested mites was recorded under binoculars. Mites that failed to move after a gentle touch by a camel hair brush were considered as dead.
Sublethal effects of bifenazate on the survival and reproduction of T. urticae females
Bifenazate was diluted with distilled water to 10% lethal concentration (4.92 µg/mL) and 20% lethal concentration (8.77 µg/mL) based on above bioassay. The cowpea leaf discs (3 cm in diameter) were put into the diluted liquid for 5 s and then allowed them to be dried, and the leaf discs were placed into Petri dishes. One newly emerged mated female was transferred to the surface of each leaf disc. In total 100 females were treated for each concentration of bifenazate. After 24 h, the survived female was transferred to new clean leaf discs without bifenazate, where a male mite was added for mating (the males was re-added if the previous male mite died). The numbers of females survived and eggs were recorded every 24 h, until all females died naturally. The leaf disc treated with distilled water only served as the control.
Sublethal effects of bifenazate on development and population parameters of the progeny generations
To evaluate further effects of bifenazate on the treated females, 100 eggs of each treatment (LC10, LC20 and control) were collected randomly (Hamedi et al. 2011), and placed on the surface of the leaf disc individually. The survivorship, growth and development were checked every 24 h. After eclosion, males and females were paired. In case there were no enough males to pair with females, males from the stock colony were also used (these males did not included in life table analysis). If the male died or escaped, another male was introduced. The survival of the mites and the number of eggs laid were checked daily until they die. The females that died because of improper handling or escape from the leaf disc were excluded from the data analysis.
Estimation of the LC50, the lethal concentrations and the regression equation for the concentration mortality line were obtained using a probit program of SPSS 16.0 for Windows (SPSS, Chicago, IL, USA). The life history raw data of all individuals were analyzed based on the age-stage, two-sex life table theories (Chi & Liu 1985), which can analyze the growth process of mite population of both females and males (Chi 1988). The means and standard errors of the population parameters were estimated by using the bootstrap technology (Huang & Chi 2013). The computer program TWOSEX-MSChart (Chi 2015) was used for life table analysis. The developmental duration for immature stages and adult longevity, the reproductive period and the total female fecundity, the age-specific survival rate (lx), the age-specific fecundity (mx) and the population parameters: the intrinsic rate of increase from the Euler-Lotka equation: Σ e- rm(x+1)lxmx=1; finite rate of increase λ = e rm;. net reproduction rate R0 = Σ lxmx; the mean generation time Tc = (In R0)/rm, were calculated accordingly. Means, variances and standard errors of longevity, fecundity, and duration of immature stage among different treatments were estimated with the bootstrap technique using 10,000 replications to generate less-variable results.
Concentration response bioassay
The regression equation between concentration and mortality of bifenazate against females of T. urticae was y = -2.49 + 1.75 x; [y = mortality (probit), x = logarithm of concentration (ppm)], with the correlation coefficient value of 0.9890], The LC50, LC20 and LC10 of bifenazate against T. urticae were 26.54, 8.77 and 4.92 µg/mL, respectively. No mortality was observed in the controls.
Sublethal effects of bifenazate on the survival and reproductions of T. urticae females
The results showed the 10% lethal concentration (LC10) and LC20 of bifenazate significantly affected the female oviposition periods, average oviposition, and longevity (Table 1 ). The number of females that laid eggs (ovipositing individual) and the survival rate were distinctly reduced after exposure to bifenazate at LC10 and LC20. After a 24 h exposure to bifenazate, the proportion of females that survived from treatment was 0.91 (LC10) and 0.87 (LC20), while there were no dead females in control; after 10 days the survival rate in both LC10 and LC20 treatments were lower than 0.1, far below the control (Figure 1 ). The fecundity of the treated female was obviously less than the control, and fecundity of female treated with LC20 of bifenazate was higher than that with LC10 during 5 to 10 d (Figure 2).
Oviposition per female, oviposition period, female longevity and survival of T. urticae females treated with LC10 and LC20 of bifenazate, estimated using all individuals and the bootstrap technique.
Sublethal effects of bifenazate on development and populatin parameters of the progeny generations
The preadult survival rate of progeny generation in both LC10 and LC20 of bifenazate treatment were significantly lower than that of the control. The duration of immature stages of females of the progeny generation were significantly prolonged both in the treatments of LC10 and LC20 of bifenazate compared with the control, whereas the oviposition period, female average oviposition and female longevity were not significantly different from the control (Table 2). The duration of immature stages of the male of the progeny generation in treatment of LC10 was significantly prolonged compared with the control, whereas that of LC20 had no significant difference with that of the control. On the contrary, the male adult longevity of progeny generation in treatment of LC20 was significantly shortened compared to the control, whereas that of LC10 had no significant difference with that of the control (Table 2).
Developmental durations of immature stages, oviposition periods, longevity of offspring from female T. urticae treated with LC10 and LC20 of bifenazate. estimated using all individuals and the bootstrap technique.
The rm and λ of population treated with the LC10 and LC20 of bifenazate were both significantly decreased, and the progeny generation had a longer Tc, which these three population parameters of progeny generation treated between LC10 and LC20 of bifenazate were not different significantly (Table 3). The value of R0 of progeny population treated with the 20% lethal concentration was reduced significantly. Nevertheless, the value of R0 of population treated with the 10% lethal concentration had no significant difference from the control (Table 3). The lx of female offspring decreased with the increasing concentrations of bifenazate, however, the lx of male offspring of LC20 was higher than that of LC10, although both of them were lower than lx of male offspring of the control (Figure 3). The mx,fx and lxmx of the female offspring showed the synchronization for both two lethal concentrations compared with the control (Figure 4).
