Bifenthrin is a broad-spectrum insecticide and acaricide that is widely used in China. We evaluated the effects of sublethal concentrations (LC10 and LC25) of bifenthrin on the eggs and adult females of the two-spotted spider mite, Tetranychus urticae, in the laboratory at 26±1°C, 80% RH, and a 16 h: 8 h (L: D) photoperiod. The sublethal doses of bifenthrin decreased the intrinsic and finite rate of increase, net reproductive rate, survival rate, and reproductive value. The sublethal doses also increased the mean generation time, total pre-ovipositional period, and duration of the larval and nymphal stages. The intrinsic rate of increase dropped from 0.252/day in the control to 0.222 and 0.208/day in response to LC10 and LC25 treatments, respectively. Following LC10 and LC25 treatments, the net reproductive rate dropped from 60.65 offspring/individual in the control to 45.19 and 40.81, respectively. These laboratory results indicate that sublethal concentrations of bifenthrin may decrease the developmental rate of T. urticae, are unlikely to result in the resurgence of T. urtciae populations, and might therefore be useful in the integrated management of this pest.
The two-spotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae), is a cosmopolitan and destructive pest of agricultural crops in China and elsewhere. Pesticides are widely used against T. urticae. Such pesticides include bifenthrin, which is a pyrethroid insecticide and acaricide that is widely used against many insect and mite pests of the agricultural crops and orchards. The current study concerns the effects of sublethal concentrations of bifenthrin on T. urticae.
When too little pesticide has been applied or when the pesticide has degraded, pests are likely to be exposed to sublethal concentrations. In some cases, sublethal concentrations of pesticides can contribute to pest management. For example, sublethal pesticide concentrations may increase pest developmental time and reduce adult longevity and fecundity (Wang et al. 2009; Song et al. 2013; He et al. 2013). In other cases, however, sublethal doses of insecticides can cause a resurgence of the pest population (Hall 1979; Liu et al. 1998). Therefore, an understanding of sublethal effects is fundamental to understanding the efficacy and risk of pesticide application (Desneux et al. 2007).
Pyrethroid insecticides like bifenthrin interfere with the insect nervous system, resulting in trembling or paralysis, which is usually followed by death. Because of their rapid action and excellent contact toxicity against a broad-spectrum of arthropod pests, pyrethroids are often used to control insects and spider mites (Herron et al. 2001; Zhang et al. 2012). The effects of lethal and sublethal concentrations of some pyrethroid insecticides on various mite species have been investigated (Liu et al. 1998; Bowi et al. 2001; Zhang et al. 2012), and most studies have found that pyrethroid application induces resurgence of the pest population (Gerson & Cohen 1989; Dutcher 2007). In a laboratory study with the mite T. cinnabarinus, deltamethrin increased oviposition and cypermethrin increased population growth, suggesting that application of these pyrethroids probably contributed to the population resurgence observed in treated fields (Liu et al. 1998). In a field study, deltamethrin application increased T. cinnabarinus numbers (Gao et al. 1991). The results of other studies, however, did not indicate that the effects of sublethal pesticide concentrations would result in the resurgence of mite populations (Bowi et al. 2001; Zhang et al. 2012).
As noted earlier, the pyrethroid bifenthrin is widely used against many insect and mite pests including T. urticae. Because the effects of sublethal concentrations of bifenthrin on T. urticae are unknown, we conducted a laboratory study to determine the sublethal effects of bifenthrin on eggs and adult females of T. urticae. The results obtained will provide fundamental information for the management of this important pest. More specifically, the results will increase our understanding of whether bifenthrin application contributes to the resurgence of T. urticae populations.
2. Materials and methods
2.1 Mite and insecticide
Specimens of T. urticae were originally obtained from an apple orchard in Tai'an Shandong Province, China, in June 2009. The population was maintained on bean leaf discs (var. Bifeng) on moist sponges in Petri dishes (12-cm-diameter dishes) in an incubator at 26±1°C, 80% RH, and a photoperiod of 16 h: 8 h (L: D). Cotton strips placed around each leaf disc prevented mite escape. These incubator conditions were used for all experiments in this study.
The bifenthrin formulation used in this study was an emulsifiable concentrate (Bayer Cropscience China Co., Ltd., China) containing 100 g/kg of active ingredient.
2.2 Bioassay and determination of sublethal concentrations
Bioassays were conducted with eggs and adult females of T. urticae using the leaf dipping method (He et al., 2011). Serial dilutions of bifenthrin were prepared with pure water. Six concentrations (including the water control) were used. Each bean leaf disc (2 cm in diameter), which contained either 30 recently deposited eggs or 30 24-h-old adult females, was dipped into a solution for 5 s and then quickly dried using small pieces of filter paper (the filter paper absorbed the excess acaricide solution attached to the surface of the mites and leaf disc). Each leaf disc was then placed on a sponge in a Petri dish, and the dishes were placed in the incubator. One Petri dish with one disc was regarded as a replicate, and four replicate dishes were used for each concentration. Mortality of adult females was assessed after 24 h; female mites that could not crawl and were non-functional when touched with a camel hair brush were scored as dead. For the eggs, mortality was assessed daily starting with the eclosion of the first protonymph (about 6 d after treatment) and continuing for five successive days. When more than 90% eggs in the controls had eclosed except those that had died because of physiological causes, eggs that had not developed into larvae were scored as dead. Egg mortality was determined by subtracting the number of protonymphs from the total number of eggs.
Mortality data for adult females and eggs were corrected using the Abbott's formula (Abbott 1925), and the LC10 and LC25 values and their 95% fiducial limits and slope ± SE were calculated from probit analysis using Polo Plus Version 1.0 software (LeOra Software, Berkeley, CA,USA). According t