Population parameters, mean ± the standard error, of offspring T. urticae from female treated with LC10 and LC20 of bifenazate, estimated using all individuals and the bootstrap technique.
Bifenazate had acaricidal activity against T. urticae (Ochiai et al. 2007), and the resistance mechanism of T. urticae to bifenazate was thoroughly illuminated by several studies (Van Leeuwen et al. 2007; Van Leeuwen et al. 2008; Van Nieuwenhuyse et al. 2009; Van Leeuwen et al. 2011). Our study here is the first comprehensive investigation on the sublethal effects of bifenazate on life table parameters of progeny generation of T. urticae. Our results demonstrated that bifenazate had a strong adulticidal activity on T. urticae, which was in accordance with the previous report (Ochiai et al. 2007). Furthermore, our results found that LC10 and LC20 of bifenazate could reduce the survival rate, oviposition period, average oviposition and longevity of the female of T. urticae. This was also consistent with previous studies. For example, Marcic (2005) concluded that females of T. urticae survived from the treatment of tebufenpyrad at protonymphal or deutonymphal stage had the lower fertility, and the females of two-spotted spider mite treated with the sublethal concentration/dose of spirodiclofen (Marcic 2007), spirotetramat (Marcic et al. 2012), Beauveria bassiana (Seyed-Talebi et al. 2012), fenpyroximate and pyridaben (Kim et al. 2006) had shorter or lower longevity, reproduction period and fecundity than the controls.
It is known that exposure to pesticides can lead to hereditary malfunctions and malformations, and hence can lead to significant disturbances of insect or mite development in the next generation (Adamski et al. 2009). Our results demonstrated that exposure to lethal concentrations of bifenazate at LC10 and LC20 during the female adult stage had negative effects on the two-spotted mite population increase of progeny generation (i.e. lower rm, and λ values, but higher Tc). According to He et al. (2011), exposure to sublethal concentrations of avermectin during the adult stage had negative effects on the population increase (i.e. lower rm, R0, and λ values, longer Dt) of the progeny generation of Panonychus citri (McGregor). Li et al. (2006) also reported that the values of rm in offspring of abamectin-treated adult females of Amphitetranychus viennensis decreased significantly. However, Landeros et al. (2002) reported that there was a significant increase in the R0 of T. urticae after applications of abamectin at LC10. In this study, the mites treated with LC20 of bifenazate had negative effects on R0of the mites. LC20 treatment caused significant changes in the progeny including the prolonged immature stages, decreased preadult survival rate and adult male longevity.
The rm is the most important parameter to describe the growth potential of a pest population. The results of this study showed that values of rm of progeny generation of the treated mites significantly decreased. Although the preadult survival rate and population growth of progeny generation of treated mites was substantially lower, the female adult longevity, oviposition period and average oviposition per female were not different from those of population of the control. In other words, the surviving females of the progeny generation treated with LC10 and LC20 of bifenazate were not affected. This phenomenon may refer to the arcaricide resistance of bifenazate. Van Leeuwen et al. (2010) reported the progeny of T. urticae could absorb small amount of bifenazate from female treated with the sublethal concentration of bifenazate by maternal inheritance referring to the bifenazate resistance. In some other studies, low concentration of bifenazate caused an effect called hormesis, which is referred to biphasic dose response relationship with stimulatory effect at low doses and inhibitory effect at high doses of a stressor or pesticide (Luckey 1968; Guedes & Cutler 2014). There are two hypotheses which provided the potential mechanistic explanation for hormesis: the growth hormesis theory (Stebbing 2000), and the principle of physiological resource allocation (Weitje et al. 2005). According to the growth theory, this hypothesis is based on control systems and growth analysis (Stebbing 1998; Stebbing 2000). The control mechanisms that regulate growth counteract the perturbing or inhibitory effects of toxic agents (Guedes & Cutler 2014). This idea remains highly speculative, focused on growth as a life-history trait and lacking a general mechanistic underpinning (Thayer et al. 2005; Mushak 2007; Jager et al. 2013). The principle of resource allocation is based on physiological energetic models that predict trade-offs in resource allocation among different physiological processes (Forbes 2000; Guedes & Cutler 2014). Hormesis by acquisition is explained by an unlimited increase in energy uptake from the environment, when such a resource is limitless (Jager et al. 2013). This increased energy uptake favors growth rate, and possibly maximizes size, which increase reproduction and population growth rate (Guedes & Cutler 2014). The results of our study showed that, the LC10 and LC20 of bifenazate had no negative effects on the offspring of the treated female T. urticae in fecundity and longevity. This situation may be explained by resource allocation, but further studies are necessary to confirm this conjecture.
Investigations on sublethal effects of an acaricide aim to discover the negative non-lethal impacts of the acaricide on various life table parameters that might affect population dynamics (Stark & Banks 2003), and help us gain more information about the acaricide and put it to better use. Our study is the first step to explore the sublethal effect of bifenazate on progeny generation of T. urticae. Based on our results, the survival and the population growth of progeny generation of T. urticae was substantially decreased, indicating that low concentration of bifenazate could pay an important role when this arcaricide was combined with other biological control agents like natural enemies. However, future field investigation will provide more information to guide the rational use of this acaricide.
This study was partially supported by the Innovation Fund for Social Programs and People’s Livelihood Guarantee of Chongqing (cstc2015shms-ztzx80011). We thank Professor Hsin Chi (Laboratory of Theoretical and Applied Ecology, Department of Entomology, National Chung Hsing University, Taiwan) for providing the software and for assistance with data formatting.